io
Standard IO Server Interface Functions
This module provides an interface to standard Erlang IO servers.
The output functions all return ok
if they are successful,
or exit if they are not.
In the following description, all functions have an optional
parameter IoDevice
. If included, it must be the pid of a
process which handles the IO protocols. Normally, it is the
IoDevice
returned by
file:open/2.
For a description of the IO protocols refer to Armstrong, Virding and Williams, 'Concurrent Programming in Erlang', Chapter 13, unfortunately now very outdated, but the general principles still apply.
DATA TYPES
io_device() as returned by file:open/2, a process handling IO protocols
Functions
columns([IoDevice]) -> {ok,int()} | {error, enotsup}
IoDevice = io_device()
Retrieves the number of columns of the
IoDevice
(i.e. the width of a terminal). The function
only succeeds for terminal devices, for all other devices
the function returns {error, enotsup}
put_chars([IoDevice,] IoData) -> ok
IoDevice = io_device()
IoData = iodata() -- see erlang(3)
Writes the characters of IoData
to the standard output
(IoDevice
).
nl([IoDevice]) -> ok
IoDevice = io_device()
Writes new line to the standard output (IoDevice
).
get_chars([IoDevice,] Prompt, Count) -> string() | eof
IoDevice = io_device()
Prompt = atom() | string()
Count = int()
Reads Count
characters from standard input
(IoDevice
), prompting it with Prompt
. It
returns:
String
The input characters.
eof
End of file was encountered.
get_line([IoDevice,] Prompt) -> string() | eof | {error,Reason}
IoDevice = io_device()
Prompt = atom() | string()
Reads a line from the standard input (IoDevice
),
prompting it with Prompt
. It returns:
String
The characters in the line terminated by a LF (or end of file).
eof
End of file was encountered.
{error,Reason}
Other (rare) error condition, for instance {error,estale}
if reading from an NFS file system.
setopts([IoDevice,] Opts) -> ok | {error, Reason}
IoDevice = io_device()
Opts = [Opt]
Opt = binary | list
Reason = term()
Set options for standard input/output (IoDevice
).
Possible options are:
binary
Makes get_chars/2,3
and get_line/1,2
return
binaries instead of lists of chars.
list
Makes get_chars/2,3
and get_line/1,2
return
lists of chars, which is the default.
expand_fun
Provide a function for tab-completion (expansion)
like the erlang shell. This function is called
when the user presses the Tab key. The expansion is
active when calling line-reading functions such as
get_line/1,2
.
The function is called with the current line, upto
the cursor, as a reversed string. It should return a
three-tuple: {yes|no, string(), [string(), ...]}
. The
first element gives a beep if no
, otherwise the
expansion is silent, the second is a string that will be
entered at the cursor position, and the third is a list of
possible expansions. If this list is non-empty, the list
will be printed and the current input line will be written
once again.
Trivial example (beep on anything except empty line, which is expanded to "quit"):
fun("") -> {yes, "quit", []}; (_) -> {no, "", ["quit"]} end
Note!
The binary
option does not work against IO servers
on remote nodes running an older version of Erlang/OTP than
R9C.
write([IoDevice,] Term) -> ok
IoDevice = io_device()
Term = term()
Writes the term Term
to the standard output
(IoDevice
).
read([IoDevice,] Prompt) -> Result
IoDevice = io_device()
Prompt = atom() | string()
Result = {ok, Term} | eof | {error, ErrorInfo}
Term = term()
ErrorInfo -- see section Error Information below
Reads a term Term
from the standard input
(IoDevice
), prompting it with Prompt
. It
returns:
{ok, Term}
The parsing was successful.
eof
End of file was encountered.
{error, ErrorInfo}
The parsing failed.
read(IoDevice, Prompt, StartLine) -> Result
IoDevice = io_device()
Prompt = atom() | string()
StartLine = int()
Result = {ok, Term, EndLine} | {eof, EndLine} | {error, ErrorInfo, EndLine}
Term = term()
EndLine = int()
ErrorInfo -- see section Error Information below
Reads a term Term
from IoDevice
, prompting it
with Prompt
. Reading starts at line number
StartLine
. It returns:
{ok, Term, EndLine}
The parsing was successful.
{eof, EndLine}
End of file was encountered.
{error, ErrorInfo, EndLine}
The parsing failed.
fwrite(Format) ->
fwrite([IoDevice,] Format, Data) -> ok
format(Format) ->
format([IoDevice,] Format, Data) -> ok
IoDevice = io_device()
Format = atom() | string() | binary()
Data = [term()]
Writes the items in Data
([]
) on the standard
output (IoDevice
) in accordance with Format
.
Format
contains plain characters which are copied to
the output device, and control sequences for formatting, see
below. If Format
is an atom or a binary, it is first
converted to a list with the aid of atom_to_list/1
or binary_to_list/1
.
1> io:fwrite("Hello world!~n", []).
Hello world!
ok
The general format of a control sequence is ~F.P.PadC
.
The character C
determines the type of control sequence
to be used, F
and P
are optional numeric
arguments. If F
, P
, or Pad
is *
,
the next argument in Data
is used as the numeric value
of F
or P
.
F
is the field width
of the printed argument. A
negative value means that the argument will be left justified
within the field, otherwise it will be right justified. If no
field width is specified, the required print width will be
used. If the field width specified is too small, then the
whole field will be filled with *
characters.
P
is the precision
of the printed argument. A
default value is used if no precision is specified. The
interpretation of precision depends on the control sequences.
Unless otherwise specified, the argument within
is used
to determine print width.
Pad
is the padding character. This is the character
used to pad the printed representation of the argument so that
it conforms to the specified field width and precision. Only
one padding character can be specified and, whenever
applicable, it is used for both the field width and precision.
The default padding character is ' '
(space).
The following control sequences are available:
~
The character ~
is written.
c
The argument is a number that will be interpreted as an ASCII code. The precision is the number of times the character is printed and it defaults to the field width, which in turn defaults to 1. The following example illustrates:
2> io:fwrite("|~10.5c|~-10.5c|~5c|~n", [$a, $b, $c]).
| aaaaa|bbbbb |ccccc|
ok
f
The argument is a float which is written as
[-]ddd.ddd
, where the precision is the number of
digits after the decimal point. The default precision is 6
and it cannot be less than 1.
e
The argument is a float which is written as
[-]d.ddde+-ddd
, where the precision is the number
of digits written. The default precision is 6 and it
cannot be less than 2.
g
The argument is a float which is written as f
, if
it is >= 0.1 and < 10000.0. Otherwise, it is written
in the e
format. The precision is the number of
significant digits. It defaults to 6 and should not be
less than 2. If the absolute value of the float does not
allow it to be written in the f
format with the
desired number of significant digits, it is also written
in the e
format.
s
Prints the argument with the string
syntax. The
argument is an I/O list, a binary, or an atom. The characters
are printed without quotes. In this format, the printed
argument is truncated to the given precision and field
width.
This format can be used for printing any object and truncating the output so it fits a specified field:
3>io:fwrite("|~10w|~n", [{hey, hey, hey}]).
|**********| ok 4>io:fwrite("|~10s|~n", [io_lib:write({hey, hey, hey})]).
|{hey,hey,h| ok
w
Writes data with the standard syntax. This is used to output Erlang terms. Atoms are printed within quotes if they contain embedded non-printable characters, and floats are printed accurately as the shortest, correctly rounded string.
p
Writes the data with standard syntax in the same way as
~w
, but breaks terms whose printed representation
is longer than one line into many lines and indents each
line sensibly. It also tries to detect lists of printable
characters and to output these as strings. For example:
5>T = [{attributes,[[{id,age,1.50000},{mode,explicit},
{typename,"INTEGER"}], [{id,cho},{mode,explicit},{typename,'Cho'}]]},
{typename,'Person'},{tag,{'PRIVATE',3}},{mode,implicit}].
... 6>io:fwrite("~w~n", [T]).
[{attributes,[[{id,age,1.5},{mode,explicit},{typename, [73,78,84,69,71,69,82]}],[{id,cho},{mode,explicit},{typena me,'Cho'}]]},{typename,'Person'},{tag,{'PRIVATE',3}},{mode ,implicit}] ok 7>io:fwrite("~62p~n", [T]).
[{attributes,[[{id,age,1.5}, {mode,explicit}, {typename,"INTEGER"}], [{id,cho},{mode,explicit},{typename,'Cho'}]]}, {typename,'Person'}, {tag,{'PRIVATE',3}}, {mode,implicit}] ok
The field width specifies the maximum line length. It
defaults to 80. The precision specifies the initial
indentation of the term. It defaults to the number of
characters printed on this line in the same
call to
io:fwrite
or io:format
. For example, using
T
above:
8> io:fwrite("Here T = ~62p~n", [T]).
Here T = [{attributes,[[{id,age,1.5},
{mode,explicit},
{typename,"INTEGER"}],
[{id,cho},
{mode,explicit},
{typename,'Cho'}]]},
{typename,'Person'},
{tag,{'PRIVATE',3}},
{mode,implicit}]
ok
W
Writes data in the same way as ~w
, but takes an
extra argument which is the maximum depth to which terms
are printed. Anything below this depth is replaced with
...
. For example, using T
above:
9> io:fwrite("~W~n", [T,9]).
[{attributes,[[{id,age,1.5},{mode,explicit},{typename,...}],
[{id,cho},{mode,...},{...}]]},{typename,'Person'},
{tag,{'PRIVATE',3}},{mode,implicit}]
ok
If the maximum depth has been reached, then it is
impossible to read in the resultant output. Also, the
,...
form in a tuple denotes that there are more
elements in the tuple but these are below the print depth.
P
Writes data in the same way as ~p
, but takes an
extra argument which is the maximum depth to which terms
are printed. Anything below this depth is replaced with
...
. For example:
10> io:fwrite("~62P~n", [T,9]).
[{attributes,[[{id,age,1.5},{mode,explicit},{typename,...}],
[{id,cho},{mode,...},{...}]]},
{typename,'Person'},
{tag,{'PRIVATE',3}},
{mode,implicit}]
ok
B
Writes an integer in base 2..36, the default base is 10. A leading dash is printed for negative integers.
The precision field selects base. For example:
11>io:fwrite("~.16B~n", [31]).
1F ok 12>io:fwrite("~.2B~n", [-19]).
-10011 ok 13>io:fwrite("~.36B~n", [5*36+35]).
5Z ok
X
Like B
, but takes an extra argument that is a
prefix to insert before the number, but after the leading
dash, if any.
The prefix can be a possibly deep list of characters or an atom.
14>io:fwrite("~X~n", [31,"10#"]).
10#31 ok 15>io:fwrite("~.16X~n", [-31,"0x"]).
-0x1F ok
#
Like B
, but prints the number with an Erlang style
'#'-separated base prefix.
16>io:fwrite("~.10#~n", [31]).
10#31 ok 17>io:fwrite("~.16#~n", [-31]).
-16#1F ok
b
Like B
, but prints lowercase letters.
x
Like X
, but prints lowercase letters.
+
Like #
, but prints lowercase letters.
n
Writes a new line.
i
Ignores the next term.
Returns:
ok
The formatting succeeded.
If an error occurs, there is no output. For example:
18>io:fwrite("~s ~w ~i ~w ~c ~n",['abc def', 'abc def', {foo, 1},{foo, 1}, 65]).
abc def 'abc def' {foo,1} A ok 19>io:fwrite("~s", [65]).
** exception exit: {badarg,[{io,format,[<0.22.0>,"~s","A"]}, {erl_eval,do_apply,5}, {shell,exprs,6}, {shell,eval_exprs,6}, {shell,eval_loop,3}]} in function io:o_request/2
In this example, an attempt was made to output the single character '65' with the aid of the string formatting directive "~s".
fread([IoDevice,] Prompt, Format) -> Result
IoDevice = io_device()
Prompt = atom() | string()
Format = string()
Result = {ok, Terms} | eof | {error, What}
Terms = [term()]
What = term()
Reads characters from the standard input (IoDevice
),
prompting it with Prompt
. Interprets the characters in
accordance with Format
. Format
contains control
sequences which directs the interpretation of the input.
Format
may contain:
-
White space characters (SPACE, TAB and NEWLINE) which cause input to be read to the next non-white space character.
-
Ordinary characters which must match the next input character.
-
Control sequences, which have the general format
~*FC
. The character*
is an optional return suppression character. It provides a method to specify a field which is to be omitted.F
is thefield width
of the input field andC
determines the type of control sequence.Unless otherwise specified, leading white-space is ignored for all control sequences. An input field cannot be more than one line wide. The following control sequences are available:
~
A single
~
is expected in the input.d
A decimal integer is expected.
u
An unsigned integer in base 2..36 is expected. The field width parameter is used to specify base. Leading white-space characters are not skipped.
-
An optional sign character is expected. A sign character '-' gives the return value
-1
. Sign character '+' or none gives1
. The field width parameter is ignored. Leading white-space characters are not skipped.#
An integer in base 2..36 with Erlang-style base prefix (for example
"16#ffff"
) is expected.f
A floating point number is expected. It must follow the Erlang floating point number syntax.
s
A string of non-white-space characters is read. If a field width has been specified, this number of characters are read and all trailing white-space characters are stripped. An Erlang string (list of characters) is returned.
a
Similar to
s
, but the resulting string is converted into an atom.c
The number of characters equal to the field width are read (default is 1) and returned as an Erlang string. However, leading and trailing white-space characters are not omitted as they are with
s
. All characters are returned.l
Returns the number of characters which have been scanned up to that point, including white-space characters.
It returns:
{ok, Terms}
The read was successful and
Terms
is the list of successfully matched and read items.eof
End of file was encountered.
{error, What}
The read operation failed and the parameter
What
gives a hint about the error.
Examples:
20>io:fread('enter>', "~f~f~f").
enter>1.9 35.5e3 15.0
{ok,[1.9,3.55e4,15.0]} 21>io:fread('enter>', "~10f~d").
enter>5.67899
{ok,[5.678,99]} 22>io:fread('enter>', ":~10s:~10c:").
enter>:
alan
:
joe
:
{ok, ["alan", " joe "]}
rows([IoDevice]) -> {ok,int()} | {error, enotsup}
IoDevice = io_device()
Retrieves the number of rows of the
IoDevice
(i.e. the height of a terminal). The function
only succeeds for terminal devices, for all other devices
the function returns {error, enotsup}
scan_erl_exprs(Prompt) ->
scan_erl_exprs([IoDevice,] Prompt, StartLine) -> Result
IoDevice = io_device()
Prompt = atom() | string()
StartLine = int()
Result = {ok, Tokens, EndLine} | {eof, EndLine} | {error, ErrorInfo, EndLine}
Tokens -- see erl_scan(3)
EndLine = int()
ErrorInfo -- see section Error Information below
Reads data from the standard input (IoDevice
),
prompting it with Prompt
. Reading starts at line number
StartLine
(1). The data is tokenized as if it were a
sequence of Erlang expressions until a final '.'
is
reached. This token is also returned. It returns:
{ok, Tokens, EndLine}
The tokenization succeeded.
{eof, EndLine}
End of file was encountered.
{error, ErrorInfo, EndLine}
An error occurred.
Example:
23>io:scan_erl_exprs('enter>').
enter>abc(), "hey".
{ok,[{atom,1,abc},{'(',1},{')',1},{',',1},{string,1,"hey"},{dot,1}],2} 24>io:scan_erl_exprs('enter>').
enter>1.0er.
{error,{1,erl_scan,{illegal,float}},2}
scan_erl_form(Prompt) ->
scan_erl_form([IoDevice,] Prompt, StartLine) -> Result
IoDevice = io_device()
Prompt = atom() | string()
StartLine = int()
Result = {ok, Tokens, EndLine} | {eof, EndLine} | {error, ErrorInfo, EndLine}
Tokens -- see erl_scan(3)
EndLine = int()
ErrorInfo -- see section Error Information below
Reads data from the standard input (IoDevice
),
prompting it with Prompt
. Starts reading at line number
StartLine
(1). The data is tokenized as if it were an
Erlang form - one of the valid Erlang expressions in an
Erlang source file - until a final '.'
is reached.
This last token is also returned. The return values are the
same as for scan_erl_exprs/1,2,3
above.
parse_erl_exprs(Prompt) ->
parse_erl_exprs([IoDevice,] Prompt, StartLine) -> Result
IoDevice = io_device()
Prompt = atom() | string()
StartLine = int()
Result = {ok, Expr_list, EndLine} | {eof, EndLine} | {error, ErrorInfo, EndLine}
Expr_list -- see erl_parse(3)
EndLine = int()
ErrorInfo -- see section Error Information below
Reads data from the standard input (IoDevice
),
prompting it with Prompt
. Starts reading at line number
StartLine
(1). The data is tokenized and parsed as if
it were a sequence of Erlang expressions until a final '.' is
reached. It returns:
{ok, Expr_list, EndLine}
The parsing was successful.
{eof, EndLine}
End of file was encountered.
{error, ErrorInfo, EndLine}
An error occurred.
Example:
25>io:parse_erl_exprs('enter>').
enter>abc(), "hey".
{ok, [{call,1,{atom,1,abc},[]},{string,1,"hey"}],2} 26>io:parse_erl_exprs ('enter>').
enter>abc("hey".
{error,{1,erl_parse,["syntax error before: ",["'.'"]]},2}
parse_erl_form(Prompt) ->
parse_erl_form([IoDevice,] Prompt, StartLine) -> Result
IoDevice = io_device()
Prompt = atom() | string()
StartLine = int()
Result = {ok, AbsForm, EndLine} | {eof, EndLine} | {error, ErrorInfo, EndLine}
AbsForm -- see erl_parse(3)
EndLine = int()
ErrorInfo -- see section Error Information below
Reads data from the standard input (IoDevice
),
prompting it with Prompt
. Starts reading at line number
StartLine
(1). The data is tokenized and parsed as if
it were an Erlang form - one of the valid Erlang expressions
in an Erlang source file - until a final '.' is reached. It
returns:
{ok, AbsForm, EndLine}
The parsing was successful.
{eof, EndLine}
End of file was encountered.
{error, ErrorInfo, EndLine}
An error occurred.
Standard Input/Output
All Erlang processes have a default standard IO device. This
device is used when no IoDevice
argument is specified in
the above function calls. However, it is sometimes desirable to
use an explicit IoDevice
argument which refers to the
default IO device. This is the case with functions that can
access either a file or the default IO device. The atom
standard_io
has this special meaning. The following example
illustrates this:
27>io:read('enter>').
enter>foo.
{ok,foo} 28>io:read(standard_io, 'enter>').
enter>bar.
{ok,bar}
There is always a process registered under the name of
user
. This can be used for sending output to the user.
Error Information
The ErrorInfo
mentioned above is the standard
ErrorInfo
structure which is returned from all IO modules.
It has the format:
{ErrorLine, Module, ErrorDescriptor}
A string which describes the error is obtained with the following call:
apply(Module, format_error, ErrorDescriptor)