erlang

The Erlang BIFs

By convention, most built-in functions (BIFs) are seen as being in the module erlang. A number of the BIFs are viewed more or less as part of the Erlang programming language and are auto-imported. Thus, it is not necessary to specify the module name and both the calls atom_to_list(Erlang) and erlang:atom_to_list(Erlang) are identical.

In the text, auto-imported BIFs are listed without module prefix. BIFs listed with module prefix are not auto-imported.

BIFs may fail for a variety of reasons. All BIFs fail with reason badarg if they are called with arguments of an incorrect type. The other reasons that may make BIFs fail are described in connection with the description of each individual BIF.

Some BIFs may be used in guard tests, these are marked with "Allowed in guard tests".

Types


ext_binary()

A binary data object, structured according to the Erlang external term format.

timestamp/0

See now/0.

Functions


abs/1

Returns an integer or float which is the arithmetical absolute value of Float or Int.

> abs(-3.33).
3.33
> abs(-3).
3

Allowed in guard tests.

adler32/1

Computes and returns the adler32 checksum for Data.

adler32/2

Continue computing the adler32 checksum by combining the previous checksum, OldAdler, with the checksum of Data.

The following code:

X = erlang:adler32(Data1), Y = erlang:adler32(X,Data2).

- would assign the same value to Y as this would:

Y = erlang:adler32([Data1,Data2]).

adler32_combine/3

Combines two previously computed adler32 checksums. This computation requires the size of the data object for the second checksum to be known.

The following code:

Y = erlang:adler32(Data1), Z = erlang:adler32(Y,Data2).

- would assign the same value to Z as this would:

X = erlang:adler32(Data1), Y = erlang:adler32(Data2), Z = erlang:adler32_combine(X,Y,iolist_size(Data2)).

append_element/2

Returns a new tuple which has one element more than Tuple1, and contains the elements in Tuple1 followed by Term as the last element. Semantically equivalent to list_to_tuple(tuple_to_list(Tuple1) ++ [Term]), but much faster.

> erlang:append_element({one, two}, three).
{one,two,three}

apply/2

Call a fun, passing the elements in Args as arguments.

Note: If the number of elements in the arguments are known at compile-time, the call is better written as Fun(Arg1, Arg2, ... ArgN).

Warning!

Earlier, Fun could also be given as {Module, Function}, equivalent to apply(Module, Function, Args). This usage is deprecated and will stop working in a future release of Erlang/OTP.

apply/3

Returns the result of applying Function in Module to Args. The applied function must be exported from Module. The arity of the function is the length of Args.

> apply(lists, reverse, [[a, b, c]]).
[c,b,a]

apply can be used to evaluate BIFs by using the module name erlang.

> apply(erlang, atom_to_list, ['Erlang']).
"Erlang"

Note: If the number of arguments are known at compile-time, the call is better written as Module:Function(Arg1, Arg2, ..., ArgN).

Failure: error_handler:undefined_function/3 is called if the applied function is not exported. The error handler can be redefined (see process_flag/2). If the error_handler is undefined, or if the user has redefined the default error_handler so the replacement module is undefined, an error with the reason undef is generated.

atom_to_binary/2

Returns a binary which corresponds to the text representation of Atom. If Encoding is latin1, there will be one byte for each character in the text representation. If Encoding is utf8 or unicode, the characters will be encoded using UTF-8 (meaning that characters from 16#80 up to 0xFF will be encoded in two bytes).

Note!

Currently, atom_to_binary(Atom, latin1) can never fail because the text representation of an atom can only contain characters from 0 to 16#FF. In a future release, the text representation of atoms might be allowed to contain any Unicode character and atom_to_binary(Atom, latin1) will fail if the text representation for the Atom contains a Unicode character greater than 16#FF.

> atom_to_binary('Erlang', latin1).
<<"Erlang">>

atom_to_list/1

Returns a string which corresponds to the text representation of Atom.

> atom_to_list('Erlang').
"Erlang"

binary_part/2

Extracts the part of the binary described by PosLen.

Negative length can be used to extract bytes at the end of a binary:

1> Bin = <<1,2,3,4,5,6,7,8,9,10>>. 2> binary_part(Bin,{byte_size(Bin), -5}). <<6,7,8,9,10>>

If PosLen in any way references outside the binary, a badarg exception is raised.

Start is zero-based, i.e.:

1> Bin = <<1,2,3>> 2> binary_part(Bin,{0,2}). <<1,2>>

See the STDLIB module binary for details about the PosLen semantics.

Allowed in guard tests.

binary_part/3

The same as binary_part(Subject, {Start, Length}).

Allowed in guard tests.

binary_to_atom/2

Returns the atom whose text representation is Binary. If Encoding is latin1, no translation of bytes in the binary is done. If Encoding is utf8 or unicode, the binary must contain valid UTF-8 sequences; furthermore, only Unicode characters up to 0xFF are allowed.

Note!

binary_to_atom(Binary, utf8) will fail if the binary contains Unicode characters greater than 16#FF. In a future release, such Unicode characters might be allowed and binary_to_atom(Binary, utf8) will not fail in that case. For more information on Unicode support in atoms see note on UTF-8 encoded atoms in the chapter about the external term format in the ERTS User's Guide.

> binary_to_atom(<<"Erlang">>, latin1).
'Erlang'
> binary_to_atom(<<1024/utf8>>, utf8).
** exception error: bad argument
     in function  binary_to_atom/2
        called as binary_to_atom(<<208,128>>,utf8)

binary_to_existing_atom/2

Works like binary_to_atom/2, but the atom must already exist.

Failure: badarg if the atom does not already exist.

binary_to_float/1

Returns the float whose text representation is Binary.

> binary_to_float(<<"2.2017764e+0">>).
2.2017764

Failure: badarg if Binary contains a bad representation of a float.

binary_to_integer/1

Returns an integer whose text representation is Binary.

> binary_to_integer(<<"123">>).
123

Failure: badarg if Binary contains a bad representation of an integer.

binary_to_integer/2

Returns an integer whose text representation in base Base is Binary.

> binary_to_integer(<<"3FF">>, 16).
1023

Failure: badarg if Binary contains a bad representation of an integer.

binary_to_list/1

Returns a list of integers which correspond to the bytes of Binary.

binary_to_list/3

1..byte_size(Binary)

As binary_to_list/1, but returns a list of integers corresponding to the bytes from position Start to position Stop in Binary. Positions in the binary are numbered starting from 1.

Note!

This function's indexing style of using one-based indices for binaries is deprecated. New code should use the functions in the STDLIB module binary instead. They consequently use the same (zero-based) style of indexing.

bitstring_to_list/1

Returns a list of integers which correspond to the bytes of Bitstring. If the number of bits in the binary is not divisible by 8, the last element of the list will be a bitstring containing the remaining bits (1 up to 7 bits).

binary_to_term/1

Returns an Erlang term which is the result of decoding the binary object Binary, which must be encoded according to the Erlang external term format.

Warning!

When decoding binaries from untrusted sources, consider using binary_to_term/2 to prevent denial of service attacks.

See also term_to_binary/1 and binary_to_term/2.

binary_to_term/2

As binary_to_term/1, but takes options that affect decoding of the binary.

safe

Use this option when receiving binaries from an untrusted source.

When enabled, it prevents decoding data that may be used to attack the Erlang system. In the event of receiving unsafe data, decoding fails with a badarg error.

Currently, this prevents creation of new atoms directly, creation of new atoms indirectly (as they are embedded in certain structures like pids, refs, funs, etc.), and creation of new external function references. None of those resources are currently garbage collected, so unchecked creation of them can exhaust available memory.

Failure: badarg if safe is specified and unsafe data is decoded.

See also term_to_binary/1, binary_to_term/1, and list_to_existing_atom/1.

bit_size/1

Returns an integer which is the size in bits of Bitstring.

> bit_size(<<433:16,3:3>>).
19
> bit_size(<<1,2,3>>).
24

Allowed in guard tests.

bump_reductions/1

This implementation-dependent function increments the reduction counter for the calling process. In the Beam emulator, the reduction counter is normally incremented by one for each function and BIF call, and a context switch is forced when the counter reaches the maximum number of reductions for a process (2000 reductions in R12B).

Warning!

This BIF might be removed in a future version of the Beam machine without prior warning. It is unlikely to be implemented in other Erlang implementations.

byte_size/1

Returns an integer which is the number of bytes needed to contain Bitstring. (That is, if the number of bits in Bitstring is not divisible by 8, the resulting number of bytes will be rounded up.)

> byte_size(<<433:16,3:3>>).
3
> byte_size(<<1,2,3>>).
3

Allowed in guard tests.

cancel_timer/1

Cancels a timer, where TimerRef was returned by either erlang:send_after/3 or erlang:start_timer/3. If the timer is there to be removed, the function returns the time in milliseconds left until the timer would have expired, otherwise false (which means that TimerRef was never a timer, that it has already been cancelled, or that it has already delivered its message).

See also erlang:send_after/3, erlang:start_timer/3, and erlang:read_timer/1.

Note: Cancelling a timer does not guarantee that the message has not already been delivered to the message queue.

check_old_code/1

Returns true if the Module has old code, and false otherwise.

See also code(3).

check_process_code/2

Returns true if the process Pid is executing old code for Module. That is, if the current call of the process executes old code for this module, or if the process has references to old code for this module, or if the process contains funs that references old code for this module. Otherwise, it returns false.

> check_process_code(Pid, lists).
false

See also code(3).

crc32/1

Computes and returns the crc32 (IEEE 802.3 style) checksum for Data.

crc32/2

Continue computing the crc32 checksum by combining the previous checksum, OldCrc, with the checksum of Data.

The following code:

X = erlang:crc32(Data1), Y = erlang:crc32(X,Data2).

- would assign the same value to Y as this would:

Y = erlang:crc32([Data1,Data2]).

crc32_combine/3

Combines two previously computed crc32 checksums. This computation requires the size of the data object for the second checksum to be known.

The following code:

Y = erlang:crc32(Data1), Z = erlang:crc32(Y,Data2).

- would assign the same value to Z as this would:

X = erlang:crc32(Data1), Y = erlang:crc32(Data2), Z = erlang:crc32_combine(X,Y,iolist_size(Data2)).

date/0

Returns the current date as {Year, Month, Day}.

The time zone and daylight saving time correction depend on the underlying OS.

> date().
{1995,2,19}

decode_packet/3

Decodes the binary Bin according to the packet protocol specified by Type. Very similar to the packet handling done by sockets with the option {packet,Type}.

If an entire packet is contained in Bin it is returned together with the remainder of the binary as {ok,Packet,Rest}.

If Bin does not contain the entire packet, {more,Length} is returned. Length is either the expected total size of the packet or undefined if the expected packet size is not known. decode_packet can then be called again with more data added.

If the packet does not conform to the protocol format {error,Reason} is returned.

The following values of Type are valid:

raw | 0

No packet handling is done. Entire binary is returned unless it is empty.

1 | 2 | 4

Packets consist of a header specifying the number of bytes in the packet, followed by that number of bytes. The length of header can be one, two, or four bytes; the order of the bytes is big-endian. The header will be stripped off when the packet is returned.

line

A packet is a line terminated with newline. The newline character is included in the returned packet unless the line was truncated according to the option line_length.

asn1 | cdr | sunrm | fcgi | tpkt

The header is not stripped off.

The meanings of the packet types are as follows:

asn1 - ASN.1 BER
sunrm - Sun's RPC encoding
cdr - CORBA (GIOP 1.1)
fcgi - Fast CGI
tpkt - TPKT format [RFC1006]
http | httph | http_bin | httph_bin

The Hypertext Transfer Protocol. The packets are returned with the format according to HttpPacket described above. A packet is either a request, a response, a header or an end of header mark. Invalid lines are returned as HttpError.

Recognized request methods and header fields are returned as atoms. Others are returned as strings. Strings of unrecognized header fields are formatted with only capital letters first and after hyphen characters (like "Sec-Websocket-Key").

The protocol type http should only be used for the first line when a HttpRequest or a HttpResponse is expected. The following calls should use httph to get HttpHeader's until http_eoh is returned that marks the end of the headers and the beginning of any following message body.

The variants http_bin and httph_bin will return strings (HttpString) as binaries instead of lists.

The following options are available:

{packet_size, integer() >= 0}

Sets the max allowed size of the packet body. If the packet header indicates that the length of the packet is longer than the max allowed length, the packet is considered invalid. Default is 0 which means no size limit.

{line_length, integer() >= 0}

For packet type line, truncate lines longer than the indicated length.

Option line_length also applies to http* packet types as an alias for option packet_size in the case when packet_size itself is not set. This usage is only intended for backward compatibility.

> erlang:decode_packet(1,<<3,"abcd">>,[]).
{ok,<<"abc">>,<<"d">>}
> erlang:decode_packet(1,<<5,"abcd">>,[]).
{more,6}

delete_element/2

1..tuple_size(Tuple1)

Returns a new tuple with element at Index removed from tuple Tuple1.

> erlang:delete_element(2, {one, two, three}).
{one,three}

delete_module/1

Makes the current code for Module become old code, and deletes all references for this module from the export table. Returns undefined if the module does not exist, otherwise true.

Warning!

This BIF is intended for the code server (see code(3)) and should not be used elsewhere.

Failure: badarg if there is already an old version of Module.

demonitor/1

If MonitorRef is a reference which the calling process obtained by calling monitor/2, this monitoring is turned off. If the monitoring is already turned off, nothing happens.

Once demonitor(MonitorRef) has returned it is guaranteed that no {'DOWN', MonitorRef, _, _, _} message due to the monitor will be placed in the caller's message queue in the future. A {'DOWN', MonitorRef, _, _, _} message might have been placed in the caller's message queue prior to the call, though. Therefore, in most cases, it is advisable to remove such a 'DOWN' message from the message queue after monitoring has been stopped. demonitor(MonitorRef, [flush]) can be used instead of demonitor(MonitorRef) if this cleanup is wanted.

Note!

Prior to OTP release R11B (erts version 5.5) demonitor/1 behaved completely asynchronous, i.e., the monitor was active until the "demonitor signal" reached the monitored entity. This had one undesirable effect, though. You could never know when you were guaranteed not to receive a DOWN message due to the monitor.

Current behavior can be viewed as two combined operations: asynchronously send a "demonitor signal" to the monitored entity and ignore any future results of the monitor.

Failure: It is an error if MonitorRef refers to a monitoring started by another process. Not all such cases are cheap to check; if checking is cheap, the call fails with badarg (for example if MonitorRef is a remote reference).

demonitor/2

The returned value is true unless info is part of OptionList.

demonitor(MonitorRef, []) is equivalent to demonitor(MonitorRef).

Currently the following Options are valid:

flush

Remove (one) {_, MonitorRef, _, _, _} message, if there is one, from the caller's message queue after monitoring has been stopped.

Calling demonitor(MonitorRef, [flush]) is equivalent to the following, but more efficient:


    demonitor(MonitorRef),
    receive
        {_, MonitorRef, _, _, _} ->
            true
    after 0 ->
            true
    end
info

The returned value is one of the following:

true

The monitor was found and removed. In this case no 'DOWN' message due to this monitor have been nor will be placed in the message queue of the caller.

false

The monitor was not found and could not be removed. This probably because someone already has placed a 'DOWN' message corresponding to this monitor in the caller's message queue.

If the info option is combined with the flush option, false will be returned if a flush was needed; otherwise, true.

Note!

More options may be added in the future.

Failure: badarg if OptionList is not a list, or if Option is not a valid option, or the same failure as for demonitor/1

disconnect_node/1

Forces the disconnection of a node. This will appear to the node Node as if the local node has crashed. This BIF is mainly used in the Erlang network authentication protocols. Returns true if disconnection succeeds, otherwise false. If the local node is not alive, the function returns ignored.

display/1

Prints a text representation of Term on the standard output.

Warning!

This BIF is intended for debugging only.

element/2

1..tuple_size(Tuple)

Returns the Nth element (numbering from 1) of Tuple.

> element(2, {a, b, c}).
b

Allowed in guard tests.

erase/0

Returns the process dictionary and deletes it.

> put(key1, {1, 2, 3}),
put(key2, [a, b, c]),
erase().
[{key1,{1,2,3}},{key2,[a,b,c]}]

erase/1

Returns the value Val associated with Key and deletes it from the process dictionary. Returns undefined if no value is associated with Key.

> put(key1, {merry, lambs, are, playing}),
X = erase(key1),
{X, erase(key1)}.
{{merry,lambs,are,playing},undefined}

error/1

Stops the execution of the calling process with the reason Reason, where Reason is any term. The actual exit reason will be {Reason, Where}, where Where is a list of the functions most recently called (the current function first). Since evaluating this function causes the process to terminate, it has no return value.

> catch error(foobar).
{'EXIT',{foobar,[{erl_eval,do_apply,5},
                 {erl_eval,expr,5},
                 {shell,exprs,6},
                 {shell,eval_exprs,6},
                 {shell,eval_loop,3}]}}

error/2

Stops the execution of the calling process with the reason Reason, where Reason is any term. The actual exit reason will be {Reason, Where}, where Where is a list of the functions most recently called (the current function first). Args is expected to be the list of arguments for the current function; in Beam it will be used to provide the actual arguments for the current function in the Where term. Since evaluating this function causes the process to terminate, it has no return value.

exit/1

Stops the execution of the calling process with the exit reason Reason, where Reason is any term. Since evaluating this function causes the process to terminate, it has no return value.

> exit(foobar).
** exception exit: foobar
> catch exit(foobar).
{'EXIT',foobar}

exit/2

Sends an exit signal with exit reason Reason to the process or port identified by Pid.

The following behavior apply if Reason is any term except normal or kill:

If Pid is not trapping exits, Pid itself will exit with exit reason Reason. If Pid is trapping exits, the exit signal is transformed into a message {'EXIT', From, Reason} and delivered to the message queue of Pid. From is the pid of the process which sent the exit signal. See also process_flag/2.

If Reason is the atom normal, Pid will not exit. If it is trapping exits, the exit signal is transformed into a message {'EXIT', From, normal} and delivered to its message queue.

If Reason is the atom kill, that is if exit(Pid, kill) is called, an untrappable exit signal is sent to Pid which will unconditionally exit with exit reason killed.

external_size/1

Calculates, without doing the encoding, the maximum byte size for a term encoded in the Erlang external term format. The following condition applies always:

> Size1 = byte_size(term_to_binary(Term)),
> Size2 = erlang:external_size(Term),
> true = Size1 =< Size2.
true
          

This is equivalent to a call to: erlang:external_size(Term, [])

external_size/2

Calculates, without doing the encoding, the maximum byte size for a term encoded in the Erlang external term format. The following condition applies always:

> Size1 = byte_size(term_to_binary(Term, Options)),
> Size2 = erlang:external_size(Term, Options),
> true = Size1 =< Size2.
true
          

The option {minor_version, Version} specifies how floats are encoded. See term_to_binary/2 for a more detailed description.

float/1

Returns a float by converting Number to a float.

> float(55).
55.0

Allowed in guard tests.

Note!

Note that if used on the top-level in a guard, it will test whether the argument is a floating point number; for clarity, use is_float/1 instead.

When float/1 is used in an expression in a guard, such as 'float(A) == 4.0', it converts a number as described above.

float_to_binary/1

The same as float_to_binary(Float,[{scientific,20}]).

float_to_binary/2

Returns a binary which corresponds to the text representation of Float using fixed decimal point formatting. The Options behave in the same way as float_to_list/2.

> float_to_binary(7.12, [{decimals, 4}]).
<<"7.1200">>
> float_to_binary(7.12, [{decimals, 4}, compact]).
<<"7.12">>

float_to_list/1

The same as float_to_list(Float,[{scientific,20}]).

float_to_list/2

Returns a string which corresponds to the text representation of Float using fixed decimal point formatting. When decimals option is specified the returned value will contain at most Decimals number of digits past the decimal point. If the number doesn't fit in the internal static buffer of 256 bytes, the function throws badarg. When compact option is provided the trailing zeros at the end of the list are truncated (this option is only meaningful together with the decimals option). When scientific option is provided, the float will be formatted using scientific notation with Decimals digits of precision. If Options is [] the function behaves like float_to_list/1.

> float_to_list(7.12, [{decimals, 4}]).
"7.1200"
> float_to_list(7.12, [{decimals, 4}, compact]).
"7.12"

fun_info/1

Returns a list containing information about the fun Fun. Each element of the list is a tuple. The order of the tuples is not defined, and more tuples may be added in a future release.

Warning!

This BIF is mainly intended for debugging, but it can occasionally be useful in library functions that might need to verify, for instance, the arity of a fun.

There are two types of funs with slightly different semantics:

A fun created by fun M:F/A is called an external fun. Calling it will always call the function F with arity A in the latest code for module M. Note that module M does not even need to be loaded when the fun fun M:F/A is created.

All other funs are called local. When a local fun is called, the same version of the code that created the fun will be called (even if newer version of the module has been loaded).

The following elements will always be present in the list for both local and external funs:

{type, Type}

Type is either local or external.

{module, Module}

Module (an atom) is the module name.

If Fun is a local fun, Module is the module in which the fun is defined.

If Fun is an external fun, Module is the module that the fun refers to.

{name, Name}

Name (an atom) is a function name.

If Fun is a local fun, Name is the name of the local function that implements the fun. (This name was generated by the compiler, and is generally only of informational use. As it is a local function, it is not possible to call it directly.) If no code is currently loaded for the fun, [] will be returned instead of an atom.

If Fun is an external fun, Name is the name of the exported function that the fun refers to.

{arity, Arity}

Arity is the number of arguments that the fun should be called with.

{env, Env}

Env (a list) is the environment or free variables for the fun. (For external funs, the returned list is always empty.)

The following elements will only be present in the list if Fun is local:

{pid, Pid}

Pid is the pid of the process that originally created the fun.

{index, Index}

Index (an integer) is an index into the module's fun table.

{new_index, Index}

Index (an integer) is an index into the module's fun table.

{new_uniq, Uniq}

Uniq (a binary) is a unique value for this fun. It is calculated from the compiled code for the entire module.

{uniq, Uniq}

Uniq (an integer) is a unique value for this fun. Starting in the R15 release, this integer is calculated from the compiled code for the entire module. Before R15, this integer was based on only the body of the fun.

fun_info/2

Returns information about Fun as specified by Item, in the form {Item,Info}.

For any fun, Item can be any of the atoms module, name, arity, env, or type.

For a local fun, Item can also be any of the atoms index, new_index, new_uniq, uniq, and pid. For an external fun, the value of any of these items is always the atom undefined.

See erlang:fun_info/1.

fun_to_list/1

Returns a string which corresponds to the text representation of Fun.

function_exported/3

Returns true if the module Module is loaded and contains an exported function Function/Arity; otherwise false.

Returns false for any BIF (functions implemented in C rather than in Erlang).

garbage_collect/0

Forces an immediate garbage collection of the currently executing process. The function should not be used, unless it has been noticed -- or there are good reasons to suspect -- that the spontaneous garbage collection will occur too late or not at all. Improper use may seriously degrade system performance.

Compatibility note: In versions of OTP prior to R7, the garbage collection took place at the next context switch, not immediately. To force a context switch after a call to erlang:garbage_collect(), it was sufficient to make any function call.

garbage_collect/1

Works like erlang:garbage_collect() but on any process. The same caveats apply. Returns false if Pid refers to a dead process; true otherwise.

get/0

Returns the process dictionary as a list of {Key, Val} tuples.

> put(key1, merry),
put(key2, lambs),
put(key3, {are, playing}),
get().
[{key1,merry},{key2,lambs},{key3,{are,playing}}]

get/1

Returns the value Valassociated with Key in the process dictionary, or undefined if Key does not exist.

> put(key1, merry),
put(key2, lambs),
put({any, [valid, term]}, {are, playing}),
get({any, [valid, term]}).
{are,playing}

get_cookie/0

Returns the magic cookie of the local node, if the node is alive; otherwise the atom nocookie.

get_keys/1

Returns a list of keys which are associated with the value Val in the process dictionary.

> put(mary, {1, 2}),
put(had, {1, 2}),
put(a, {1, 2}),
put(little, {1, 2}),
put(dog, {1, 3}),
put(lamb, {1, 2}),
get_keys({1, 2}).
[mary,had,a,little,lamb]

get_stacktrace/0

Get the call stack back-trace (stacktrace) of the last exception in the calling process as a list of {Module,Function,Arity,Location} tuples. The Arity field in the first tuple may be the argument list of that function call instead of an arity integer, depending on the exception.

If there has not been any exceptions in a process, the stacktrace is []. After a code change for the process, the stacktrace may also be reset to [].

The stacktrace is the same data as the catch operator returns, for example:

{'EXIT',{badarg,Stacktrace}} = catch abs(x)

Location is a (possibly empty) list of two-tuples that may indicate the location in the source code of the function. The first element is an atom that describes the type of information in the second element. Currently the following items may occur:

file

The second element of the tuple is a string (list of characters) representing the filename of the source file of the function.

line

The second element of the tuple is the line number (an integer greater than zero) in the source file where the exception occurred or the function was called.

See also erlang:error/1 and erlang:error/2.

group_leader/0

Returns the pid of the group leader for the process which evaluates the function.

Every process is a member of some process group and all groups have a group leader. All IO from the group is channeled to the group leader. When a new process is spawned, it gets the same group leader as the spawning process. Initially, at system start-up, init is both its own group leader and the group leader of all processes.

group_leader/2

Sets the group leader of Pid to GroupLeader. Typically, this is used when a processes started from a certain shell should have another group leader than init.

See also group_leader/0.

halt/0

The same as halt(0, []).

> halt().
os_prompt% 

halt/1

The same as halt(Status, []).

> halt(17).
os_prompt% echo $?
17
os_prompt% 

halt/2

Status must be a non-negative integer, a string, or the atom abort. Halts the Erlang runtime system. Has no return value. Depending on Status:

integer()
The runtime system exits with the integer value Status as status code to the calling environment (operating system).
string()
An erlang crash dump is produced with Status as slogan, and then the runtime system exits with status code 1.
abort
The runtime system aborts producing a core dump, if that is enabled in the operating system.

Note that on many platforms, only the status codes 0-255 are supported by the operating system.

For integer Status the Erlang runtime system closes all ports and allows async threads to finish their operations before exiting. To exit without such flushing use Option as {flush,false}.

For statuses string() and abort the flush option is ignored and flushing is not done.

hash/2

Returns a hash value for Term within the range 1..Range. The allowed range is 1..2^27-1.

Warning!

This BIF is deprecated as the hash value may differ on different architectures. Also the hash values for integer terms larger than 2^27 as well as large binaries are very poor. The BIF is retained for backward compatibility reasons (it may have been used to hash records into a file), but all new code should use one of the BIFs erlang:phash/2 or erlang:phash2/1,2 instead.

hd/1

Returns the head of List, that is, the first element.

> hd([1,2,3,4,5]).
1

Allowed in guard tests.

Failure: badarg if List is the empty list [].

hibernate/3

Puts the calling process into a wait state where its memory allocation has been reduced as much as possible, which is useful if the process does not expect to receive any messages in the near future.

The process will be awaken when a message is sent to it, and control will resume in Module:Function with the arguments given by Args with the call stack emptied, meaning that the process will terminate when that function returns. Thus erlang:hibernate/3 will never return to its caller.

If the process has any message in its message queue, the process will be awaken immediately in the same way as described above.

In more technical terms, what erlang:hibernate/3 does is the following. It discards the call stack for the process. Then it garbage collects the process. After the garbage collection, all live data is in one continuous heap. The heap is then shrunken to the exact same size as the live data which it holds (even if that size is less than the minimum heap size for the process).

If the size of the live data in the process is less than the minimum heap size, the first garbage collection occurring after the process has been awaken will ensure that the heap size is changed to a size not smaller than the minimum heap size.

Note that emptying the call stack means that any surrounding catch is removed and has to be re-inserted after hibernation. One effect of this is that processes started using proc_lib (also indirectly, such as gen_server processes), should use proc_lib:hibernate/3 instead to ensure that the exception handler continues to work when the process wakes up.

insert_element/3

1..tuple_size(Tuple1) + 1

Returns a new tuple with element Term insert at position Index in tuple Tuple1. All elements from position Index and upwards are subsequently pushed one step higher in the new tuple Tuple2.

> erlang:insert_element(2, {one, two, three}, new).
{one,new,two,three}

integer_to_binary/1

Returns a binary which corresponds to the text representation of Integer.

> integer_to_binary(77).
<<"77">>

integer_to_binary/2

Returns a binary which corresponds to the text representation of Integer in base Base.

> integer_to_binary(1023, 16).
<<"3FF">>

integer_to_list/1

Returns a string which corresponds to the text representation of Integer.

> integer_to_list(77).
"77"

integer_to_list/2

Returns a string which corresponds to the text representation of Integer in base Base.

> integer_to_list(1023, 16).
"3FF"

iolist_to_binary/1

Returns a binary which is made from the integers and binaries in IoListOrBinary.

> Bin1 = <<1,2,3>>.
<<1,2,3>>
> Bin2 = <<4,5>>.
<<4,5>>
> Bin3 = <<6>>.
<<6>>
> iolist_to_binary([Bin1,1,[2,3,Bin2],4|Bin3]).
<<1,2,3,1,2,3,4,5,4,6>>

iolist_size/1

Returns an integer which is the size in bytes of the binary that would be the result of iolist_to_binary(Item).

> iolist_size([1,2|<<3,4>>]).
4

is_alive/0

Returns true if the local node is alive; that is, if the node can be part of a distributed system. Otherwise, it returns false.

is_atom/1

Returns true if Term is an atom; otherwise returns false.

Allowed in guard tests.

is_binary/1

Returns true if Term is a binary; otherwise returns false.

A binary always contains a complete number of bytes.

Allowed in guard tests.

is_bitstring/1

Returns true if Term is a bitstring (including a binary); otherwise returns false.

Allowed in guard tests.

is_boolean/1

Returns true if Term is either the atom true or the atom false (i.e. a boolean); otherwise returns false.

Allowed in guard tests.

is_builtin/3

Returns true if Module:Function/Arity is a BIF implemented in C; otherwise returns false. This BIF is useful for builders of cross reference tools.

is_float/1

Returns true if Term is a floating point number; otherwise returns false.

Allowed in guard tests.

is_function/1

Returns true if Term is a fun; otherwise returns false.

Allowed in guard tests.

is_function/2

Returns true if Term is a fun that can be applied with Arity number of arguments; otherwise returns false.

Allowed in guard tests.

is_integer/1

Returns true if Term is an integer; otherwise returns false.

Allowed in guard tests.

is_list/1

Returns true if Term is a list with zero or more elements; otherwise returns false.

Allowed in guard tests.

is_number/1

Returns true if Term is either an integer or a floating point number; otherwise returns false.

Allowed in guard tests.

is_pid/1

Returns true if Term is a pid (process identifier); otherwise returns false.

Allowed in guard tests.

is_port/1

Returns true if Term is a port identifier; otherwise returns false.

Allowed in guard tests.

is_process_alive/1

Pid must refer to a process at the local node. Returns true if the process exists and is alive, that is, is not exiting and has not exited. Otherwise, returns false.

is_record/2

Returns true if Term is a tuple and its first element is RecordTag. Otherwise, returns false.

Note!

Normally the compiler treats calls to is_record/2 specially. It emits code to verify that Term is a tuple, that its first element is RecordTag, and that the size is correct. However, if the RecordTag is not a literal atom, the is_record/2 BIF will be called instead and the size of the tuple will not be verified.

Allowed in guard tests, if RecordTag is a literal atom.

is_record/3

RecordTag must be an atom. Returns true if Term is a tuple, its first element is RecordTag, and its size is Size. Otherwise, returns false.

Allowed in guard tests, provided that RecordTag is a literal atom and Size is a literal integer.

Note!

This BIF is documented for completeness. In most cases is_record/2 should be used.

is_reference/1

Returns true if Term is a reference; otherwise returns false.

Allowed in guard tests.

is_tuple/1

Returns true if Term is a tuple; otherwise returns false.

Allowed in guard tests.

length/1

Returns the length of List.

> length([1,2,3,4,5,6,7,8,9]).
9

Allowed in guard tests.

link/1

Creates a link between the calling process and another process (or port) PidOrPort, if there is not such a link already. If a process attempts to create a link to itself, nothing is done. Returns true.

If PidOrPort does not exist, the behavior of the BIF depends on if the calling process is trapping exits or not (see process_flag/2):

If the calling process is not trapping exits, and checking PidOrPort is cheap -- that is, if PidOrPort is local -- link/1 fails with reason noproc. Otherwise, if the calling process is trapping exits, and/or PidOrPort is remote, link/1 returns true, but an exit signal with reason noproc is sent to the calling process.

list_to_atom/1

Returns the atom whose text representation is String.

String may only contain ISO-latin-1 characters (i.e. numbers below 256) as the current implementation does not allow unicode characters >= 256 in atoms. For more information on Unicode support in atoms see note on UTF-8 encoded atoms in the chapter about the external term format in the ERTS User's Guide.

> list_to_atom("Erlang").
'Erlang'

list_to_binary/1

Returns a binary which is made from the integers and binaries in IoList.

> Bin1 = <<1,2,3>>.
<<1,2,3>>
> Bin2 = <<4,5>>.
<<4,5>>
> Bin3 = <<6>>.
<<6>>
> list_to_binary([Bin1,1,[2,3,Bin2],4|Bin3]).
<<1,2,3,1,2,3,4,5,4,6>>

list_to_bitstring/1

Returns a bitstring which is made from the integers and bitstrings in BitstringList. (The last tail in BitstringList is allowed to be a bitstring.)

> Bin1 = <<1,2,3>>.
<<1,2,3>>
> Bin2 = <<4,5>>.
<<4,5>>
> Bin3 = <<6,7:4,>>.
<<6>>
> list_to_bitstring([Bin1,1,[2,3,Bin2],4|Bin3]).
<<1,2,3,1,2,3,4,5,4,6,7:46>>

list_to_existing_atom/1

Returns the atom whose text representation is String, but only if there already exists such atom.

Failure: badarg if there does not already exist an atom whose text representation is String.

list_to_float/1

Returns the float whose text representation is String.

> list_to_float("2.2017764e+0").
2.2017764

Failure: badarg if String contains a bad representation of a float.

list_to_integer/1

Returns an integer whose text representation is String.

> list_to_integer("123").
123

Failure: badarg if String contains a bad representation of an integer.

list_to_integer/2

Returns an integer whose text representation in base Base is String.

> list_to_integer("3FF", 16).
1023

Failure: badarg if String contains a bad representation of an integer.

list_to_pid/1

Returns a pid whose text representation is String.

Warning!

This BIF is intended for debugging and for use in the Erlang operating system. It should not be used in application programs.

> list_to_pid("<0.4.1>").
<0.4.1>

Failure: badarg if String contains a bad representation of a pid.

list_to_tuple/1

Returns a tuple which corresponds to List. List can contain any Erlang terms.

> list_to_tuple([share, ['Ericsson_B', 163]]).
{share, ['Ericsson_B', 163]}

load_module/2

If Binary contains the object code for the module Module, this BIF loads that object code. Also, if the code for the module Module already exists, all export references are replaced so they point to the newly loaded code. The previously loaded code is kept in the system as old code, as there may still be processes which are executing that code. It returns either {module, Module}, or {error, Reason} if loading fails. Reason is one of the following:

badfile

The object code in Binary has an incorrect format or the object code contains code for another module than Module.

not_purged

Binary contains a module which cannot be loaded because old code for this module already exists.

Warning!

This BIF is intended for the code server (see code(3)) and should not be used elsewhere.

load_nif/2

Note!

In releases older than OTP R14B, NIFs were an experimental feature. Versions of OTP older than R14B might have different and possibly incompatible NIF semantics and interfaces. For example, in R13B03 the return value on failure was {error,Reason,Text}.

Loads and links a dynamic library containing native implemented functions (NIFs) for a module. Path is a file path to the sharable object/dynamic library file minus the OS-dependent file extension (.so for Unix and .dll for Windows). See erl_nif on how to implement a NIF library.

LoadInfo can be any term. It will be passed on to the library as part of the initialization. A good practice is to include a module version number to support future code upgrade scenarios.

The call to load_nif/2 must be made directly from the Erlang code of the module that the NIF library belongs to.

It returns either ok, or {error,{Reason,Text}} if loading fails. Reason is one of the atoms below, while Text is a human readable string that may give some more information about the failure.

load_failed

The OS failed to load the NIF library.

bad_lib

The library did not fulfil the requirements as a NIF library of the calling module.

load | reload | upgrade

The corresponding library callback was not successful.

old_code

The call to load_nif/2 was made from the old code of a module that has been upgraded. This is not allowed.

loaded/0

Returns a list of all loaded Erlang modules (current and/or old code), including preloaded modules.

See also code(3).

localtime/0

Returns the current local date and time {{Year, Month, Day}, {Hour, Minute, Second}}.

The time zone and daylight saving time correction depend on the underlying OS.

> erlang:localtime().
{{1996,11,6},{14,45,17}}

localtime_to_universaltime/1

Converts local date and time to Universal Time Coordinated (UTC), if this is supported by the underlying OS. Otherwise, no conversion is done and Localtime is returned.

> erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}).
{{1996,11,6},{13,45,17}}

Failure: badarg if Localtime does not denote a valid date and time.

localtime_to_universaltime/2

Converts local date and time to Universal Time Coordinated (UTC) just like erlang:localtime_to_universaltime/1, but the caller decides if daylight saving time is active or not.

If IsDst == true the Localtime is during daylight saving time, if IsDst == false it is not, and if IsDst == undefined the underlying OS may guess, which is the same as calling erlang:localtime_to_universaltime(Localtime).

> erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, true).
{{1996,11,6},{12,45,17}}
> erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, false).
{{1996,11,6},{13,45,17}}
> erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, undefined).
{{1996,11,6},{13,45,17}}

Failure: badarg if Localtime does not denote a valid date and time.

make_ref/0

Returns an almost unique reference.

The returned reference will re-occur after approximately 2^82 calls; therefore it is unique enough for practical purposes.

> make_ref().
#Ref<0.0.0.135>

make_tuple/2

Returns a new tuple of the given Arity, where all elements are InitialValue.

> erlang:make_tuple(4, []).
{[],[],[],[]}

make_tuple/3

erlang:make_tuple first creates a tuple of size Arity where each element has the value DefaultValue. It then fills in values from InitList. Each list element in InitList must be a two-tuple where the first element is a position in the newly created tuple and the second element is any term. If a position occurs more than once in the list, the term corresponding to last occurrence will be used.

> erlang:make_tuple(5, [], [{2,ignored},{5,zz},{2,aa}]).
{{[],aa,[],[],zz}

max/2

Return the largest of Term1 and Term2; if the terms compare equal, Term1 will be returned.

md5/1

Computes an MD5 message digest from Data, where the length of the digest is 128 bits (16 bytes). Data is a binary or a list of small integers and binaries.

See The MD5 Message Digest Algorithm (RFC 1321) for more information about MD5.

Warning!

The MD5 Message Digest Algorithm is not considered safe for code-signing or software integrity purposes.

md5_final/1

Finishes the update of an MD5 Context and returns the computed MD5 message digest.

md5_init/0

Creates an MD5 context, to be used in subsequent calls to md5_update/2.

md5_update/2

Updates an MD5 Context with Data, and returns a NewContext.

memory/0

Returns a list containing information about memory dynamically allocated by the Erlang emulator. Each element of the list is a tuple {Type, Size}. The first element Typeis an atom describing memory type. The second element Sizeis memory size in bytes. A description of each memory type follows:

total

The total amount of memory currently allocated, which is the same as the sum of memory size for processes and system.

processes

The total amount of memory currently allocated by the Erlang processes.

processes_used

The total amount of memory currently used by the Erlang processes.

This memory is part of the memory presented as processes memory.

system

The total amount of memory currently allocated by the emulator that is not directly related to any Erlang process.

Memory presented as processes is not included in this memory.

atom

The total amount of memory currently allocated for atoms.

This memory is part of the memory presented as system memory.

atom_used

The total amount of memory currently used for atoms.

This memory is part of the memory presented as atom memory.

binary

The total amount of memory currently allocated for binaries.

This memory is part of the memory presented as system memory.

code

The total amount of memory currently allocated for Erlang code.

This memory is part of the memory presented as system memory.

ets

The total amount of memory currently allocated for ets tables.

This memory is part of the memory presented as system memory.

low

Only on 64-bit halfword emulator.

The total amount of memory allocated in low memory areas that are restricted to less than 4 Gb even though the system may have more physical memory.

May be removed in future releases of halfword emulator.

maximum

The maximum total amount of memory allocated since the emulator was started.

This tuple is only present when the emulator is run with instrumentation.

For information on how to run the emulator with instrumentation see instrument(3) and/or erl(1).

Note!

The system value is not complete. Some allocated memory that should be part of the system value are not.

When the emulator is run with instrumentation, the system value is more accurate, but memory directly allocated by malloc (and friends) are still not part of the system value. Direct calls to malloc are only done from OS specific runtime libraries and perhaps from user implemented Erlang drivers that do not use the memory allocation functions in the driver interface.

Since the total value is the sum of processes and system the error in system will propagate to the total value.

The different amounts of memory that are summed are not gathered atomically which also introduce an error in the result.

The different values has the following relation to each other. Values beginning with an uppercase letter is not part of the result.

        total = processes + system
        processes = processes_used + ProcessesNotUsed
        system = atom + binary + code + ets + OtherSystem
        atom = atom_used + AtomNotUsed

        RealTotal = processes + RealSystem
        RealSystem = system + MissedSystem

More tuples in the returned list may be added in the future.

Note!

The total value is supposed to be the total amount of memory dynamically allocated by the emulator. Shared libraries, the code of the emulator itself, and the emulator stack(s) are not supposed to be included. That is, the total value is not supposed to be equal to the total size of all pages mapped to the emulator. Furthermore, due to fragmentation and pre-reservation of memory areas, the size of the memory segments which contain the dynamically allocated memory blocks can be substantially larger than the total size of the dynamically allocated memory blocks.

Note!

Since erts version 5.6.4 erlang:memory/0 requires that all erts_alloc(3) allocators are enabled (default behaviour).

Failure:

notsup
If an erts_alloc(3) allocator has been disabled.

memory/1

Returns the memory size in bytes allocated for memory of type Type. The argument can also be given as a list of memory_type() atoms, in which case a corresponding list of {memory_type(), Size :: integer >= 0} tuples is returned.

Note!

Since erts version 5.6.4 erlang:memory/1 requires that all erts_alloc(3) allocators are enabled (default behaviour).

Failures:

badarg
If Type is not one of the memory types listed in the documentation of erlang:memory/0.
badarg
If maximum is passed as Type and the emulator is not run in instrumented mode.
notsup
If an erts_alloc(3) allocator has been disabled.

See also erlang:memory/0.

min/2

Return the smallest of Term1 and Term2; if the terms compare equal, Term1 will be returned.

module_loaded/1

Returns true if the module Module is loaded, otherwise returns false. It does not attempt to load the module.

Warning!

This BIF is intended for the code server (see code(3)) and should not be used elsewhere.

monitor/2

The calling process starts monitoring Item which is an object of type Type.

Currently only processes can be monitored, i.e. the only allowed Type is process, but other types may be allowed in the future.

Item can be:

pid()

The pid of the process to monitor.

{RegName, Node}

A tuple consisting of a registered name of a process and a node name. The process residing on the node Node with the registered name RegName will be monitored.

RegName

The process locally registered as RegName will be monitored.

Note!

When a process is monitored by registered name, the process that has the registered name at the time when monitor/2 is called will be monitored. The monitor will not be effected, if the registered name is unregistered.

A 'DOWN' message will be sent to the monitoring process if Item dies, if Item does not exist, or if the connection is lost to the node which Item resides on. A 'DOWN' message has the following pattern:

{'DOWN', MonitorRef, Type, Object, Info}

where MonitorRef and Type are the same as described above, and:

Object

A reference to the monitored object:

the pid of the monitored process, if Item was specified as a pid. {RegName, Node}, if Item was specified as {RegName, Node}. {RegName, Node}, if Item was specified as RegName. Node will in this case be the name of the local node (node()).
Info

Either the exit reason of the process, noproc (non-existing process), or noconnection (no connection to Node).

Note!

If/when monitor/2 is extended (e.g. to handle other item types than process), other possible values for Object, and Info in the 'DOWN' message will be introduced.

The monitoring is turned off either when the 'DOWN' message is sent, or when demonitor/1 is called.

If an attempt is made to monitor a process on an older node (where remote process monitoring is not implemented or one where remote process monitoring by registered name is not implemented), the call fails with badarg.

Making several calls to monitor/2 for the same Item is not an error; it results in as many, completely independent, monitorings.

Note!

The format of the 'DOWN' message changed in the 5.2 version of the emulator (OTP release R9B) for monitor by registered name. The Object element of the 'DOWN' message could in earlier versions sometimes be the pid of the monitored process and sometimes be the registered name. Now the Object element is always a tuple consisting of the registered name and the node name. Processes on new nodes (emulator version 5.2 or greater) will always get 'DOWN' messages on the new format even if they are monitoring processes on old nodes. Processes on old nodes will always get 'DOWN' messages on the old format.

monitor_node/2

Monitors the status of the node Node. If Flag is true, monitoring is turned on; if Flag is false, monitoring is turned off.

Making several calls to monitor_node(Node, true) for the same Node is not an error; it results in as many, completely independent, monitorings.

If Node fails or does not exist, the message {nodedown, Node} is delivered to the process. If a process has made two calls to monitor_node(Node, true) and Node terminates, two nodedown messages are delivered to the process. If there is no connection to Node, there will be an attempt to create one. If this fails, a nodedown message is delivered.

Nodes connected through hidden connections can be monitored as any other node.

Failure: badargif the local node is not alive.

monitor_node/3

Behaves as monitor_node/2 except that it allows an extra option to be given, namely allow_passive_connect. The option allows the BIF to wait the normal net connection timeout for the monitored node to connect itself, even if it cannot be actively connected from this node (i.e. it is blocked). The state where this might be useful can only be achieved by using the kernel option dist_auto_connect once. If that kernel option is not used, the allow_passive_connect option has no effect.

Note!

The allow_passive_connect option is used internally and is seldom needed in applications where the network topology and the kernel options in effect is known in advance.

Failure: badarg if the local node is not alive or the option list is malformed.

nif_error/1

Works exactly like erlang:error/1, but Dialyzer thinks that this BIF will return an arbitrary term. When used in a stub function for a NIF to generate an exception when the NIF library is not loaded, Dialyzer will not generate false warnings.

nif_error/2

Works exactly like erlang:error/2, but Dialyzer thinks that this BIF will return an arbitrary term. When used in a stub function for a NIF to generate an exception when the NIF library is not loaded, Dialyzer will not generate false warnings.

node/0

Returns the name of the local node. If the node is not alive, nonode@nohost is returned instead.

Allowed in guard tests.

node/1

Returns the node where Arg is located. Arg can be a pid, a reference, or a port. If the local node is not alive, nonode@nohost is returned.

Allowed in guard tests.

nodes/0

Returns a list of all visible nodes in the system, excluding the local node. Same as nodes(visible).

nodes/1

Returns a list of nodes according to argument given. The result returned when the argument is a list, is the list of nodes satisfying the disjunction(s) of the list elements.

NodeType can be any of the following:

visible

Nodes connected to this node through normal connections.

hidden

Nodes connected to this node through hidden connections.

connected

All nodes connected to this node.

this

This node.

known

Nodes which are known to this node, i.e., connected, previously connected, etc.

Some equalities: [node()] = nodes(this), nodes(connected) = nodes([visible, hidden]), and nodes() = nodes(visible).

If the local node is not alive, nodes(this) == nodes(known) == [nonode@nohost], for any other Arg the empty list [] is returned.

now/0

Returns the tuple {MegaSecs, Secs, MicroSecs} which is the elapsed time since 00:00 GMT, January 1, 1970 (zero hour) on the assumption that the underlying OS supports this. Otherwise, some other point in time is chosen. It is also guaranteed that subsequent calls to this BIF returns continuously increasing values. Hence, the return value from now() can be used to generate unique time-stamps, and if it is called in a tight loop on a fast machine the time of the node can become skewed.

It can only be used to check the local time of day if the time-zone info of the underlying operating system is properly configured.

If you do not need the return value to be unique and monotonically increasing, use os:timestamp/0 instead to avoid some overhead.

open_port/2

Returns a port identifier as the result of opening a new Erlang port. A port can be seen as an external Erlang process. PortName is one of the following:

{spawn, Command}

Starts an external program. Command is the name of the external program which will be run. Command runs outside the Erlang work space unless an Erlang driver with the name Command is found. If found, that driver will be started. A driver runs in the Erlang workspace, which means that it is linked with the Erlang runtime system.

When starting external programs on Solaris, the system call vfork is used in preference to fork for performance reasons, although it has a history of being less robust. If there are problems with using vfork, setting the environment variable ERL_NO_VFORK to any value will cause fork to be used instead.

For external programs, the PATH is searched (or an equivalent method is used to find programs, depending on operating system). This is done by invoking the shell on certain platforms. The first space separated token of the command will be considered as the name of the executable (or driver). This (among other things) makes this option unsuitable for running programs having spaces in file or directory names. Use {spawn_executable, Command} instead if spaces in executable file names is desired.

{spawn_driver, Command}

Works like {spawn, Command}, but demands the first (space separated) token of the command to be the name of a loaded driver. If no driver with that name is loaded, a badarg error is raised.

{spawn_executable, FileName}

Works like {spawn, FileName}, but only runs external executables. The FileName in its whole is used as the name of the executable, including any spaces. If arguments are to be passed, the args and arg0 PortSettings can be used.

The shell is not usually invoked to start the program, it's executed directly. Neither is the PATH (or equivalent) searched. To find a program in the PATH to execute, use os:find_executable/1.

Only if a shell script or .bat file is executed, the appropriate command interpreter will implicitly be invoked, but there will still be no command argument expansion or implicit PATH search.

The name of the executable as well as the arguments given in args and arg0 is subject to Unicode file name translation if the system is running in Unicode file name mode. To avoid translation or force i.e. UTF-8, supply the executable and/or arguments as a binary in the correct encoding. See the file module, the file:native_name_encoding/0 function and the stdlib users guide for details.

Note!

The characters in the name (if given as a list) can only be > 255 if the Erlang VM is started in Unicode file name translation mode, otherwise the name of the executable is limited to the ISO-latin-1 character set.

If the FileName cannot be run, an error exception, with the posix error code as the reason, is raised. The error reason may differ between operating systems. Typically the error enoent is raised when one tries to run a program that is not found and eaccess is raised when the given file is not executable.

{fd, In, Out}

Allows an Erlang process to access any currently opened file descriptors used by Erlang. The file descriptor In can be used for standard input, and the file descriptor Out for standard output. It is only used for various servers in the Erlang operating system (shell and user). Hence, its use is very limited.

PortSettings is a list of settings for the port. Valid settings are:

{packet, N}

Messages are preceded by their length, sent in N bytes, with the most significant byte first. Valid values for N are 1, 2, or 4.

stream

Output messages are sent without packet lengths. A user-defined protocol must be used between the Erlang process and the external object.

{line, L}

Messages are delivered on a per line basis. Each line (delimited by the OS-dependent newline sequence) is delivered in one single message. The message data format is {Flag, Line}, where Flag is either eol or noeol and Line is the actual data delivered (without the newline sequence).

L specifies the maximum line length in bytes. Lines longer than this will be delivered in more than one message, with the Flag set to noeol for all but the last message. If end of file is encountered anywhere else than immediately following a newline sequence, the last line will also be delivered with the Flag set to noeol. In all other cases, lines are delivered with Flag set to eol.

The {packet, N} and {line, L} settings are mutually exclusive.

{cd, Dir}

This is only valid for {spawn, Command} and {spawn_executable, FileName}. The external program starts using Dir as its working directory. Dir must be a string.

{env, Env}

This is only valid for {spawn, Command} and {spawn_executable, FileName}. The environment of the started process is extended using the environment specifications in Env.

Env should be a list of tuples {Name, Val}, where Name is the name of an environment variable, and Val is the value it is to have in the spawned port process. Both Name and Val must be strings. The one exception is Val being the atom false (in analogy with os:getenv/1), which removes the environment variable.

If Unicode filename encoding is in effect (see the erl manual page), the strings (both Name and Value) may contain characters with codepoints > 255.

{args, [ string() | binary() ]}

This option is only valid for {spawn_executable, FileName} and specifies arguments to the executable. Each argument is given as a separate string and (on Unix) eventually ends up as one element each in the argument vector. On other platforms, similar behavior is mimicked.

The arguments are not expanded by the shell prior to being supplied to the executable, most notably this means that file wildcard expansion will not happen. Use filelib:wildcard/1 to expand wildcards for the arguments. Note that even if the program is a Unix shell script, meaning that the shell will ultimately be invoked, wildcard expansion will not happen and the script will be provided with the untouched arguments. On Windows®, wildcard expansion is always up to the program itself, why this isn't an issue.

Note also that the actual executable name (a.k.a. argv[0]) should not be given in this list. The proper executable name will automatically be used as argv[0] where applicable.

When the Erlang VM is running in Unicode file name mode, the arguments can contain any Unicode characters and will be translated into whatever is appropriate on the underlying OS, which means UTF-8 for all platforms except Windows, which has other (more transparent) ways of dealing with Unicode arguments to programs. To avoid Unicode translation of arguments, they can be supplied as binaries in whatever encoding is deemed appropriate.

Note!

The characters in the arguments (if given as a list of characters) can only be > 255 if the Erlang VM is started in Unicode file name mode, otherwise the arguments are limited to the ISO-latin-1 character set.

If one, for any reason, wants to explicitly set the program name in the argument vector, the arg0 option can be used.

{arg0, string() | binary()}

This option is only valid for {spawn_executable, FileName} and explicitly specifies the program name argument when running an executable. This might in some circumstances, on some operating systems, be desirable. How the program responds to this is highly system dependent and no specific effect is guaranteed.

The unicode file name translation rules of the args option apply to this option as well.

exit_status

This is only valid for {spawn, Command} where Command refers to an external program, and for {spawn_executable, FileName}.

When the external process connected to the port exits, a message of the form {Port,{exit_status,Status}} is sent to the connected process, where Status is the exit status of the external process. If the program aborts, on Unix the same convention is used as the shells do (i.e., 128+signal).

If the eof option has been given as well, the eof message and the exit_status message appear in an unspecified order.

If the port program closes its stdout without exiting, the exit_status option will not work.

use_stdio

This is only valid for {spawn, Command} and {spawn_executable, FileName}. It allows the standard input and output (file descriptors 0 and 1) of the spawned (UNIX) process for communication with Erlang.

nouse_stdio

The opposite of use_stdio. Uses file descriptors 3 and 4 for communication with Erlang.

stderr_to_stdout

Affects ports to external programs. The executed program gets its standard error file redirected to its standard output file. stderr_to_stdout and nouse_stdio are mutually exclusive.

overlapped_io

Affects ports to external programs on Windows® only. The standard input and standard output handles of the port program will, if this option is supplied, be opened with the flag FILE_FLAG_OVERLAPPED, so that the port program can (and has to) do overlapped I/O on its standard handles. This is not normally the case for simple port programs, but an option of value for the experienced Windows programmer. On all other platforms, this option is silently discarded.

in

The port can only be used for input.

out

The port can only be used for output.

binary

All IO from the port are binary data objects as opposed to lists of bytes.

eof

The port will not be closed at the end of the file and produce an exit signal. Instead, it will remain open and a {Port, eof} message will be sent to the process holding the port.

hide

When running on Windows, suppress creation of a new console window when spawning the port program. (This option has no effect on other platforms.)

{parallelism, Boolean}

Set scheduler hint for port parallelism. If set to true, the VM will schedule port tasks when it by this can improve the parallelism in the system. If set to false, the VM will try to perform port tasks immediately and by this improving the latency at the expense of parallelism. The default can be set on system startup by passing the +spp command line argument to erl(1).

The default is stream for all types of port and use_stdio for spawned ports.

Failure: If the port cannot be opened, the exit reason is badarg, system_limit, or the Posix error code which most closely describes the error, or einval if no Posix code is appropriate:

badarg

Bad input arguments to open_port.

system_limit

All available ports in the Erlang emulator are in use.

enomem

There was not enough memory to create the port.

eagain

There are no more available operating system processes.

enametoolong

The external command given was too long.

emfile

There are no more available file descriptors (for the operating system process that the Erlang emulator runs in).

enfile

The file table is full (for the entire operating system).

eacces

The Command given in {spawn_executable, Command} does not point out an executable file.

enoent

The FileName given in {spawn_executable, FileName} does not point out an existing file.

During use of a port opened using {spawn, Name}, {spawn_driver, Name} or {spawn_executable, Name}, errors arising when sending messages to it are reported to the owning process using signals of the form {'EXIT', Port, PosixCode}. See file(3) for possible values of PosixCode.

The maximum number of ports that can be open at the same time can be configured by passing the +Q command line flag to erl(1).

phash/2

Range = Range = 1..2^32, Hash = 1..Range

Portable hash function that will give the same hash for the same Erlang term regardless of machine architecture and ERTS version (the BIF was introduced in ERTS 4.9.1.1). Range can be between 1 and 2^32, the function returns a hash value for Term within the range 1..Range.

This BIF could be used instead of the old deprecated erlang:hash/2 BIF, as it calculates better hashes for all data-types, but consider using phash2/1,2 instead.

phash2/1

phash2/2

Range = 1..2^32
Hash = 0..Range-1

Portable hash function that will give the same hash for the same Erlang term regardless of machine architecture and ERTS version (the BIF was introduced in ERTS 5.2). Range can be between 1 and 2^32, the function returns a hash value for Term within the range 0..Range-1. When called without the Range argument, a value in the range 0..2^27-1 is returned.

This BIF should always be used for hashing terms. It distributes small integers better than phash/2, and it is faster for bignums and binaries.

Note that the range 0..Range-1 is different from the range of phash/2 (1..Range).

pid_to_list/1

Returns a string which corresponds to the text representation of Pid.

Warning!

This BIF is intended for debugging and for use in the Erlang operating system. It should not be used in application programs.

port_close/1

Closes an open port. Roughly the same as Port ! {self(), close} except for the error behaviour (see below), being synchronous, and that the port does not reply with {Port, closed}. Any process may close a port with port_close/1, not only the port owner (the connected process).

For comparison: Port ! {self(), close} fails with badarg if Port cannot be sent to (i.e., Port refers neither to a port nor to a process). If Port is a closed port nothing happens. If Port is an open port and the calling process is the port owner, the port replies with {Port, closed} when all buffers have been flushed and the port really closes, but if the calling process is not the port owner the port owner fails with badsig.

Note that any process can close a port using Port ! {PortOwner, close} just as if it itself was the port owner, but the reply always goes to the port owner.

As of OTP-R16 Port ! {PortOwner, close} is truly asynchronous. Note that this operation has always been documented as an asynchronous operation, while the underlying implementation has been synchronous. port_close/1 is however still fully synchronous. This due to its error behavior.

Failure: badarg if Port is not an open port or the registered name of an open port.

port_command/2

Sends data to a port. Same as Port ! {PortOwner, {command, Data}} except for the error behaviour and being synchronous (see below). Any process may send data to a port with port_command/2, not only the port owner (the connected process).

For comparison: Port ! {PortOwner, {command, Data}} fails with badarg if Port cannot be sent to (i.e., Port refers neither to a port nor to a process). If Port is a closed port the data message disappears without a sound. If Port is open and the calling process is not the port owner, the port owner fails with badsig. The port owner fails with badsig also if Data is not a valid IO list.

Note that any process can send to a port using Port ! {PortOwner, {command, Data}} just as if it itself was the port owner.

If the port is busy, the calling process will be suspended until the port is not busy anymore.

As of OTP-R16 Port ! {PortOwner, {command, Data}} is truly asynchronous. Note that this operation has always been documented as an asynchronous operation, while the underlying implementation has been synchronous. port_command/2 is however still fully synchronous. This due to its error behavior.

Failures:

badarg
If Port is not an open port or the registered name of an open port.
badarg
If Data is not a valid io list.

port_command/3

Sends data to a port. port_command(Port, Data, []) equals port_command(Port, Data).

If the port command is aborted false is returned; otherwise, true is returned.

If the port is busy, the calling process will be suspended until the port is not busy anymore.

Currently the following Options are valid:

force
The calling process will not be suspended if the port is busy; instead, the port command is forced through. The call will fail with a notsup exception if the driver of the port does not support this. For more information see the ERL_DRV_FLAG_SOFT_BUSY driver flag.
nosuspend
The calling process will not be suspended if the port is busy; instead, the port command is aborted and false is returned.

Note!

More options may be added in the future.

Failures:

badarg
If Port is not an open port or the registered name of an open port.
badarg
If Data is not a valid io list.
badarg
If OptionList is not a valid option list.
notsup
If the force option has been passed, but the driver of the port does not allow forcing through a busy port.

port_connect/2

Sets the port owner (the connected port) to Pid. Roughly the same as Port ! {Owner, {connect, Pid}} except for the following:

The error behavior differs, see below.

The port does not reply with {Port,connected}.

port_connect/1 is synchronous, see below.

The new port owner gets linked to the port.

The old port owner stays linked to the port and have to call unlink(Port) if this is not desired. Any process may set the port owner to be any process with port_connect/2.

For comparison: Port ! {self(), {connect, Pid}} fails with badarg if Port cannot be sent to (i.e., Port refers neither to a port nor to a process). If Port is a closed port nothing happens. If Port is an open port and the calling process is the port owner, the port replies with {Port, connected} to the old port owner. Note that the old port owner is still linked to the port, and that the new is not. If Port is an open port and the calling process is not the port owner, the port owner fails with badsig. The port owner fails with badsig also if Pid is not an existing local pid.

Note that any process can set the port owner using Port ! {PortOwner, {connect, Pid}} just as if it itself was the port owner, but the reply always goes to the port owner.

As of OTP-R16 Port ! {PortOwner, {connect, Pid}} is truly asynchronous. Note that this operation has always been documented as an asynchronous operation, while the underlying implementation has been synchronous. port_connect/2 is however still fully synchronous. This due to its error behavior.

Failure: badarg if Port is not an open port or the registered name of an open port, or if Pid is not an existing local pid.

port_control/3

Performs a synchronous control operation on a port. The meaning of Operation and Data depends on the port, i.e., on the port driver. Not all port drivers support this control feature.

Returns: a list of integers in the range 0 through 255, or a binary, depending on the port driver. The meaning of the returned data also depends on the port driver.

Failure: badarg if Port is not an open port or the registered name of an open port, if Operation cannot fit in a 32-bit integer, if the port driver does not support synchronous control operations, or if the port driver so decides for any reason (probably something wrong with Operation or Data).

port_call/3

Performs a synchronous call to a port. The meaning of Operation and Data depends on the port, i.e., on the port driver. Not all port drivers support this feature.

Port is a port identifier, referring to a driver.

Operation is an integer, which is passed on to the driver.

Data is any Erlang term. This data is converted to binary term format and sent to the port.

Returns: a term from the driver. The meaning of the returned data also depends on the port driver.

Failure: badarg if Port is not an open port or the registered name of an open port, if Operation cannot fit in a 32-bit integer, if the port driver does not support synchronous control operations, or if the port driver so decides for any reason (probably something wrong with Operation or Data).

port_info/1

Returns a list containing tuples with information about the Port, or undefined if the port is not open. The order of the tuples is not defined, nor are all the tuples mandatory.

Currently the result will containt information about the following Items: registered_name (if the port has a registered name), id, connected, links, name, input, and output. For more information about the different Items, see port_info/2.

Failure: badarg if Port is not a local port identifier, or an atom.

port_info/2

Pid is the process identifier of the process connected to the port.

If the port identified by Port is not open, undefined is returned.

Failure: badarg if Port is not a local port identifier, or an atom.

Index is the internal index of the port. This index may be used to separate ports.

If the port identified by Port is not open, undefined is returned.

Failure: badarg if Port is not a local port identifier, or an atom.

Bytes is the total number of bytes read from the port.

If the port identified by Port is not open, undefined is returned.

Failure: badarg if Port is not a local port identifier, or an atom.

Pids is a list of the process identifiers of the processes that the port is linked to.

If the port identified by Port is not open, undefined is returned.

Failure: badarg if Port is not a local port identifier, or an atom.

Locking is currently either false (emulator without SMP support), port_level (port specific locking), or driver_level (driver specific locking). Note that these results are highly implementation specific and might change in the future.

If the port identified by Port is not open, undefined is returned.

Failure: badarg if Port is not a local port identifier, or an atom.

Bytes is the total amount of memory, in bytes, allocated for this port by the runtime system. Note that the port itself might have allocated memory which is not included in Bytes.

If the port identified by Port is not open, undefined is returned.

Failure: badarg if Port is not a local port identifier, or an atom.

Monitors represent processes that this port is monitoring.

If the port identified by Port is not open, undefined is returned.

Failure: badarg if Port is not a local port identifier, or an atom.

Name is the command name set by open_port/2.

If the port identified by Port is not open, undefined is returned.

Failure: badarg if Port is not a local port identifier, or an atom.

OsPid is the process identifier (or equivalent) of an OS process created with open_port({spawn | spawn_executable, Command}, Options). If the port is not the result of spawning an OS process, the value is undefined.

If the port identified by Port is not open, undefined is returned.

Failure: badarg if Port is not a local port identifier, or an atom.

Bytes is the total number of bytes written to the port from Erlang processes using either port_command/2, port_command/3, or Port ! {Owner, {command, Data}.

If the port identified by Port is not open, undefined is returned.

Failure: badarg if Port is not a local port identifier, or an atom.

Boolean corresponds to the port parallelism hint being used by this port. For more information see the parallelism option of open_port/2.

Bytes is the total amount of data, in bytes, queued by the port using the ERTS driver queue implementation.

If the port identified by Port is not open, undefined is returned.

Failure: badarg if Port is not a local port identifier, or an atom.

RegisteredName is the registered name of the port. If the port has no registered name, [] is returned.

If the port identified by Port is not open, undefined is returned.

Failure: badarg if Port is not a local port identifier, or an atom.

port_to_list/1

Returns a string which corresponds to the text representation of the port identifier Port.

Warning!

This BIF is intended for debugging and for use in the Erlang operating system. It should not be used in application programs.

ports/0

Returns a list of port identifiers corresponding to all the ports currently existing on the local node.

Note that a port that is exiting, exists but is not open.

pre_loaded/0

Returns a list of Erlang modules which are pre-loaded in the system. As all loading of code is done through the file system, the file system must have been loaded previously. Hence, at least the module init must be pre-loaded.

process_display/2

Writes information about the local process Pid on standard error. The currently allowed value for the atom Type is backtrace, which shows the contents of the call stack, including information about the call chain, with the current function printed first. The format of the output is not further defined.

process_flag/2

When trap_exit is set to true, exit signals arriving to a process are converted to {'EXIT', From, Reason} messages, which can be received as ordinary messages. If trap_exit is set to false, the process exits if it receives an exit signal other than normal and the exit signal is propagated to its linked processes. Application processes should normally not trap exits.

Returns the old value of the flag.

See also exit/2.

This is used by a process to redefine the error handler for undefined function calls and undefined registered processes. Inexperienced users should not use this flag since code auto-loading is dependent on the correct operation of the error handling module.

Returns the old value of the flag.

This changes the minimum heap size for the calling process.

Returns the old value of the flag.

This changes the minimum binary virtual heap size for the calling process.

Returns the old value of the flag.

This sets the process priority. Level is an atom. There are currently four priority levels: low, normal, high, and max. The default priority level is normal. NOTE: The max priority level is reserved for internal use in the Erlang runtime system, and should not be used by others.

Internally in each priority level processes are scheduled in a round robin fashion.

Execution of processes on priority normal and priority low will be interleaved. Processes on priority low will be selected for execution less frequently than processes on priority normal.

When there are runnable processes on priority high no processes on priority low, or normal will be selected for execution. Note, however, that this does not mean that no processes on priority low, or normal will be able to run when there are processes on priority high running. On the runtime system with SMP support there might be more processes running in parallel than processes on priority high, i.e., a low, and a high priority process might execute at the same time.

When there are runnable processes on priority max no processes on priority low, normal, or high will be selected for execution. As with the high priority, processes on lower priorities might execute in parallel with processes on priority max.

Scheduling is preemptive. Regardless of priority, a process is preempted when it has consumed more than a certain amount of reductions since the last time it was selected for execution.

NOTE: You should not depend on the scheduling to remain exactly as it is today. Scheduling, at least on the runtime system with SMP support, is very likely to be modified in the future in order to better utilize available processor cores.

There is currently no automatic mechanism for avoiding priority inversion, such as priority inheritance, or priority ceilings. When using priorities you have to take this into account and handle such scenarios by yourself.

Making calls from a high priority process into code that you don't have control over may cause the high priority process to wait for a processes with lower priority, i.e., effectively decreasing the priority of the high priority process during the call. Even if this isn't the case with one version of the code that you don't have under your control, it might be the case in a future version of it. This might, for example, happen if a high priority process triggers code loading, since the code server runs on priority normal.

Other priorities than normal are normally not needed. When other priorities are used, they need to be used with care, especially the high priority must be used with care. A process on high priority should only perform work for short periods of time. Busy looping for long periods of time in a high priority process will most likely cause problems, since there are important servers in OTP running on priority normal.

Returns the old value of the flag.

N must be an integer in the interval 0..10000. If N > 0, call saving is made active for the process, which means that information about the N most recent global function calls, BIF calls, sends and receives made by the process are saved in a list, which can be retrieved with process_info(Pid, last_calls). A global function call is one in which the module of the function is explicitly mentioned. Only a fixed amount of information is saved: a tuple {Module, Function, Arity} for function calls, and the mere atoms send, 'receive' and timeout for sends and receives ('receive' when a message is received and timeout when a receive times out). If N = 0, call saving is disabled for the process, which is the default. Whenever the size of the call saving list is set, its contents are reset.

Returns the old value of the flag.

Set or clear the sensitive flag for the current process. When a process has been marked as sensitive by calling process_flag(sensitive, true), features in the run-time system that can be used for examining the data and/or inner working of the process are silently disabled.

Features that are disabled include (but are not limited to) the following:

Tracing: Trace flags can still be set for the process, but no trace messages of any kind will be generated. (If the sensitive flag is turned off, trace messages will again be generated if there are any trace flags set.)

Sequential tracing: The sequential trace token will be propagated as usual, but no sequential trace messages will be generated.

process_info/1,2 cannot be used to read out the message queue or the process dictionary (both will be returned as empty lists).

Stack back-traces cannot be displayed for the process.

In crash dumps, the stack, messages, and the process dictionary will be omitted.

If {save_calls,N} has been set for the process, no function calls will be saved to the call saving list. (The call saving list will not be cleared; furthermore, send, receive, and timeout events will still be added to the list.)

Returns the old value of the flag.

process_flag/3

Sets certain flags for the process Pid, in the same manner as process_flag/2. Returns the old value of the flag. The allowed values for Flag are only a subset of those allowed in process_flag/2, namely: save_calls.

Failure: badarg if Pid is not a local process.

process_info/1

Returns a list containing InfoTuples with miscellaneous information about the process identified by Pid, or undefined if the process is not alive.

The order of the InfoTuples is not defined, nor are all the InfoTuples mandatory. The InfoTuples part of the result may be changed without prior notice. Currently InfoTuples with the following items are part of the result: current_function, initial_call, status, message_queue_len, messages, links, dictionary, trap_exit, error_handler, priority, group_leader, total_heap_size, heap_size, stack_size, reductions, and garbage_collection. If the process identified by Pid has a registered name also an InfoTuple with the item registered_name will appear.

See process_info/2 for information about specific InfoTuples.

Warning!

This BIF is intended for debugging only, use process_info/2 for all other purposes.

Failure: badarg if Pid is not a local process.

process_info/2

Returns information about the process identified by Pid as specified by the Item or the ItemList, or undefined if the process is not alive.

If the process is alive and a single Item is given, the returned value is the corresponding InfoTuple unless Item =:= registered_name and the process has no registered name. In this case [] is returned. This strange behavior is due to historical reasons, and is kept for backward compatibility.

If an ItemList is given, the result is an InfoTupleList. The InfoTuples in the InfoTupleList will appear with the corresponding Items in the same order as the Items appeared in the ItemList. Valid Items may appear multiple times in the ItemList.

Note!

If registered_name is part of an ItemList and the process has no name registered a {registered_name, []} InfoTuple will appear in the resulting InfoTupleList. This behavior is different than when a single Item =:= registered_name is given, and than when process_info/1 is used.

Currently the following InfoTuples with corresponding Items are valid:

{backtrace, Bin}

The binary Bin contains the same information as the output from erlang:process_display(Pid, backtrace). Use binary_to_list/1 to obtain the string of characters from the binary.

{binary, BinInfo}

BinInfo is a list containing miscellaneous information about binaries currently being referred to by this process. This InfoTuple may be changed or removed without prior notice.

{catchlevel, CatchLevel}

CatchLevel is the number of currently active catches in this process. This InfoTuple may be changed or removed without prior notice.

{current_function, {Module, Function, Arity}}

Module, Function, Arity is the current function call of the process.

{current_location, {Module, Function, Arity, Location}}

Module, Function, Arity is the current function call of the process. Location is a list of two-tuples that describes the location in the source code.

{current_stacktrace, Stack}

Return the current call stack back-trace (stacktrace) of the process. The stack has the same format as returned by erlang:get_stacktrace/0.

{dictionary, Dictionary}

Dictionary is the dictionary of the process.

{error_handler, Module}

Module is the error handler module used by the process (for undefined function calls, for example).

{garbage_collection, GCInfo}

GCInfo is a list which contains miscellaneous information about garbage collection for this process. The content of GCInfo may be changed without prior notice.

{group_leader, GroupLeader}

GroupLeader is group leader for the IO of the process.

{heap_size, Size}

Size is the size in words of youngest heap generation of the process. This generation currently include the stack of the process. This information is highly implementation dependent, and may change if the implementation change.

{initial_call, {Module, Function, Arity}}

Module, Function, Arity is the initial function call with which the process was spawned.

{links, PidsAndPorts}

PidsAndPorts is a list of pids and port identifiers, with processes or ports to which the process has a link.

{last_calls, false|Calls}

The value is false if call saving is not active for the process (see process_flag/3). If call saving is active, a list is returned, in which the last element is the most recent called.

{memory, Size}

Size is the size in bytes of the process. This includes call stack, heap and internal structures.

{message_queue_len, MessageQueueLen}

MessageQueueLen is the number of messages currently in the message queue of the process. This is the length of the list MessageQueue returned as the info item messages (see below).

{messages, MessageQueue}

MessageQueue is a list of the messages to the process, which have not yet been processed.

{min_heap_size, MinHeapSize}

MinHeapSize is the minimum heap size for the process.

{min_bin_vheap_size, MinBinVHeapSize}

MinBinVHeapSize is the minimum binary virtual heap size for the process.

{monitored_by, Pids}

A list of pids that are monitoring the process (with monitor/2).

{monitors, Monitors}

A list of monitors (started by monitor/2) that are active for the process. For a local process monitor or a remote process monitor by pid, the list item is {process, Pid}, and for a remote process monitor by name, the list item is {process, {RegName, Node}}.

{priority, Level}

Level is the current priority level for the process. For more information on priorities see process_flag(priority, Level).

{reductions, Number}

Number is the number of reductions executed by the process.

{registered_name, Atom}

Atom is the registered name of the process. If the process has no registered name, this tuple is not present in the list.

{sequential_trace_token, [] | SequentialTraceToken}

SequentialTraceToken the sequential trace token for the process. This InfoTuple may be changed or removed without prior notice.

{stack_size, Size}

Size is the stack size of the process in words.

{status, Status}

Status is the status of the process. Status is exiting, garbage_collecting, waiting (for a message), running, runnable (ready to run, but another process is running), or suspended (suspended on a "busy" port or by the erlang:suspend_process/[1,2] BIF).

{suspending, SuspendeeList}

SuspendeeList is a list of {Suspendee, ActiveSuspendCount, OutstandingSuspendCount} tuples. Suspendee is the pid of a process that have been or is to be suspended by the process identified by Pid via the erlang:suspend_process/2 BIF, or the erlang:suspend_process/1 BIF. ActiveSuspendCount is the number of times the Suspendee has been suspended by Pid. OutstandingSuspendCount is the number of not yet completed suspend requests sent by Pid. That is, if ActiveSuspendCount =/= 0, Suspendee is currently in the suspended state, and if OutstandingSuspendCount =/= 0 the asynchronous option of erlang:suspend_process/2 has been used and the suspendee has not yet been suspended by Pid. Note that the ActiveSuspendCount and OutstandingSuspendCount are not the total suspend count on Suspendee, only the parts contributed by Pid.

{total_heap_size, Size}

Size is the total size in words of all heap fragments of the process. This currently include the stack of the process.

{trace, InternalTraceFlags}

InternalTraceFlags is an integer representing internal trace flag for this process. This InfoTuple may be changed or removed without prior notice.

{trap_exit, Boolean}

Boolean is true if the process is trapping exits, otherwise it is false.

Note however, that not all implementations support every one of the above Items.

Failure: badarg if Pid is not a local process, or if Item is not a valid Item.

processes/0

Returns a list of process identifiers corresponding to all the processes currently existing on the local node.

Note that a process that is exiting, exists but is not alive, i.e., is_process_alive/1 will return false for a process that is exiting, but its process identifier will be part of the result returned from processes/0.

> processes().
[<0.0.0>,<0.2.0>,<0.4.0>,<0.5.0>,<0.7.0>,<0.8.0>]

purge_module/1

Removes old code for Module. Before this BIF is used, erlang:check_process_code/2 should be called to check that no processes are executing old code in the module.

Warning!

This BIF is intended for the code server (see code(3)) and should not be used elsewhere.

Failure: badarg if there is no old code for Module.

put/2

Adds a new Key to the process dictionary, associated with the value Val, and returns undefined. If Key already exists, the old value is deleted and replaced by Val and the function returns the old value.

Note!

The values stored when put is evaluated within the scope of a catch will not be retracted if a throw is evaluated, or if an error occurs.

> X = put(name, walrus), Y = put(name, carpenter),
Z = get(name),
{X, Y, Z}.
{undefined,walrus,carpenter}

raise/3

Stops the execution of the calling process with an exception of given class, reason and call stack backtrace (stacktrace).

Warning!

This BIF is intended for debugging and for use in the Erlang operating system. In general, it should be avoided in applications, unless you know very well what you are doing.

Class is one of error, exit or throw, so if it were not for the stacktrace erlang:raise(Class, Reason, Stacktrace) is equivalent to erlang:Class(Reason). Reason is any term and Stacktrace is a list as returned from get_stacktrace(), that is a list of 4-tuples {Module, Function, Arity | Args, Location} where Module and Function are atoms and the third element is an integer arity or an argument list. The stacktrace may also contain {Fun, Args, Location} tuples where Fun is a local fun and Args is an argument list.

The Location element at the end is optional. Omitting it is equivalent to specifying an empty list.

The stacktrace is used as the exception stacktrace for the calling process; it will be truncated to the current maximum stacktrace depth.

Because evaluating this function causes the process to terminate, it has no return value - unless the arguments are invalid, in which case the function returns the error reason, that is badarg. If you want to be really sure not to return you can call error(erlang:raise(Class, Reason, Stacktrace)) and hope to distinguish exceptions later.

read_timer/1

TimerRef is a timer reference returned by erlang:send_after/3 or erlang:start_timer/3. If the timer is active, the function returns the time in milliseconds left until the timer will expire, otherwise false (which means that TimerRef was never a timer, that it has been cancelled, or that it has already delivered its message).

See also erlang:send_after/3, erlang:start_timer/3, and erlang:cancel_timer/1.

ref_to_list/1

Returns a string which corresponds to the text representation of Ref.

Warning!

This BIF is intended for debugging and for use in the Erlang operating system. It should not be used in application programs.

register/2

Associates the name RegName with a pid or a port identifier. RegName, which must be an atom, can be used instead of the pid / port identifier in the send operator (RegName ! Message).

> register(db, Pid).
true

Failure: badarg if PidOrPort is not an existing, local process or port, if RegName is already in use, if the process or port is already registered (already has a name), or if RegName is the atom undefined.

registered/0

Returns a list of names which have been registered using register/2.

> registered().
[code_server, file_server, init, user, my_db]

resume_process/1

Decreases the suspend count on the process identified by Suspendee. Suspendee should previously have been suspended via erlang:suspend_process/2, or erlang:suspend_process/1 by the process calling erlang:resume_process(Suspendee). When the suspend count on Suspendee reach zero, Suspendee will be resumed, i.e., the state of the Suspendee is changed from suspended into the state Suspendee was in before it was suspended.

Warning!

This BIF is intended for debugging only.

Failures:

badarg
If Suspendee isn't a process identifier.
badarg
If the process calling erlang:resume_process/1 had not previously increased the suspend count on the process identified by Suspendee.
badarg
If the process identified by Suspendee is not alive.

round/1

Returns an integer by rounding Number.

> round(5.5).
6

Allowed in guard tests.

self/0

Returns the pid (process identifier) of the calling process.

> self().
<0.26.0>

Allowed in guard tests.

send/2

Sends a message and returns Msg. This is the same as Dest ! Msg.

Dest may be a remote or local pid, a (local) port, a locally registered name, or a tuple {RegName, Node} for a registered name at another node.

send/3

Sends a message and returns ok, or does not send the message but returns something else (see below). Otherwise the same as erlang:send/2. See also erlang:send_nosuspend/2,3. for more detailed explanation and warnings.

The possible options are:

nosuspend

If the sender would have to be suspended to do the send, nosuspend is returned instead.

noconnect

If the destination node would have to be auto-connected before doing the send, noconnect is returned instead.

Warning!

As with erlang:send_nosuspend/2,3: Use with extreme care!

send_after/3

0 <= Time <= 4294967295

Starts a timer which will send the message Msg to Dest after Time milliseconds.

If Dest is a pid() it has to be a pid() of a local process, dead or alive.

The Time value can, in the current implementation, not be greater than 4294967295.

If Dest is an atom(), it is supposed to be the name of a registered process. The process referred to by the name is looked up at the time of delivery. No error is given if the name does not refer to a process.

If Dest is a pid(), the timer will be automatically canceled if the process referred to by the pid() is not alive, or when the process exits. This feature was introduced in erts version 5.4.11. Note that timers will not be automatically canceled when Dest is an atom.

See also erlang:start_timer/3, erlang:cancel_timer/1, and erlang:read_timer/1.

Failure: badarg if the arguments does not satisfy the requirements specified above.

send_nosuspend/2

The same as erlang:send(Dest, Msg, [nosuspend]), but returns true if the message was sent and false if the message was not sent because the sender would have had to be suspended.

This function is intended for send operations towards an unreliable remote node without ever blocking the sending (Erlang) process. If the connection to the remote node (usually not a real Erlang node, but a node written in C or Java) is overloaded, this function will not send the message but return false instead.

The same happens, if Dest refers to a local port that is busy. For all other destinations (allowed for the ordinary send operator '!') this function sends the message and returns true.

This function is only to be used in very rare circumstances where a process communicates with Erlang nodes that can disappear without any trace causing the TCP buffers and the drivers queue to be over-full before the node will actually be shut down (due to tick timeouts) by net_kernel. The normal reaction to take when this happens is some kind of premature shutdown of the other node.

Note that ignoring the return value from this function would result in unreliable message passing, which is contradictory to the Erlang programming model. The message is not sent if this function returns false.

Note also that in many systems, transient states of overloaded queues are normal. The fact that this function returns false does not in any way mean that the other node is guaranteed to be non-responsive, it could be a temporary overload. Also a return value of true does only mean that the message could be sent on the (TCP) channel without blocking, the message is not guaranteed to have arrived at the remote node. Also in the case of a disconnected non-responsive node, the return value is true (mimics the behaviour of the ! operator). The expected behaviour as well as the actions to take when the function returns false are application and hardware specific.

Warning!

Use with extreme care!

send_nosuspend/3

The same as erlang:send(Dest, Msg, [nosuspend | Options]), but with boolean return value.

This function behaves like erlang:send_nosuspend/2), but takes a third parameter, a list of options. The only currently implemented option is noconnect. The option noconnect makes the function return false if the remote node is not currently reachable by the local node. The normal behaviour is to try to connect to the node, which may stall the process for a shorter period. The use of the noconnect option makes it possible to be absolutely sure not to get even the slightest delay when sending to a remote process. This is especially useful when communicating with nodes who expect to always be the connecting part (i.e. nodes written in C or Java).

Whenever the function returns false (either when a suspend would occur or when noconnect was specified and the node was not already connected), the message is guaranteed not to have been sent.

Warning!

Use with extreme care!

set_cookie/2

Sets the magic cookie of Node to the atom Cookie. If Node is the local node, the function also sets the cookie of all other unknown nodes to Cookie (see Distributed Erlang in the Erlang Reference Manual).

Failure: function_clause if the local node is not alive.

setelement/3

1..tuple_size(Tuple1)

Returns a tuple which is a copy of the argument Tuple1 with the element given by the integer argument Index (the first element is the element with index 1) replaced by the argument Value.

> setelement(2, {10, green, bottles}, red).
{10,red,bottles}

size/1

Returns an integer which is the size of the argument Item, which must be either a tuple or a binary.

> size({morni, mulle, bwange}).
3

Allowed in guard tests.

spawn/1

Returns the pid of a new process started by the application of Fun to the empty list []. Otherwise works like spawn/3.

spawn/2

Returns the pid of a new process started by the application of Fun to the empty list [] on Node. If Node does not exist, a useless pid is returned. Otherwise works like spawn/3.

spawn/3

Returns the pid of a new process started by the application of Module:Function to Args. The new process created will be placed in the system scheduler queue and be run some time later.

error_handler:undefined_function(Module, Function, Args) is evaluated by the new process if Module:Function/Arity does not exist (where Arity is the length of Args). The error handler can be redefined (see process_flag/2). If error_handler is undefined, or the user has redefined the default error_handler its replacement is undefined, a failure with the reason undef will occur.

> spawn(speed, regulator, [high_speed, thin_cut]).
<0.13.1>

spawn/4

Returns the pid of a new process started by the application of Module:Function to Args on Node. If Node does not exists, a useless pid is returned. Otherwise works like spawn/3.

spawn_link/1

Returns the pid of a new process started by the application of Fun to the empty list []. A link is created between the calling process and the new process, atomically. Otherwise works like spawn/3.

spawn_link/2

Returns the pid of a new process started by the application of Fun to the empty list [] on Node. A link is created between the calling process and the new process, atomically. If Node does not exist, a useless pid is returned (and due to the link, an exit signal with exit reason noconnection will be received). Otherwise works like spawn/3.

spawn_link/3

Returns the pid of a new process started by the application of Module:Function to Args. A link is created between the calling process and the new process, atomically. Otherwise works like spawn/3.

spawn_link/4

Returns the pid of a new process started by the application of Module:Function to Args on Node. A link is created between the calling process and the new process, atomically. If Node does not exist, a useless pid is returned (and due to the link, an exit signal with exit reason noconnection will be received). Otherwise works like spawn/3.

spawn_monitor/1

Returns the pid of a new process started by the application of Fun to the empty list [] and reference for a monitor created to the new process. Otherwise works like spawn/3.

spawn_monitor/3

A new process is started by the application of Module:Function to Args, and the process is monitored at the same time. Returns the pid and a reference for the monitor. Otherwise works like spawn/3.

spawn_opt/2

Returns the pid of a new process started by the application of Fun to the empty list []. Otherwise works like spawn_opt/4.

If the option monitor is given, the newly created process will be monitored and both the pid and reference for the monitor will be returned.

spawn_opt/3

Returns the pid of a new process started by the application of Fun to the empty list [] on Node. If Node does not exist, a useless pid is returned. Otherwise works like spawn_opt/4.

spawn_opt/4

Works exactly like spawn/3, except that an extra option list is given when creating the process.

If the option monitor is given, the newly created process will be monitored and both the pid and reference for the monitor will be returned.

link

Sets a link to the parent process (like spawn_link/3 does).

monitor

Monitor the new process (just like monitor/2 does).

{priority, Level}

Sets the priority of the new process. Equivalent to executing process_flag(priority, Level) in the start function of the new process, except that the priority will be set before the process is selected for execution for the first time. For more information on priorities see process_flag(priority, Level).

{fullsweep_after, Number}

This option is only useful for performance tuning. In general, you should not use this option unless you know that there is problem with execution times and/or memory consumption, and you should measure to make sure that the option improved matters.

The Erlang runtime system uses a generational garbage collection scheme, using an "old heap" for data that has survived at least one garbage collection. When there is no more room on the old heap, a fullsweep garbage collection will be done.

The fullsweep_after option makes it possible to specify the maximum number of generational collections before forcing a fullsweep even if there is still room on the old heap. Setting the number to zero effectively disables the general collection algorithm, meaning that all live data is copied at every garbage collection.

Here are a few cases when it could be useful to change fullsweep_after. Firstly, if binaries that are no longer used should be thrown away as soon as possible. (Set Number to zero.) Secondly, a process that mostly have short-lived data will be fullsweeped seldom or never, meaning that the old heap will contain mostly garbage. To ensure a fullsweep once in a while, set Number to a suitable value such as 10 or 20. Thirdly, in embedded systems with limited amount of RAM and no virtual memory, one might want to preserve memory by setting Number to zero. (The value may be set globally, see erlang:system_flag/2.)

{min_heap_size, Size}

This option is only useful for performance tuning. In general, you should not use this option unless you know that there is problem with execution times and/or memory consumption, and you should measure to make sure that the option improved matters.

Gives a minimum heap size in words. Setting this value higher than the system default might speed up some processes because less garbage collection is done. Setting too high value, however, might waste memory and slow down the system due to worse data locality. Therefore, it is recommended to use this option only for fine-tuning an application and to measure the execution time with various Size values.

{min_bin_vheap_size, VSize}

This option is only useful for performance tuning. In general, you should not use this option unless you know that there is problem with execution times and/or memory consumption, and you should measure to make sure that the option improved matters.

Gives a minimum binary virtual heap size in words. Setting this value higher than the system default might speed up some processes because less garbage collection is done. Setting too high value, however, might waste memory. Therefore, it is recommended to use this option only for fine-tuning an application and to measure the execution time with various VSize values.

spawn_opt/5

Returns the pid of a new process started by the application of Module:Function to Args on Node. If Node does not exist, a useless pid is returned. Otherwise works like spawn_opt/4.

split_binary/2

Pos = 0..byte_size(Bin)

Returns a tuple containing the binaries which are the result of splitting Bin into two parts at position Pos. This is not a destructive operation. After the operation, there will be three binaries altogether.

> B = list_to_binary("0123456789").
<<"0123456789">>
> byte_size(B).
10
> {B1, B2} = split_binary(B,3).
{<<"012">>,<<"3456789">>}
> byte_size(B1).
3
> byte_size(B2).
7

start_timer/3

0 <= Time <= 4294967295

Starts a timer which will send the message {timeout, TimerRef, Msg} to Dest after Time milliseconds.

If Dest is a pid() it has to be a pid() of a local process, dead or alive.

The Time value can, in the current implementation, not be greater than 4294967295.

If Dest is an atom(), it is supposed to be the name of a registered process. The process referred to by the name is looked up at the time of delivery. No error is given if the name does not refer to a process.

If Dest is a pid(), the timer will be automatically canceled if the process referred to by the pid() is not alive, or when the process exits. This feature was introduced in erts version 5.4.11. Note that timers will not be automatically canceled when Dest is an atom().

See also erlang:send_after/3, erlang:cancel_timer/1, and erlang:read_timer/1.

Failure: badarg if the arguments does not satisfy the requirements specified above.

statistics/1

ContextSwitches is the total number of context switches since the system started.

Note!

statistics(exact_reductions) is a more expensive operation than statistics(reductions) especially on an Erlang machine with SMP support.

This information may not be valid for all implementations.

> statistics(garbage_collection).
{85,23961,0}

Input is the total number of bytes received through ports, and Output is the total number of bytes output to ports.

Note!

Since erts-5.5 (OTP release R11B) this value does not include reductions performed in current time slices of currently scheduled processes. If an exact value is wanted, use statistics(exact_reductions).

> statistics(reductions).
{2046,11}

Returns the length of the run queue, that is, the number of processes that are ready to run.

Note that the run-time is the sum of the run-time for all threads in the Erlang run-time system and may therefore be greater than the wall-clock time.

> statistics(runtime).
{1690,1620}

Returns a list of tuples with {SchedulerId, ActiveTime, TotalTime}, where SchedulerId is an integer id of the scheduler, ActiveTime is the duration the scheduler has been busy, TotalTime is the total time duration since scheduler_wall_time activation. The time unit is not defined and may be subject to change between releases, operating systems and system restarts. scheduler_wall_time should only be used to calculate relative values for scheduler-utilization. ActiveTime can never exceed TotalTime.

The definition of a busy scheduler is when it is not idle or not scheduling (selecting) a process or port, meaning; executing process code, executing linked-in-driver or NIF code, executing built-in-functions or any other runtime handling, garbage collecting or handling any other memory management. Note, a scheduler may also be busy even if the operating system has scheduled out the scheduler thread.

Returns undefined if the system flag scheduler_wall_time is turned off.

The list of scheduler information is unsorted and may appear in different order between calls.

Using scheduler_wall_time to calculate scheduler utilization.

> erlang:system_flag(scheduler_wall_time, true).
false
> Ts0 = lists:sort(erlang:statistics(scheduler_wall_time)), ok.
ok

Some time later we will take another snapshot and calculate scheduler-utilization per scheduler.

> Ts1 = lists:sort(erlang:statistics(scheduler_wall_time)), ok.
ok
> lists:map(fun({{I, A0, T0}, {I, A1, T1}}) ->
	{I, (A1 - A0)/(T1 - T0)} end, lists:zip(Ts0,Ts1)).
[{1,0.9743474730177548},
 {2,0.9744843782751444},
 {3,0.9995902361669045},
 {4,0.9738012596572161},
 {5,0.9717956667018103},
 {6,0.9739235846420741},
 {7,0.973237033077876},
 {8,0.9741297293248656}]

Using the same snapshots to calculate a total scheduler-utilization.

> {A, T} = lists:foldl(fun({{_, A0, T0}, {_, A1, T1}}, {Ai,Ti}) ->
	{Ai + (A1 - A0), Ti + (T1 - T0)} end, {0, 0}, lists:zip(Ts0,Ts1)), A/T.
0.9769136803764825

Note!

scheduler_wall_time is by default disabled. Use erlang:system_flag(scheduler_wall_time, true) to enable it.

wall_clock can be used in the same manner as runtime, except that real time is measured as opposed to runtime or CPU time.

suspend_process/2

Increases the suspend count on the process identified by Suspendee and puts it in the suspended state if it isn't already in the suspended state. A suspended process will not be scheduled for execution until the process has been resumed.

A process can be suspended by multiple processes and can be suspended multiple times by a single process. A suspended process will not leave the suspended state until its suspend count reach zero. The suspend count of Suspendee is decreased when erlang:resume_process(Suspendee) is called by the same process that called erlang:suspend_process(Suspendee). All increased suspend counts on other processes acquired by a process will automatically be decreased when the process terminates.

Currently the following options (Opts) are available:

asynchronous
A suspend request is sent to the process identified by Suspendee. Suspendee will eventually suspend unless it is resumed before it was able to suspend. The caller of erlang:suspend_process/2 will return immediately, regardless of whether the Suspendee has suspended yet or not. Note that the point in time when the Suspendee will actually suspend cannot be deduced from other events in the system. The only guarantee given is that the Suspendee will eventually suspend (unless it is resumed). If the asynchronous option has not been passed, the caller of erlang:suspend_process/2 will be blocked until the Suspendee has actually suspended.
unless_suspending
The process identified by Suspendee will be suspended unless the calling process already is suspending the Suspendee. If unless_suspending is combined with the asynchronous option, a suspend request will be sent unless the calling process already is suspending the Suspendee or if a suspend request already has been sent and is in transit. If the calling process already is suspending the Suspendee, or if combined with the asynchronous option and a send request already is in transit, false is returned and the suspend count on Suspendee will remain unchanged.

If the suspend count on the process identified by Suspendee was increased, true is returned; otherwise, false is returned.

Warning!

This BIF is intended for debugging only.

Failures:

badarg
If Suspendee isn't a process identifier.
badarg
If the process identified by Suspendee is same the process as the process calling erlang:suspend_process/2.
badarg
If the process identified by Suspendee is not alive.
badarg
If the process identified by Suspendee resides on another node.
badarg
If OptList isn't a proper list of valid Opts.
system_limit
If the process identified by Suspendee has been suspended more times by the calling process than can be represented by the currently used internal data structures. The current system limit is larger than 2 000 000 000 suspends, and it will never be less than that.

suspend_process/1

Suspends the process identified by Suspendee. The same as calling erlang:suspend_process(Suspendee, []). For more information see the documentation of erlang:suspend_process/2.

Warning!

This BIF is intended for debugging only.

system_flag/2

Sets the maximum depth of call stack back-traces in the exit reason element of 'EXIT' tuples.

Returns the old value of the flag.

Warning!

This argument is deprecated and scheduled for removal in erts-5.10/OTP-R16. Instead of using this argument you are advised to use the erl command line argument +sct. When this argument has been removed a final CPU topology to use will be determined at emulator boot time.

Sets the user defined CpuTopology. The user defined CPU topology will override any automatically detected CPU topology. By passing undefined as CpuTopology the system will revert back to the CPU topology automatically detected. The returned value equals the value returned from erlang:system_info(cpu_topology) before the change was made.

Returns the old value of the flag.

The CPU topology is used when binding schedulers to logical processors. If schedulers are already bound when the CPU topology is changed, the schedulers will be sent a request to rebind according to the new CPU topology.

The user defined CPU topology can also be set by passing the +sct command line argument to erl.

For information on the CpuTopology type and more, see the documentation of erlang:system_info(cpu_topology), and the erl +sct and +sbt command line flags.

Number is a non-negative integer which indicates how many times generational garbage collections can be done without forcing a fullsweep collection. The value applies to new processes; processes already running are not affected.

Returns the old value of the flag.

In low-memory systems (especially without virtual memory), setting the value to 0 can help to conserve memory.

An alternative way to set this value is through the (operating system) environment variable ERL_FULLSWEEP_AFTER.

Sets the default minimum heap size for processes. The size is given in words. The new min_heap_size only effects processes spawned after the change of min_heap_size has been made. The min_heap_size can be set for individual processes by use of spawn_opt/N or process_flag/2.

Returns the old value of the flag.

Sets the default minimum binary virtual heap size for processes. The size is given in words. The new min_bin_vhheap_size only effects processes spawned after the change of min_bin_vhheap_size has been made. The min_bin_vheap_size can be set for individual processes by use of spawn_opt/N or process_flag/2.

Returns the old value of the flag.

If multi-scheduling is enabled, more than one scheduler thread is used by the emulator. Multi-scheduling can be blocked. When multi-scheduling has been blocked, only one scheduler thread will schedule Erlang processes.

If BlockState =:= block, multi-scheduling will be blocked. If BlockState =:= unblock and no-one else is blocking multi-scheduling and this process has only blocked one time, multi-scheduling will be unblocked. One process can block multi-scheduling multiple times. If a process has blocked multiple times, it has to unblock exactly as many times as it has blocked before it has released its multi-scheduling block. If a process that has blocked multi-scheduling exits, it will release its blocking of multi-scheduling.

The return values are disabled, blocked, or enabled. The returned value describes the state just after the call to erlang:system_flag(multi_scheduling, BlockState) has been made. The return values are described in the documentation of erlang:system_info(multi_scheduling).

NOTE: Blocking of multi-scheduling should normally not be needed. If you feel that you need to block multi-scheduling, think through the problem at least a couple of times again. Blocking multi-scheduling should only be used as a last resort since it will most likely be a very inefficient way to solve the problem.

See also erlang:system_info(multi_scheduling), erlang:system_info(multi_scheduling_blockers), and erlang:system_info(schedulers).

Warning!

This argument is deprecated and scheduled for removal in erts-5.10/OTP-R16. Instead of using this argument you are advised to use the erl command line argument +sbt. When this argument has been removed a final scheduler bind type to use will be determined at emulator boot time.

Controls if and how schedulers are bound to logical processors.

When erlang:system_flag(scheduler_bind_type, How) is called, an asynchronous signal is sent to all schedulers online which causes them to try to bind or unbind as requested. NOTE: If a scheduler fails to bind, this will often be silently ignored. This since it isn't always possible to verify valid logical processor identifiers. If an error is reported, it will be reported to the error_logger. If you want to verify that the schedulers actually have bound as requested, call erlang:system_info(scheduler_bindings).

Schedulers can currently only be bound on newer Linux, Solaris, FreeBSD, and Windows systems, but more systems will be supported in the future.

In order for the runtime system to be able to bind schedulers, the CPU topology needs to be known. If the runtime system fails to automatically detect the CPU topology, it can be defined. For more information on how to define the CPU topology, see the erl +sct command line flag.

The runtime system will by default not bind schedulers to logical processors.

NOTE: If the Erlang runtime system is the only operating system process that binds threads to logical processors, this improves the performance of the runtime system. However, if other operating system processes (as for example another Erlang runtime system) also bind threads to logical processors, there might be a performance penalty instead. In some cases this performance penalty might be severe. If this is the case, you are advised to not bind the schedulers.

Schedulers can be bound in different ways. The How argument determines how schedulers are bound. How can currently be one of:

unbound

Same as the erl command line argument +sbt u.

no_spread

Same as the erl command line argument +sbt ns.

thread_spread

Same as the erl command line argument +sbt ts.

processor_spread

Same as the erl command line argument +sbt ps.

spread

Same as the erl command line argument +sbt s.

no_node_thread_spread

Same as the erl command line argument +sbt nnts.

no_node_processor_spread

Same as the erl command line argument +sbt nnps.

thread_no_node_processor_spread

Same as the erl command line argument +sbt tnnps.

default_bind

Same as the erl command line argument +sbt db.

The value returned equals How before the scheduler_bind_type flag was changed.

Failure:

notsup

If binding of schedulers is not supported.

badarg

If How isn't one of the documented alternatives.

badarg

If no CPU topology information is available.

The scheduler bind type can also be set by passing the +sbt command line argument to erl.

For more information, see erlang:system_info(scheduler_bind_type), erlang:system_info(scheduler_bindings), the erl +sbt and +sct command line flags.

Turns on/off scheduler wall time measurements.

For more information see, erlang:statistics(scheduler_wall_time).

Sets the amount of schedulers online. Valid range is 1 <= SchedulersOnline <= erlang:system_info(schedulers).

Returns the old value of the flag.

For more information see, erlang:system_info(schedulers), and erlang:system_info(schedulers_online).

Sets the value of the node's trace control word to TCW. TCW should be an unsigned integer. For more information see documentation of the set_tcw function in the match specification documentation in the ERTS User's Guide.

Returns the old value of the flag.

system_info/1

Returns various information about the allocators of the current system (emulator) as specified by Item:

allocated_areas

Returns a list of tuples with information about miscellaneous allocated memory areas.

Each tuple contains an atom describing type of memory as first element and amount of allocated memory in bytes as second element. In those cases when there is information present about allocated and used memory, a third element is present. This third element contains the amount of used memory in bytes.

erlang:system_info(allocated_areas) is intended for debugging, and the content is highly implementation dependent. The content of the results will therefore change when needed without prior notice.

Note: The sum of these values is not the total amount of memory allocated by the emulator. Some values are part of other values, and some memory areas are not part of the result. If you are interested in the total amount of memory allocated by the emulator see erlang:memory/0,1.

allocator

Returns {Allocator, Version, Features, Settings}.

Explanation:

Allocator corresponds to the malloc() implementation used. If Allocator equals undefined, the malloc() implementation used could not be identified. Currently glibc can be identified.

Version is a list of integers (but not a string) representing the version of the malloc() implementation used.

Features is a list of atoms representing allocation features used.

Settings is a list of subsystems, their configurable parameters, and used values. Settings may differ between different combinations of platforms, allocators, and allocation features. Memory sizes are given in bytes.

See also "System Flags Effecting erts_alloc" in erts_alloc(3).

alloc_util_allocators

Returns a list of the names of all allocators using the ERTS internal alloc_util framework as atoms. For more information see the "the alloc_util framework" section in the erts_alloc(3) documentation.

{allocator, Alloc}

Returns information about the specified allocator. As of erts version 5.6.1 the return value is a list of {instance, InstanceNo, InstanceInfo} tuples where InstanceInfo contains information about a specific instance of the allocator. If Alloc is not a recognized allocator, undefined is returned. If Alloc is disabled, false is returned.

Note: The information returned is highly implementation dependent and may be changed, or removed at any time without prior notice. It was initially intended as a tool when developing new allocators, but since it might be of interest for others it has been briefly documented.

The recognized allocators are listed in erts_alloc(3). After reading the erts_alloc(3) documentation, the returned information should more or less speak for itself. But it can be worth explaining some things. Call counts are presented by two values. The first value is giga calls, and the second value is calls. mbcs, and sbcs are abbreviations for, respectively, multi-block carriers, and single-block carriers. Sizes are presented in bytes. When it is not a size that is presented, it is the amount of something. Sizes and amounts are often presented by three values, the first is current value, the second is maximum value since the last call to erlang:system_info({allocator, Alloc}), and the third is maximum value since the emulator was started. If only one value is present, it is the current value. fix_alloc memory block types are presented by two values. The first value is memory pool size and the second value used memory size.

{allocator_sizes, Alloc}

Returns various size information for the specified allocator. The information returned is a subset of the information returned by erlang:system_info({allocator, Alloc}).

All LevelEntrys of a list must contain the same LevelTag, except on the top level where both node and processor LevelTags may co-exist. {LevelTag, SubLevel} == {LevelTag, [], SubLevel} More LevelTags may be introduced in the future. The info_list() may be extended in the future.

Returns various information about the CPU topology of the current system (emulator) as specified by Item:

cpu_topology

Returns the CpuTopology which currently is used by the emulator. The CPU topology is used when binding schedulers to logical processors. The CPU topology used is the user defined CPU topology if such exists; otherwise, the automatically detected CPU topology if such exists. If no CPU topology exists, undefined is returned.

node refers to NUMA (non-uniform memory access) nodes, and thread refers to hardware threads (e.g. Intels hyper-threads).

A level in the CpuTopology term can be omitted if only one entry exists and the InfoList is empty.

thread can only be a sub level to core. core can be a sub level to either processor or node. processor can either be on the top level or a sub level to node. node can either be on the top level or a sub level to processor. That is, NUMA nodes can be processor internal or processor external. A CPU topology can consist of a mix of processor internal and external NUMA nodes, as long as each logical CPU belongs to one and only one NUMA node. Cache hierarchy is not part of the CpuTopology type yet, but will be in the future. Other things may also make it into the CPU topology in the future. In other words, expect the CpuTopology type to change.

{cpu_topology, defined}

Returns the user defined CpuTopology. For more information see the documentation of the erl +sct command line flag, and the documentation of the cpu_topology argument.

{cpu_topology, detected}

Returns the automatically detected CpuTopology. The emulator currently only detects the CPU topology on some newer Linux, Solaris, FreeBSD, and Windows systems. On Windows system with more than 32 logical processors the CPU topology is not detected.

For more information see the documentation of the cpu_topology argument.

{cpu_topology, used}

Returns the CpuTopology which is used by the emulator. For more information see the documentation of the cpu_topology argument.

Returns various information about the current system (emulator) as specified by Item:

allocated_areas, allocator, alloc_util_allocators, allocator_sizes

See above.

build_type

Returns an atom describing the build type of the runtime system. This is normally the atom opt for optimized. Other possible return values are debug, purify, quantify, purecov, gcov, valgrind, gprof, and lcnt. Possible return values may be added and/or removed at any time without prior notice.

c_compiler_used

Returns a two-tuple describing the C compiler used when compiling the runtime system. The first element is an atom describing the name of the compiler, or undefined if unknown. The second element is a term describing the version of the compiler, or undefined if unknown.

check_io

Returns a list containing miscellaneous information regarding the emulators internal I/O checking. Note, the content of the returned list may vary between platforms and over time. The only thing guaranteed is that a list is returned.

compat_rel

Returns the compatibility mode of the local node as an integer. The integer returned represents the Erlang/OTP release which the current emulator has been set to be backward compatible with. The compatibility mode can be configured at startup by using the command line flag +R, see erl(1).

cpu_topology

See above.

creation

Returns the creation of the local node as an integer. The creation is changed when a node is restarted. The creation of a node is stored in process identifiers, port identifiers, and references. This makes it (to some extent) possible to distinguish between identifiers from different incarnations of a node. Currently valid creations are integers in the range 1..3, but this may (probably will) change in the future. If the node is not alive, 0 is returned.

debug_compiled

Returns true if the emulator has been debug compiled; otherwise, false.

dist

Returns a binary containing a string of distribution information formatted as in Erlang crash dumps. For more information see the "How to interpret the Erlang crash dumps" chapter in the ERTS User's Guide.

dist_ctrl

Returns a list of tuples {Node, ControllingEntity}, one entry for each connected remote node. The Node is the name of the node and the ControllingEntity is the port or pid responsible for the communication to that node. More specifically, the ControllingEntity for nodes connected via TCP/IP (the normal case) is the socket actually used in communication with the specific node.

driver_version

Returns a string containing the erlang driver version used by the runtime system. It will be on the form "<major ver>.<minor ver>".

dynamic_trace

Returns an atom describing the dynamic trace framework compiled into the virtual machine. It can currently be either dtrace, systemtap or none. For a commercial or standard build, this is always none, the other return values indicate a custom configuration (e.g. ./configure --with-dynamic-trace=dtrace). See the dyntrace manual page and the README.dtrace/README.systemtap files in the Erlang source code top directory for more information about dynamic tracing.

dynamic_trace_probes

Returns a boolean() indicating if dynamic trace probes (either dtrace or systemtap) are built into the emulator. This can only be true if the virtual machine was built for dynamic tracing (i.e. system_info(dynamic_trace) returns dtrace or systemtap).

elib_malloc

This option will be removed in a future release. The return value will always be false since the elib_malloc allocator has been removed.

dist_buf_busy_limit

Returns the value of the distribution buffer busy limit in bytes. This limit can be set on startup by passing the +zdbbl command line flag to erl.

fullsweep_after

Returns {fullsweep_after, integer() >= 0} which is the fullsweep_after garbage collection setting used by default. For more information see garbage_collection described below.

garbage_collection

Returns a list describing the default garbage collection settings. A process spawned on the local node by a spawn or spawn_link will use these garbage collection settings. The default settings can be changed by use of system_flag/2. spawn_opt/4 can spawn a process that does not use the default settings.

heap_sizes

Returns a list of integers representing valid heap sizes in words. All Erlang heaps are sized from sizes in this list.

heap_type

Returns the heap type used by the current emulator. Currently only the following heap type exists:

private

Each process has a heap reserved for its use and no references between heaps of different processes are allowed. Messages passed between processes are copied between heaps.

info

Returns a binary containing a string of miscellaneous system information formatted as in Erlang crash dumps. For more information see the "How to interpret the Erlang crash dumps" chapter in the ERTS User's Guide.

kernel_poll

Returns true if the emulator uses some kind of kernel-poll implementation; otherwise, false.

loaded

Returns a binary containing a string of loaded module information formatted as in Erlang crash dumps. For more information see the "How to interpret the Erlang crash dumps" chapter in the ERTS User's Guide.

logical_processors

Returns the detected number of logical processors configured on the system. The return value is either an integer, or the atom unknown if the emulator wasn't able to detect logical processors configured.

logical_processors_available

Returns the detected number of logical processors available to the Erlang runtime system. The return value is either an integer, or the atom unknown if the emulator wasn't able to detect logical processors available. The number of logical processors available is less than or equal to the number of logical processors online.

logical_processors_online

Returns the detected number of logical processors online on the system. The return value is either an integer, or the atom unknown if the emulator wasn't able to detect logical processors online. The number of logical processors online is less than or equal to the number of logical processors configured.

machine

Returns a string containing the Erlang machine name.

min_heap_size

Returns {min_heap_size, MinHeapSize} where MinHeapSize is the current system wide minimum heap size for spawned processes.

min_bin_vheap_size

Returns {min_bin_vheap_size, MinBinVHeapSize} where MinBinVHeapSize is the current system wide minimum binary virtual heap size for spawned processes.

modified_timing_level

Returns the modified timing level (an integer) if modified timing has been enabled; otherwise, undefined. See the +T command line flag in the documentation of the erl(1) command for more information on modified timing.

multi_scheduling

Returns disabled, blocked, or enabled. A description of the return values:

disabled

The emulator has only one scheduler thread. The emulator does not have SMP support, or have been started with only one scheduler thread.

blocked

The emulator has more than one scheduler thread, but all scheduler threads but one have been blocked, i.e., only one scheduler thread will schedule Erlang processes and execute Erlang code.

enabled

The emulator has more than one scheduler thread, and no scheduler threads have been blocked, i.e., all available scheduler threads will schedule Erlang processes and execute Erlang code.

See also erlang:system_flag(multi_scheduling, BlockState), erlang:system_info(multi_scheduling_blockers), and erlang:system_info(schedulers).

multi_scheduling_blockers

Returns a list of PIDs when multi-scheduling is blocked; otherwise, the empty list. The PIDs in the list is PIDs of the processes currently blocking multi-scheduling. A PID will only be present once in the list, even if the corresponding process has blocked multiple times.

See also erlang:system_flag(multi_scheduling, BlockState), erlang:system_info(multi_scheduling), and erlang:system_info(schedulers).

otp_release

Returns a string containing the OTP release number.

port_parallelism

Returns the default port parallelism scheduling hint used. For more information see the +spp command line argument of erl(1).

port_count

Returns the number of ports currently existing at the local node as an integer. The same value as length(erlang:ports()) returns, but more efficient.

port_limit

Returns the maximum number of simultaneously existing ports at the local node as an integer. This limit can be configured at startup by using the +Q command line flag of erl(1).

process_count

Returns the number of processes currently existing at the local node as an integer. The same value as length(processes()) returns, but more efficient.

process_limit

Returns the maximum number of simultaneously existing processes at the local node as an integer. This limit can be configured at startup by using the +P command line flag of erl(1).

procs

Returns a binary containing a string of process and port information formatted as in Erlang crash dumps. For more information see the "How to interpret the Erlang crash dumps" chapter in the ERTS User's Guide.

scheduler_bind_type

Returns information on how user has requested schedulers to be bound or not bound.

NOTE: Even though user has requested schedulers to be bound, they might have silently failed to bind. In order to inspect actual scheduler bindings call erlang:system_info(scheduler_bindings).

For more information, see the erl +sbt command line argument, and erlang:system_info(scheduler_bindings).

scheduler_bindings

Returns information on currently used scheduler bindings.

A tuple of a size equal to erlang:system_info(schedulers) is returned. The elements of the tuple are integers or the atom unbound. Logical processor identifiers are represented as integers. The Nth element of the tuple equals the current binding for the scheduler with the scheduler identifier equal to N. E.g., if the schedulers have been bound, element(erlang:system_info(scheduler_id), erlang:system_info(scheduler_bindings)) will return the identifier of the logical processor that the calling process is executing on.

Note that only schedulers online can be bound to logical processors.

For more information, see the erl +sbt command line argument, erlang:system_info(schedulers_online).

scheduler_id

Returns the scheduler id (SchedulerId) of the scheduler thread that the calling process is executing on. SchedulerId is a positive integer; where 1 <= SchedulerId <= erlang:system_info(schedulers). See also erlang:system_info(schedulers).

schedulers

Returns the number of scheduler threads used by the emulator. Scheduler threads online schedules Erlang processes and Erlang ports, and execute Erlang code and Erlang linked in driver code.

The number of scheduler threads is determined at emulator boot time and cannot be changed after that. The amount of schedulers online can however be changed at any time.

See also erlang:system_flag(schedulers_online, SchedulersOnline), erlang:system_info(schedulers_online), erlang:system_info(scheduler_id), erlang:system_flag(multi_scheduling, BlockState), erlang:system_info(multi_scheduling), and and erlang:system_info(multi_scheduling_blockers).

schedulers_online

Returns the amount of schedulers online. The scheduler identifiers of schedulers online satisfy the following relationship: 1 <= SchedulerId <= erlang:system_info(schedulers_online).

For more information, see erlang:system_info(schedulers), and erlang:system_flag(schedulers_online, SchedulersOnline).

smp_support

Returns true if the emulator has been compiled with smp support; otherwise, false.

system_version

Returns a string containing version number and some important properties such as the number of schedulers.

system_architecture

Returns a string containing the processor and OS architecture the emulator is built for.

threads

Returns true if the emulator has been compiled with thread support; otherwise, false is returned.

thread_pool_size

Returns the number of async threads in the async thread pool used for asynchronous driver calls (driver_async()) as an integer.

trace_control_word

Returns the value of the node's trace control word. For more information see documentation of the function get_tcw in "Match Specifications in Erlang", ERTS User's Guide.

update_cpu_info

The runtime system rereads the CPU information available and updates its internally stored information about the detected CPU topology and the amount of logical processors configured, online, and available. If the CPU information has changed since the last time it was read, the atom changed is returned; otherwise, the atom unchanged is returned. If the CPU information has changed you probably want to adjust the amount of schedulers online. You typically want to have as many schedulers online as logical processors available.

version

Returns a string containing the version number of the emulator.

wordsize

Same as {wordsize, internal}.

{wordsize, internal}

Returns the size of Erlang term words in bytes as an integer, i.e. on a 32-bit architecture 4 is returned, and on a pure 64-bit architecture 8 is returned. On a halfword 64-bit emulator, 4 is returned, as the Erlang terms are stored using a virtual wordsize of half the system's wordsize.

{wordsize, external}

Returns the true wordsize of the emulator, i.e. the size of a pointer, in bytes as an integer. On a pure 32-bit architecture 4 is returned, on both a halfword and pure 64-bit architecture, 8 is returned.

Note!

The scheduler argument has changed name to scheduler_id. This in order to avoid mixup with the schedulers argument. The scheduler argument was introduced in ERTS version 5.5 and renamed in ERTS version 5.5.1.

system_monitor/0

Returns the current system monitoring settings set by erlang:system_monitor/2 as {MonitorPid, Options}, or undefined if there are no settings. The order of the options may be different from the one that was set.

system_monitor/1

When called with the argument undefined, all system performance monitoring settings are cleared.

Calling the function with {MonitorPid, Options} as argument, is the same as calling erlang:system_monitor(MonitorPid, Options).

Returns the previous system monitor settings just like erlang:system_monitor/0.

system_monitor/2

Sets system performance monitoring options. MonitorPid is a local pid that will receive system monitor messages, and the second argument is a list of monitoring options:

{long_gc, Time}

If a garbage collection in the system takes at least Time wallclock milliseconds, a message {monitor, GcPid, long_gc, Info} is sent to MonitorPid. GcPid is the pid that was garbage collected and Info is a list of two-element tuples describing the result of the garbage collection. One of the tuples is {timeout, GcTime} where GcTime is the actual time for the garbage collection in milliseconds. The other tuples are tagged with heap_size, heap_block_size, stack_size, mbuf_size, old_heap_size, and old_heap_block_size. These tuples are explained in the documentation of the gc_start trace message (see erlang:trace/3). New tuples may be added, and the order of the tuples in the Info list may be changed at any time without prior notice.

{long_schedule, Time}

If a process or port in the system runs uninterrupted for at least Time wall clock milliseconds, a message {monitor, PidOrPort, long_schedule, Info} is sent to MonitorPid. PidOrPort is the process or port that was running and Info is a list of two-element tuples describing the event. In case of a pid(), the tuples {timeout, Millis}, {in, Location} and {out, Location} will be present, where Location is either an MFA ({Module, Function, Arity}) describing the function where the process was scheduled in/out, or the atom undefined. In case of a port(), the tuples {timeout, Millis} and {port_op,Op} will be present. Op will be one of proc_sig, timeout, input, output, event or dist_cmd, depending on which driver callback was executing. proc_sig is an internal operation and should never appear, while the others represent the corresponding driver callbacks timeout, ready_input, ready_output, event and finally outputv (when the port is used by distribution). The Millis value in the timeout tuple will tell you the actual uninterrupted execution time of the process or port, which will always be >= the Time value supplied when starting the trace. New tuples may be added to the Info list in the future, and the order of the tuples in the list may be changed at any time without prior notice.

This can be used to detect problems with NIF's or drivers that take too long to execute. Generally, 1 ms is considered a good maximum time for a driver callback or a NIF. However, a time sharing system should usually consider everything below 100 ms as "possible" and fairly "normal". Schedule times above that might however indicate swapping or a NIF/driver that is misbehaving. Misbehaving NIF's and drivers could cause bad resource utilization and bad overall performance of the system.

{large_heap, Size}

If a garbage collection in the system results in the allocated size of a heap being at least Size words, a message {monitor, GcPid, large_heap, Info} is sent to MonitorPid. GcPid and Info are the same as for long_gc above, except that the tuple tagged with timeout is not present. Note: As of erts version 5.6 the monitor message is sent if the sum of the sizes of all memory blocks allocated for all heap generations is equal to or larger than Size. Previously the monitor message was sent if the memory block allocated for the youngest generation was equal to or larger than Size.

busy_port

If a process in the system gets suspended because it sends to a busy port, a message {monitor, SusPid, busy_port, Port} is sent to MonitorPid. SusPid is the pid that got suspended when sending to Port.

busy_dist_port

If a process in the system gets suspended because it sends to a process on a remote node whose inter-node communication was handled by a busy port, a message {monitor, SusPid, busy_dist_port, Port} is sent to MonitorPid. SusPid is the pid that got suspended when sending through the inter-node communication port Port.

Returns the previous system monitor settings just like erlang:system_monitor/0.

Note!

If a monitoring process gets so large that it itself starts to cause system monitor messages when garbage collecting, the messages will enlarge the process's message queue and probably make the problem worse.

Keep the monitoring process neat and do not set the system monitor limits too tight.

Failure: badarg if MonitorPid does not exist or is not a local process.

system_profile/0

Returns the current system profiling settings set by erlang:system_profile/2 as {ProfilerPid, Options}, or undefined if there are no settings. The order of the options may be different from the one that was set.

system_profile/2

Sets system profiler options. ProfilerPid is a local pid or port that will receive profiling messages. The receiver is excluded from all profiling. The second argument is a list of profiling options:

exclusive

If a synchronous call to a port from a process is done, the calling process is considered not runnable during the call runtime to the port. The calling process is notified as inactive and subsequently active when the port callback returns.

runnable_procs

If a process is put into or removed from the run queue a message, {profile, Pid, State, Mfa, Ts}, is sent to ProfilerPid. Running processes that is reinserted into the run queue after having been preemptively scheduled out will not trigger this message.

runnable_ports

If a port is put into or removed from the run queue a message, {profile, Port, State, 0, Ts}, is sent to ProfilerPid.

scheduler

If a scheduler is put to sleep or awoken a message, {profile, scheduler, Id, State, NoScheds, Ts}, is sent to ProfilerPid.

Note!

erlang:system_profile is considered experimental and its behaviour may change in the future.

term_to_binary/1

Returns a binary data object which is the result of encoding Term according to the Erlang external term format.

This can be used for a variety of purposes, for example writing a term to a file in an efficient way, or sending an Erlang term to some type of communications channel not supported by distributed Erlang.

See also binary_to_term/1.

term_to_binary/2

Returns a binary data object which is the result of encoding Term according to the Erlang external term format.

If the option compressed is provided, the external term format will be compressed. The compressed format is automatically recognized by binary_to_term/1 in R7B and later.

It is also possible to specify a compression level by giving the option {compressed, Level}, where Level is an integer from 0 through 9. 0 means that no compression will be done (it is the same as not giving any compressed option); 1 will take the least time but may not compress as well as the higher levels; 9 will take the most time and may produce a smaller result. Note the "mays" in the preceding sentence; depending on the input term, level 9 compression may or may not produce a smaller result than level 1 compression.

Currently, compressed gives the same result as {compressed, 6}.

The option {minor_version, Version} can be use to control some details of the encoding. This option was introduced in R11B-4. Currently, the allowed values for Version are 0 and 1.

{minor_version, 1} forces any floats in the term to be encoded in a more space-efficient and exact way (namely in the 64-bit IEEE format, rather than converted to a textual representation). binary_to_term/1 in R11B-4 and later is able decode the new representation.

{minor_version, 0} is currently the default, meaning that floats will be encoded using a textual representation; this option is useful if you want to ensure that releases prior to R11B-4 can decode resulting binary.

See also binary_to_term/1.

throw/1

A non-local return from a function. If evaluated within a catch, catch will return the value Any.

> catch throw({hello, there}).
{hello,there}

Failure: nocatch if not evaluated within a catch.

time/0

Returns the current time as {Hour, Minute, Second}.

The time zone and daylight saving time correction depend on the underlying OS.

> time().
{9,42,44}

tl/1

Returns the tail of List, that is, the list minus the first element.

> tl([geesties, guilies, beasties]).
[guilies, beasties]

Allowed in guard tests.

Failure: badarg if List is the empty list [].

trace/3

Turns on (if How == true) or off (if How == false) the trace flags in FlagList for the process or processes represented by PidSpec.

PidSpec is either a pid for a local process, or one of the following atoms:

existing

All processes currently existing.

new

All processes that will be created in the future.

all

All currently existing processes and all processes that will be created in the future.

FlagList can contain any number of the following flags (the "message tags" refers to the list of messages following below):

all

Set all trace flags except {tracer, Tracer} and cpu_timestamp that are in their nature different than the others.

send

Trace sending of messages.

Message tags: send, send_to_non_existing_process.

'receive'

Trace receiving of messages.

Message tags: 'receive'.

procs

Trace process related events.

Message tags: spawn, exit, register, unregister, link, unlink, getting_linked, getting_unlinked.

call

Trace certain function calls. Specify which function calls to trace by calling erlang:trace_pattern/3.

Message tags: call, return_from.

silent

Used in conjunction with the call trace flag. The call, return_from and return_to trace messages are inhibited if this flag is set, but if there are match specs they are executed as normal.

Silent mode is inhibited by executing erlang:trace(_, false, [silent|_]), or by a match spec executing the {silent, false} function.

The silent trace flag facilitates setting up a trace on many or even all processes in the system. Then the interesting trace can be activated and deactivated using the {silent,Bool} match spec function, giving a high degree of control of which functions with which arguments that triggers the trace.

Message tags: call, return_from, return_to. Or rather, the absence of.

return_to

Used in conjunction with the call trace flag. Trace the actual return from a traced function back to its caller. Only works for functions traced with the local option to erlang:trace_pattern/3.

The semantics is that a trace message is sent when a call traced function actually returns, that is, when a chain of tail recursive calls is ended. There will be only one trace message sent per chain of tail recursive calls, why the properties of tail recursiveness for function calls are kept while tracing with this flag. Using call and return_to trace together makes it possible to know exactly in which function a process executes at any time.

To get trace messages containing return values from functions, use the {return_trace} match_spec action instead.

Message tags: return_to.

running

Trace scheduling of processes.

Message tags: in, and out.

exiting

Trace scheduling of an exiting processes.

Message tags: in_exiting, out_exiting, and out_exited.

garbage_collection

Trace garbage collections of processes.

Message tags: gc_start, gc_end.

timestamp

Include a time stamp in all trace messages. The time stamp (Ts) is of the same form as returned by erlang:now().

cpu_timestamp

A global trace flag for the Erlang node that makes all trace timestamps be in CPU time, not wallclock. It is only allowed with PidSpec==all. If the host machine operating system does not support high resolution CPU time measurements, trace/3 exits with badarg.

arity

Used in conjunction with the call trace flag. {M, F, Arity} will be specified instead of {M, F, Args} in call trace messages.

set_on_spawn

Makes any process created by a traced process inherit its trace flags, including the set_on_spawn flag.

set_on_first_spawn

Makes the first process created by a traced process inherit its trace flags, excluding the set_on_first_spawn flag.

set_on_link

Makes any process linked by a traced process inherit its trace flags, including the set_on_link flag.

set_on_first_link

Makes the first process linked to by a traced process inherit its trace flags, excluding the set_on_first_link flag.

{tracer, Tracer}

Specify where to send the trace messages. Tracer must be the pid of a local process or the port identifier of a local port. If this flag is not given, trace messages will be sent to the process that called erlang:trace/3.

The effect of combining set_on_first_link with set_on_link is the same as having set_on_first_link alone. Likewise for set_on_spawn and set_on_first_spawn.

If the timestamp flag is not given, the tracing process will receive the trace messages described below. Pid is the pid of the traced process in which the traced event has occurred. The third element of the tuple is the message tag.

If the timestamp flag is given, the first element of the tuple will be trace_ts instead and the timestamp is added last in the tuple.

{trace, Pid, 'receive', Msg}

When Pid receives the message Msg.

{trace, Pid, send, Msg, To}

When Pid sends the message Msg to the process To.

{trace, Pid, send_to_non_existing_process, Msg, To}

When Pid sends the message Msg to the non-existing process To.

{trace, Pid, call, {M, F, Args}}

When Pid calls a traced function. The return values of calls are never supplied, only the call and its arguments.

Note that the trace flag arity can be used to change the contents of this message, so that Arity is specified instead of Args.

{trace, Pid, return_to, {M, F, Arity}}

When Pid returns to the specified function. This trace message is sent if both the call and the return_to flags are set, and the function is set to be traced on local function calls. The message is only sent when returning from a chain of tail recursive function calls where at least one call generated a call trace message (that is, the functions match specification matched and {message, false} was not an action).

{trace, Pid, return_from, {M, F, Arity}, ReturnValue}

When Pid returns from the specified function. This trace message is sent if the call flag is set, and the function has a match specification with a return_trace or exception_trace action.

{trace, Pid, exception_from, {M, F, Arity}, {Class, Value}}

When Pid exits from the specified function due to an exception. This trace message is sent if the call flag is set, and the function has a match specification with an exception_trace action.

{trace, Pid, spawn, Pid2, {M, F, Args}}

When Pid spawns a new process Pid2 with the specified function call as entry point.

Note that Args is supposed to be the argument list, but may be any term in the case of an erroneous spawn.

{trace, Pid, exit, Reason}

When Pid exits with reason Reason.

{trace, Pid, link, Pid2}

When Pid links to a process Pid2.

{trace, Pid, unlink, Pid2}

When Pid removes the link from a process Pid2.

{trace, Pid, getting_linked, Pid2}

When Pid gets linked to a process Pid2.

{trace, Pid, getting_unlinked, Pid2}

When Pid gets unlinked from a process Pid2.

{trace, Pid, register, RegName}

When Pid gets the name RegName registered.

{trace, Pid, unregister, RegName}

When Pid gets the name RegName unregistered. Note that this is done automatically when a registered process exits.

{trace, Pid, in, {M, F, Arity} | 0}

When Pid is scheduled to run. The process will run in function {M, F, Arity}. On some rare occasions the current function cannot be determined, then the last element Arity is 0.

{trace, Pid, out, {M, F, Arity} | 0}

When Pid is scheduled out. The process was running in function {M, F, Arity}. On some rare occasions the current function cannot be determined, then the last element Arity is 0.

{trace, Pid, gc_start, Info}

Sent when garbage collection is about to be started. Info is a list of two-element tuples, where the first element is a key, and the second is the value. You should not depend on the tuples have any defined order. Currently, the following keys are defined:

heap_size
The size of the used part of the heap.
heap_block_size
The size of the memory block used for storing the heap and the stack.
old_heap_size
The size of the used part of the old heap.
old_heap_block_size
The size of the memory block used for storing the old heap.
stack_size
The actual size of the stack.
recent_size
The size of the data that survived the previous garbage collection.
mbuf_size
The combined size of message buffers associated with the process.
bin_vheap_size
The total size of unique off-heap binaries referenced from the process heap.
bin_vheap_block_size
The total size of binaries, in words, allowed in the virtual heap in the process before doing a garbage collection.
bin_old_vheap_size
The total size of unique off-heap binaries referenced from the process old heap.
bin_vheap_block_size
The total size of binaries, in words, allowed in the virtual old heap in the process before doing a garbage collection.

All sizes are in words.

{trace, Pid, gc_end, Info}

Sent when garbage collection is finished. Info contains the same kind of list as in the gc_start message, but the sizes reflect the new sizes after garbage collection.

If the tracing process dies, the flags will be silently removed.

Only one process can trace a particular process. For this reason, attempts to trace an already traced process will fail.

Returns: A number indicating the number of processes that matched PidSpec. If PidSpec is a pid, the return value will be 1. If PidSpec is all or existing the return value will be the number of processes running, excluding tracer processes. If PidSpec is new, the return value will be 0.

Failure: If specified arguments are not supported. For example cpu_timestamp is not supported on all platforms.

trace_delivered/1

The delivery of trace messages is dislocated on the time-line compared to other events in the system. If you know that the Tracee has passed some specific point in its execution, and you want to know when at least all trace messages corresponding to events up to this point have reached the tracer you can use erlang:trace_delivered(Tracee). A {trace_delivered, Tracee, Ref} message is sent to the caller of erlang:trace_delivered(Tracee) when it is guaranteed that all trace messages have been delivered to the tracer up to the point that the Tracee had reached at the time of the call to erlang:trace_delivered(Tracee).

Note that the trace_delivered message does not imply that trace messages have been delivered; instead, it implies that all trace messages that should be delivered have been delivered. It is not an error if Tracee isn't, and hasn't been traced by someone, but if this is the case, no trace messages will have been delivered when the trace_delivered message arrives.

Note that Tracee has to refer to a process currently, or previously existing on the same node as the caller of erlang:trace_delivered(Tracee) resides on. The special Tracee atom all denotes all processes that currently are traced in the node.

An example: Process A is Tracee, port B is tracer, and process C is the port owner of B. C wants to close B when A exits. C can ensure that the trace isn't truncated by calling erlang:trace_delivered(A) when A exits and wait for the {trace_delivered, A, Ref} message before closing B.

Failure: badarg if Tracee does not refer to a process (dead or alive) on the same node as the caller of erlang:trace_delivered(Tracee) resides on.

trace_info/2

Returns trace information about a process or function.

To get information about a process, PidOrFunc should be a pid or the atom new. The atom new means that the default trace state for processes to be created will be returned. Item must have one of the following values:

flags

Return a list of atoms indicating what kind of traces is enabled for the process. The list will be empty if no traces are enabled, and one or more of the followings atoms if traces are enabled: send, 'receive', set_on_spawn, call, return_to, procs, set_on_first_spawn, set_on_link, running, garbage_collection, timestamp, and arity. The order is arbitrary.

tracer

Return the identifier for process or port tracing this process. If this process is not being traced, the return value will be [].

To get information about a function, PidOrFunc should be a three-element tuple: {Module, Function, Arity} or the atom on_load. No wildcards are allowed. Returns undefined if the function does not exist or false if the function is not traced at all. Item must have one of the following values:

traced

Return global if this function is traced on global function calls, local if this function is traced on local function calls (i.e local and global function calls), and false if neither local nor global function calls are traced.

match_spec

Return the match specification for this function, if it has one. If the function is locally or globally traced but has no match specification defined, the returned value is [].

meta

Return the meta trace tracer process or port for this function, if it has one. If the function is not meta traced the returned value is false, and if the function is meta traced but has once detected that the tracer proc is invalid, the returned value is [].

meta_match_spec

Return the meta trace match specification for this function, if it has one. If the function is meta traced but has no match specification defined, the returned value is [].

call_count

Return the call count value for this function or true for the pseudo function on_load if call count tracing is active. Return false otherwise. See also erlang:trace_pattern/3.

call_time

Return the call time values for this function or true for the pseudo function on_load if call time tracing is active. Returns false otherwise. The call time values returned, [{Pid, Count, S, Us}], is a list of each process that has executed the function and its specific counters. See also erlang:trace_pattern/3.

all

Return a list containing the {Item, Value} tuples for all other items, or return false if no tracing is active for this function.

The actual return value will be {Item, Value}, where Value is the requested information as described above. If a pid for a dead process was given, or the name of a non-existing function, Value will be undefined.

If PidOrFunc is the on_load, the information returned refers to the default value for code that will be loaded.

trace_pattern/2

The same as erlang:trace_pattern(MFA, MatchSpec, []), retained for backward compatibility.

trace_pattern/3

This BIF is used to enable or disable call tracing for exported functions. It must be combined with erlang:trace/3 to set the call trace flag for one or more processes.

Conceptually, call tracing works like this: Inside the Erlang virtual machine there is a set of processes to be traced and a set of functions to be traced. Tracing will be enabled on the intersection of the set. That is, if a process included in the traced process set calls a function included in the traced function set, the trace action will be taken. Otherwise, nothing will happen.

Use erlang:trace/3 to add or remove one or more processes to the set of traced processes. Use erlang:trace_pattern/2 to add or remove exported functions to the set of traced functions.

The erlang:trace_pattern/3 BIF can also add match specifications to an exported function. A match specification comprises a pattern that the arguments to the function must match, a guard expression which must evaluate to true and an action to be performed. The default action is to send a trace message. If the pattern does not match or the guard fails, the action will not be executed.

The MFA argument should be a tuple like {Module, Function, Arity} or the atom on_load (described below). It can be the module, function, and arity for an exported function (or a BIF in any module). The '_' atom can be used to mean any of that kind. Wildcards can be used in any of the following ways:

{Module,Function,'_'}

All exported functions of any arity named Function in module Module.

{Module,'_','_'}

All exported functions in module Module.

{'_','_','_'}

All exported functions in all loaded modules.

Other combinations, such as {Module,'_',Arity}, are not allowed. Local functions will match wildcards only if the local option is in the FlagList.

If the MFA argument is the atom on_load, the match specification and flag list will be used on all modules that are newly loaded.

The MatchSpec argument can take any of the following forms:

false

Disable tracing for the matching function(s). Any match specification will be removed.

true

Enable tracing for the matching function(s).

MatchSpecList

A list of match specifications. An empty list is equivalent to true. See the ERTS User's Guide for a description of match specifications.

restart

For the FlagList option call_count and call_time: restart the existing counters. The behaviour is undefined for other FlagList options.

pause

For the FlagList option call_count and call_time: pause the existing counters. The behaviour is undefined for other FlagList options.

The FlagList parameter is a list of options. The following options are allowed:

global

Turn on or off call tracing for global function calls (that is, calls specifying the module explicitly). Only exported functions will match and only global calls will generate trace messages. This is the default.

local

Turn on or off call tracing for all types of function calls. Trace messages will be sent whenever any of the specified functions are called, regardless of how they are called. If the return_to flag is set for the process, a return_to message will also be sent when this function returns to its caller.

meta | {meta, Pid}

Turn on or off meta tracing for all types of function calls. Trace messages will be sent to the tracer process or port Pid whenever any of the specified functions are called, regardless of how they are called. If no Pid is specified, self() is used as a default tracer process.

Meta tracing traces all processes and does not care about the process trace flags set by trace/3, the trace flags are instead fixed to [call, timestamp].

The match spec function {return_trace} works with meta trace and send its trace message to the same tracer process.

call_count

Starts (MatchSpec == true) or stops (MatchSpec == false) call count tracing for all types of function calls. For every function a counter is incremented when the function is called, in any process. No process trace flags need to be activated.

If call count tracing is started while already running, the count is restarted from zero. Running counters can be paused with MatchSpec == pause. Paused and running counters can be restarted from zero with MatchSpec == restart.

The counter value can be read with erlang:trace_info/2.

call_time

Starts (MatchSpec == true) or stops (MatchSpec == false) call time tracing for all types of function calls. For every function a counter is incremented when the function is called. Time spent in the function is accumulated in two other counters, seconds and micro-seconds. The counters are stored for each call traced process.

If call time tracing is started while already running, the count and time is restarted from zero. Running counters can be paused with MatchSpec == pause. Paused and running counters can be restarted from zero with MatchSpec == restart.

The counter value can be read with erlang:trace_info/2.

The global and local options are mutually exclusive and global is the default (if no options are specified). The call_count and meta options perform a kind of local tracing, and can also not be combined with global. A function can be either globally or locally traced. If global tracing is specified for a specified set of functions; local, meta, call time and call count tracing for the matching set of local functions will be disabled, and vice versa.

When disabling trace, the option must match the type of trace that is set on the function, so that local tracing must be disabled with the local option and global tracing with the global option (or no option at all), and so forth.

There is no way to directly change part of a match specification list. If a function has a match specification, you can replace it with a completely new one. If you need to change an existing match specification, use the erlang:trace_info/2 BIF to retrieve the existing match specification.

Returns the number of exported functions that matched the MFA argument. This will be zero if none matched at all.

trunc/1

Returns an integer by the truncating Number.

> trunc(5.5).
5

Allowed in guard tests.

tuple_size/1

Returns an integer which is the number of elements in Tuple.

> tuple_size({morni, mulle, bwange}).
3

Allowed in guard tests.

tuple_to_list/1

Returns a list which corresponds to Tuple. Tuple may contain any Erlang terms.

> tuple_to_list({share, {'Ericsson_B', 163}}).
[share,{'Ericsson_B',163}]

universaltime/0

Returns the current date and time according to Universal Time Coordinated (UTC), also called GMT, in the form {{Year, Month, Day}, {Hour, Minute, Second}} if supported by the underlying operating system. If not, erlang:universaltime() is equivalent to erlang:localtime().

> erlang:universaltime().
{{1996,11,6},{14,18,43}}

universaltime_to_localtime/1

Converts Universal Time Coordinated (UTC) date and time to local date and time, if this is supported by the underlying OS. Otherwise, no conversion is done, and Universaltime is returned.

> erlang:universaltime_to_localtime({{1996,11,6},{14,18,43}}).
{{1996,11,7},{15,18,43}}

Failure: badarg if Universaltime does not denote a valid date and time.

unlink/1

Removes the link, if there is one, between the calling process and the process or port referred to by Id.

Returns true and does not fail, even if there is no link to Id, or if Id does not exist.

Once unlink(Id) has returned it is guaranteed that the link between the caller and the entity referred to by Id has no effect on the caller in the future (unless the link is setup again). If caller is trapping exits, an {'EXIT', Id, _} message due to the link might have been placed in the caller's message queue prior to the call, though. Note, the {'EXIT', Id, _} message can be the result of the link, but can also be the result of Id calling exit/2. Therefore, it may be appropriate to cleanup the message queue when trapping exits after the call to unlink(Id), as follow:


    unlink(Id),
    receive
        {'EXIT', Id, _} ->
            true
    after 0 ->
            true
    end

Note!

Prior to OTP release R11B (erts version 5.5) unlink/1 behaved completely asynchronous, i.e., the link was active until the "unlink signal" reached the linked entity. This had one undesirable effect, though. You could never know when you were guaranteed not to be effected by the link.

Current behavior can be viewed as two combined operations: asynchronously send an "unlink signal" to the linked entity and ignore any future results of the link.

unregister/1

Removes the registered name RegName, associated with a pid or a port identifier.

> unregister(db).
true

Users are advised not to unregister system processes.

Failure: badarg if RegName is not a registered name.

whereis/1

Returns the pid or port identifier with the registered name RegName. Returns undefined if the name is not registered.

> whereis(db).
<0.43.0>

yield/0

Voluntarily let other processes (if any) get a chance to execute. Using erlang:yield() is similar to receive after 1 -> ok end, except that yield() is faster.

Warning!

There is seldom or never any need to use this BIF, especially in the SMP-emulator as other processes will have a chance to run in another scheduler thread anyway. Using this BIF without a thorough grasp of how the scheduler works may cause performance degradation.