ei
(erl_interface)Note!
The support for VxWorks is deprecated as of OTP 22, and will be removed in OTP 23.
The library ei
contains macros and functions to encode
and decode the Erlang binary term format.
ei
allows you to convert atoms, lists, numbers, and
binaries to and from the binary format. This is useful when
writing port programs and drivers. ei
uses a given
buffer, no dynamic memory (except
ei_decode_fun()
) and is often quite fast.
ei
also handles C-nodes, C-programs that talks Erlang
distribution with Erlang nodes (or other C-nodes) using the
Erlang distribution format. The difference between ei
and erl_interface
is that ei
uses
the binary format directly when sending and receiving terms. It is also
thread safe, and using threads, one process can handle multiple
C-nodes. The erl_interface
library is built on top of
ei
, but of legacy reasons, it does not allow for
multiple C-nodes. In general, ei
is the preferred way
of doing C-nodes.
The decode and encode functions use a buffer and an index into the buffer, which points at the point where to encode and decode. The index is updated to point right after the term encoded/decoded. No checking is done whether the term fits in the buffer or not. If encoding goes outside the buffer, the program can crash.
All functions take two parameters:
buf
is a pointer to the buffer where the binary data is or will be.index
is a pointer to an index into the buffer. This parameter is incremented with the size of the term decoded/encoded.
The data is thus at buf[*index]
when an
ei
function is called.
All encode functions assume that the buf
and
index
parameters point to a buffer large enough for
the data. To get the size of an encoded term, without encoding it,
pass NULL
instead of a buffer pointer. Parameter
index
is incremented, but nothing will be encoded. This
is the way in ei
to "preflight" term encoding.
There are also encode functions that use a dynamic buffer. It
is often more convenient to use these to encode data. All encode
functions comes in two versions; those starting with
ei_x
use a dynamic buffer.
All functions return 0
if successful, otherwise
-1
(for example, if a term is not of the expected
type, or the data to decode is an invalid Erlang term).
Some of the decode functions need a pre-allocated buffer. This
buffer must be allocated large enough, and for non-compound types
the ei_get_type()
function returns the size required (notice that for strings an
extra byte is needed for the NULL
-terminator).
Data Types
typedef enum { ERLANG_ASCII = 1, ERLANG_LATIN1 = 2, ERLANG_UTF8 = 4 } erlang_char_encoding;
The character encodings used for atoms. ERLANG_ASCII
represents 7-bit ASCII. Latin-1 and UTF-8 are different extensions
of 7-bit ASCII. All 7-bit ASCII characters are valid Latin-1 and
UTF-8 characters. ASCII and Latin-1 both represent each character
by one byte. An UTF-8 character can consist of 1-4 bytes.
Notice that these constants are bit-flags and can be combined with
bitwise OR.
Functions
Decodes an atom from the binary format. The NULL
-terminated
name of the atom is placed at p
. At most
MAXATOMLEN
bytes can be placed in the buffer.
Decodes an atom from the binary format. The NULL
-terminated
name of the atom is placed in buffer at p
of length plen
bytes.
The wanted string encoding is specified by
want
.
The original encoding used in the binary format (Latin-1 or UTF-8) can
be obtained from *was
. The encoding of the resulting string
(7-bit ASCII, Latin-1, or UTF-8) can be obtained from *result
.
Both was
and result
can be NULL
. *result
can differ from want
if want
is a bitwise OR'd
combination like ERLANG_LATIN1|ERLANG_UTF8
or if
*result
turns out to be pure 7-bit ASCII
(compatible with both Latin-1 and UTF-8).
This function fails if the atom is too long for the buffer
or if it cannot be represented with encoding want
.
This function was introduced in Erlang/OTP R16 as part of a first step to support UTF-8 atoms.
Decodes an integer in the binary format to a GMP
mpz_t
integer. To use this function, the ei
library must be configured and compiled to use the GMP library.
Decodes a binary from the binary format. Parameter
len
is set to the actual size of the
binary. Notice that ei_decode_binary()
assumes that
there is enough room for the binary. The size required can be
fetched by ei_get_type()
.
Decodes a bit string from the binary format.
pp
Either NULL
or *pp
returns a pointer to
the first byte of the bit string. The returned bit string is
readable as long as the buffer pointed to by buf
is
readable and not written to.
bitoffsp
Either NULL
or *bitoffsp
returns the
number of unused bits in the first byte pointed to by
*pp
. The value of *bitoffsp
is between 0 and 7.
Unused bits in the first byte are the most significant bits.
nbitsp
Either NULL
or *nbitsp
returns the length
of the bit string in bits.
Returns 0
if it was a bit string term.
The number of bytes pointed to by *pp
, which are
part of the bit string, is (*bitoffsp + *nbitsp + 7)/8
. If
(*bitoffsp + *bitsp)%8 > 0
then only (*bitoffsp +
*bitsp)%8
bits of the last byte are used. Unused bits in
the last byte are the least significant bits.
The values of unused bits in the first and last byte are undefined and cannot be relied on.
Number of bits may be divisible by 8, which means a binary
decodable by ei_decode_binary
is also decodable by
ei_decode_bitstring
.
Decodes a boolean value from the binary format.
A boolean is actually an atom, true
decodes 1
and false
decodes 0.
Decodes a char (8-bit) integer between 0-255 from the binary format.
For historical reasons the returned integer is of
type char
. Your C code is to consider the
returned value to be of type unsigned char
even if
the C compilers and system can define char
to be
signed.
Decodes a double-precision (64-bit) floating point number from the binary format.
Decodes any term, or at least tries to. If the term
pointed at by *index
in buf
fits
in the term
union, it is decoded, and the
appropriate field in term->value
is set, and
*index
is incremented by the term size.
The function returns 1
on successful decoding, -1
on
error, and 0
if the term seems alright, but does not fit in the
term
structure. If 1
is returned, the
index
is incremented, and term
contains the decoded term.
The term
structure contains the arity for a tuple
or list, size for a binary, string, or atom. It contains
a term if it is any of the following: integer, float, atom,
pid, port, or ref.
Decodes a fun from the binary format. Parameter
p
is to be NULL
or point to an
erlang_fun
structure. This is the only decode
function that allocates memory. When the erlang_fun
is no longer needed, it is to be freed with
free_fun
. (This has to do with the arbitrary size
of the environment for a fun.)
Decodes a list header from the binary
format. The number of elements is returned in
arity
. The arity+1
elements
follow (the last one is the tail of the list, normally an empty list).
If arity
is 0
, it is an empty
list.
Notice that lists are encoded as strings if they consist
entirely of integers in the range 0..255. This function do
not decode such strings, use ei_decode_string()
instead.
Decodes a long integer from the binary format.
If the code is 64 bits, the function ei_decode_long()
is
the same as ei_decode_longlong()
.
Decodes a GCC long long
or Visual C++
__int64
(64-bit) integer from the binary format. This
function is missing in the VxWorks port.
Decodes a map header from the binary
format. The number of key-value pairs is returned in
*arity
. Keys and values follow in this order:
K1, V1, K2, V2, ..., Kn, Vn
. This makes a total of
arity*2
terms. If arity
is zero, it is an empty map.
A correctly encoded map does not have duplicate keys.
Decodes a process identifier (pid) from the binary format.
Decodes a port identifier from the binary format.
Decodes a reference from the binary format.
Decodes a string from the binary format. A
string in Erlang is a list of integers between 0 and
255. Notice that as the string is just a list, sometimes
lists are encoded as strings by term_to_binary/1
,
even if it was not intended.
The string is copied to p
, and enough space must
be allocated. The returned string is NULL
-terminated, so you
must add an extra byte to the memory requirement.
Decodes a term from the binary format. The term
is return in t
as a ETERM*
, so
t
is actually an ETERM**
(see
erl_eterm
).
The term is later to be deallocated.
Note!
This function is deprecated as of OTP 22 and will be removed in
OTP 23 together with the old legacy erl_interface
library (functions
with prefix erl_
).
Decodes an Erlang trace token from the binary format.
Decodes a tuple header, the number of elements
is returned in arity
. The tuple elements follow
in order in the buffer.
Decodes an unsigned long integer from the binary format.
If the code is 64 bits, the function ei_decode_ulong()
is
the same as ei_decode_ulonglong()
.
Decodes a GCC unsigned long long
or Visual C++
unsigned __int64
(64-bit) integer from the binary
format. This function is missing in the VxWorks port.
Decodes the version magic number for the Erlang binary term format. It must be the first token in a binary term.
Encodes an atom in the binary format. Parameter p
is the name of the atom in Latin-1 encoding. Only up to
MAXATOMLEN-1
bytes
are encoded. The name is to be NULL
-terminated, except for
the ei_x_encode_atom_len()
function.
Encodes an atom in the binary format. Parameter p
is the name of the atom with
character encoding
from_enc
(ASCII, Latin-1, or UTF-8). The name must either be NULL
-terminated or
a function variant with a len
parameter must be used.
The encoding fails if p
is not a valid string in encoding
from_enc
.
Argument to_enc
is ignored. As from Erlang/OTP 20 the encoding is always
done in UTF-8 which is readable by nodes as old as Erlang/OTP R16.
Encodes a GMP mpz_t
integer to binary format.
To use this function, the ei
library must be configured and
compiled to use the GMP library.
Encodes a binary in the binary format. The data is at
p
, of len
bytes length.
Encodes a bit string in the binary format.
The data is at p
. The length of the bit string is nbits
bits. The first bitoffs
bits of the data at p
are unused.
The first byte which is part of the bit string is
p[bitoffs/8]
. The bitoffs%8
most significant bits of
the first byte p[bitoffs/8]
are unused.
The number of bytes which is part of the bit string is (bitoffs +
nbits + 7)/8
. If (bitoffs + nbits)%8 > 0
then only (bitoffs +
nbits)%8
bits of the last byte are used. Unused bits in
the last byte are the least significant bits.
The values of unused bits are disregarded and does not need to be cleared.
Encodes a boolean value as the atom true
if
p
is not zero, or false
if p
is
zero.
Encodes a char (8-bit) as an integer between 0-255 in the binary
format. For historical reasons the integer argument is of
type char
. Your C code is to consider the specified
argument to be of type unsigned char
even if
the C compilers and system may define char
to be
signed.
Encodes a double-precision (64-bit) floating point number in the binary format.
Returns -1
if the floating point
number is not finite.
Encodes an empty list. It is often used at the tail of a list.
Encodes a fun in the binary format. Parameter p
points to an erlang_fun
structure. The
erlang_fun
is not freed automatically, the
free_fun
is to be called if the fun is not needed
after encoding.
Encodes a list header, with a specified
arity. The next arity+1
terms are the elements
(actually its arity
cons cells) and the tail of the
list. Lists and tuples are encoded recursively, so that a
list can contain another list or tuple.
For example, to encode the list
[c, d, [e | f]]
:
ei_encode_list_header(buf, &i, 3); ei_encode_atom(buf, &i, "c"); ei_encode_atom(buf, &i, "d"); ei_encode_list_header(buf, &i, 1); ei_encode_atom(buf, &i, "e"); ei_encode_atom(buf, &i, "f"); ei_encode_empty_list(buf, &i);
Note!
It may seem that there is no way to create a list without
knowing the number of elements in advance. But indeed
there is a way. Notice that the list [a, b, c]
can be written as [a | [b | [c]]]
.
Using this, a list can be written as conses.
To encode a list, without knowing the arity in advance:
while (something()) { ei_x_encode_list_header(&x, 1); ei_x_encode_ulong(&x, i); /* just an example */ } ei_x_encode_empty_list(&x);
Encodes a long integer in the binary format.
If the code is 64 bits, the function ei_encode_long()
is
the same as ei_encode_longlong()
.
Encodes a GCC long long
or Visual C++
__int64
(64-bit) integer in the binary format.
This function is missing in the VxWorks port.
Encodes a map header, with a specified arity. The next
arity*2
terms encoded will be the keys and values of the map
encoded in the following order: K1, V1, K2, V2, ..., Kn, Vn
.
For example, to encode the map #{a => "Apple", b =>
"Banana"}
:
ei_x_encode_map_header(&x, 2); ei_x_encode_atom(&x, "a"); ei_x_encode_string(&x, "Apple"); ei_x_encode_atom(&x, "b"); ei_x_encode_string(&x, "Banana");
A correctly encoded map cannot have duplicate keys.
Encodes an Erlang process identifier (pid) in the binary
format. Parameter p
points to an
erlang_pid
structure (which should have been
obtained earlier with ei_decode_pid()
).
Encodes an Erlang port in the binary format. Parameter
p
points to a erlang_port
structure (which should have been obtained earlier with
ei_decode_port()
).
Encodes an Erlang reference in the binary format. Parameter
p
points to a erlang_ref
structure (which should have been obtained earlier with
ei_decode_ref()
).
Encodes a string in the binary format. (A string in Erlang
is a list, but is encoded as a character array in the binary
format.) The string is to be NULL
-terminated, except for
the ei_x_encode_string_len()
function.
Encodes an ETERM
, as obtained from
erl_interface
. Parameter t
is
actually an ETERM
pointer. This function
does not free the ETERM
.
Note!
These functions are deprecated as of OTP 22 and will be removed in
OTP 23 together with the old legacy erl_interface
library
(functions with prefix erl_
).
Encodes an Erlang trace token in the binary format.
Parameter p
points to a
erlang_trace
structure (which should have been
obtained earlier with ei_decode_trace()
).
Encodes a tuple header, with a specified
arity. The next arity
terms encoded will be the
elements of the tuple. Tuples and lists are encoded
recursively, so that a tuple can contain another tuple or list.
For example, to encode the tuple {a, {b, {}}}
:
ei_encode_tuple_header(buf, &i, 2); ei_encode_atom(buf, &i, "a"); ei_encode_tuple_header(buf, &i, 2); ei_encode_atom(buf, &i, "b"); ei_encode_tuple_header(buf, &i, 0);
Encodes an unsigned long integer in the binary format.
If the code is 64 bits, the function ei_encode_ulong()
is
the same as ei_encode_ulonglong()
.
Encodes a GCC unsigned long long
or Visual C++
unsigned __int64
(64-bit) integer in the binary
format. This function is missing in the VxWorks port.
Encodes a version magic number for the binary format. Must be the first token in a binary term.
Returns the type in *type
and size in
*size
of the encoded term. For strings and atoms,
size is the number of characters not including the
terminating NULL
. For binaries and bitstrings, *size
is
the number of bytes. For lists, tuples and maps, *size
is the
arity of the object. For other types, *size
is 0. In all
cases, index
is left unchanged.
Initialize the ei
library. This function should be called once
(and only once) before calling any other functionality in the ei
library. However, note the exception below.
If the ei
library is used together with the erl_interface
library, this function should not be called directly. It will be
called by the erl_init()
function which should be used to initialize
the combination of the two libraries instead.
On success zero is returned. On failure a posix error code is returned.
Prints a term, in clear text, to the file
specified by fp
, or the buffer pointed to by
s
. It
tries to resemble the term printing in the Erlang shell.
In ei_s_print_term()
, parameter
s
is to
point to a dynamically (malloc) allocated string of
BUFSIZ
bytes or a NULL
pointer. The string
can be reallocated (and *s
can be updated) by this
function if the result is more than BUFSIZ
characters. The string returned is NULL
-terminated.
The return value is the number of characters written to the file
or string, or -1
if buf[index]
does not
contain a valid term.
Unfortunately, I/O errors on fp
is not checked.
Argument index
is updated, that is, this function
can be viewed as a decode function that decodes a term into a
human-readable format.
unsigned release_number;
In general, the ei
library is guaranteed
to be compatible with other Erlang/OTP components that are 2 major
releases older or newer than the ei
library itself.
Sometimes an exception to the above rule has to be made to make new
features (or even bug fixes) possible. A call to
ei_set_compat_rel(release_number)
sets
the ei
library in compatibility mode of OTP release
release_number
.
The only useful value for release_number
is currently
21
. This will only be useful and have an effect if bit
strings or export funs are received from a connected
node. Before OTP 22, bit strings and export funs were not supported by
ei
. They were instead encoded using an undocumented fallback
tuple format when sent from the emulator to ei
:
Bit string
The term <<42, 1:1>>
was encoded as
{<<42, 128>>, 1}
. The first element of the tuple is a
binary and the second element denotes how many bits of the last bytes
are part of the bit string. In this example only the most significant
bit of the last byte (128) is part of the bit string.
Export fun
The term fun lists:map/2
was encoded as
{lists,map}
. A tuple with the module, function and a missing
arity.
If ei_set_compat_rel(21)
is not called then a connected
emulator will send bit strings and export funs correctly encoded. The
functions ei_decode_bitstring
and ei_decode_fun
has to be used to decode such terms. Calling
ei_set_compat_rel(21)
should only be done as a workaround to
keep an old implementation alive, which expects to receive the
undocumented tuple formats for bit strings and/or export funs.
Note!
If this function is called, it can only be called once
and must be called before any other functions in the
ei
library are called.
Skips a term in the specified buffer; recursively skips elements of lists and tuples, so that a full term is skipped. This is a way to get the size of an Erlang term.
buf
is the buffer.
index
is updated to point right after the term
in the buffer.
Note!
This can be useful when you want to hold arbitrary terms: skip them and copy the binary term data to some buffer.
Returns 0
on success, otherwise
-1
.
Appends data at the end of buffer x
.
Formats a term, given as a string, to a buffer.
Works like a sprintf for Erlang terms.
fmt
contains a format string, with arguments like
~d
, to insert terms from variables. The following
formats are supported (with the C types given):
~a An atom, char* ~c A character, char ~s A string, char* ~i An integer, int ~l A long integer, long int ~u A unsigned long integer, unsigned long int ~f A float, float ~d A double float, double float ~p An Erlang pid, erlang_pid*
For example, to encode a tuple with some stuff:
ei_x_format("{~a,~i,~d}", "numbers", 12, 3.14159) encodes the tuple {numbers,12,3.14159}
ei_x_format_wo_ver()
formats into a buffer,
without the initial version byte.
Frees an ei_x_buff
buffer.
The memory used by the buffer is returned to the OS.
Allocates a new ei_x_buff
buffer. The
fields of the structure pointed to by parameter x
is filled in, and a default buffer is allocated.
ei_x_new_with_version()
also puts an initial
version byte, which is used in the binary format (so that
ei_x_encode_version()
will not be needed.)
Debug Information
Some tips on what to check when the emulator does not seem to receive the terms that you send:
- Be careful with the version header, use
ei_x_new_with_version()
when appropriate. - Turn on distribution tracing on the Erlang node.
- Check the result codes from
ei_decode_-calls
.