| io(3erl) | Erlang Module Definition | io(3erl) |
io - Standard I/O server interface functions.
This module provides an interface to standard Erlang I/O servers. The output functions all return ok if they are successful, or exit if they are not.
All functions in this module have an optional parameter IoDevice. If included, it must be the pid of a process that handles the I/O protocols. Normally, it is the IoDevice returned by file:open/2.
For a description of the I/O protocols, see section The Erlang I/O Protocol in the User's Guide.
If an I/O device is set in binary mode, functions get_chars/2,3 and get_line/1,2 can return binaries instead of lists. The binaries are, as from Erlang/OTP R13A, encoded in UTF-8.
To work with binaries in ISO Latin-1 encoding, use the file module instead.
For conversion functions between character encodings, see the unicode module.
device() = atom() | pid()
An I/O device, either standard_io, standard_error, a registered name, or a pid handling I/O protocols (returned from file:open/2).
opt_pair() =
{binary, boolean()} |
{echo, boolean()} |
{expand_fun, expand_fun()} |
{encoding, encoding()}
expand_fun() =
fun((term()) -> {yes | no, string(), [string(), ...]})
encoding() =
latin1 |
unicode |
utf8 |
utf16 |
utf32 |
{utf16, big | little} |
{utf32, big | little}
setopt() = binary | list | opt_pair()
format() = atom() | string() | binary()
location() = erl_anno:location()
prompt() = atom() | unicode:chardata()
server_no_data() = {error, ErrorDescription :: term()} | eof
What the I/O server sends when there is no data.
columns() -> {ok, integer() >= 1} | {error, enotsup}
columns(IoDevice) -> {ok, integer() >= 1} | {error, enotsup}
Types:
Retrieves the number of columns of the IoDevice (that is, the width of a terminal). The function succeeds for terminal devices and returns {error, enotsup} for all other I/O devices.
format(Format) -> ok
format(Format, Data) -> ok
format(IoDevice, Format, Data) -> ok
fwrite(Format) -> ok
fwrite(Format, Data) -> ok
fwrite(IoDevice, Format, Data) -> ok
Types:
Writes the items in Data ([]) on the standard output (IoDevice) in accordance with Format. Format contains plain characters that 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. Example:
1> io:fwrite("Hello world!~n", []).
Hello world!
ok
The general format of a control sequence is ~F.P.PadModC.
The character C determines the type of control sequence to be used. It is the only required field. All of F, P, Pad, and Mod are optional. For example, to use a # for Pad but use the default values for F and P, you can write ~..#C.
If F, P, or Pad is a * character, the next argument in Data is used as the value. For example:
1> io:fwrite("~*.*.0f~n",[9, 5, 3.14159265]).
003.14159
ok
To use a literal * character as Pad, it must be passed as an argument:
2> io:fwrite("~*.*.*f~n",[9, 5, $*, 3.14159265]).
**3.14159
ok
Available control sequences:
1> io:fwrite("|~10.5c|~-10.5c|~5c|~n", [$a, $b, $c]).
| aaaaa|bbbbb |ccccc|
ok
If the Unicode translation modifier (t) is in effect, the integer argument can be any number representing a valid Unicode codepoint, otherwise it is to be an integer less than or equal to 255, otherwise it is masked with 16#FF:
2> io:fwrite("~tc~n",[1024]).
\x{400}
ok
3> io:fwrite("~c~n",[1024]).
^@
ok
This format can be used for printing any object and truncating the output so it fits a specified field:
1> io:fwrite("|~10w|~n", [{hey, hey, hey}]).
|**********|
ok
2> io:fwrite("|~10s|~n", [io_lib:write({hey, hey, hey})]).
|{hey,hey,h|
3> io:fwrite("|~-10.8s|~n", [io_lib:write({hey, hey, hey})]).
|{hey,hey |
ok
A list with integers > 255 is considered an error if the Unicode translation modifier is not specified:
4> io:fwrite("~ts~n",[[1024]]).
\x{400}
ok
5> io:fwrite("~s~n",[[1024]]).
** exception error: bad argument
in function io:format/3
called as io:format(<0.53.0>,"~s~n",[[1024]])
1> T = [{attributes,[[{id,age,1.50000},{mode,explicit},
{typename,"INTEGER"}], [{id,cho},{mode,explicit},{typename,'Cho'}]]},
{typename,'Person'},{tag,{'PRIVATE',3}},{mode,implicit}].
...
2> 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
3> 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 write/1 or format/1,2,3. For example, using T above:
4> 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
As from Erlang/OTP 21.0, a field width of value 0 can be used for specifying that a line is infinitely long, which means that no line breaks are inserted. For example:
5> io:fwrite("~0p~n", [lists:seq(1, 30)]).
[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30]
ok
When the modifier l is specified, no detection of printable character lists takes place, for example:
6> S = [{a,"a"}, {b, "b"}], io:fwrite("~15p~n", [S]).
[{a,"a"},
{b,"b"}]
ok
7> io:fwrite("~15lp~n", [S]).
[{a,[97]},
{b,[98]}]
ok
The Unicode translation modifier t specifies how to treat characters outside the Latin-1 range of codepoints, in atoms, strings, and binaries. For example, printing an atom containing a character > 255:
8> io:fwrite("~p~n",[list_to_atom([1024])]).
ok
9> io:fwrite("~tp~n",[list_to_atom([1024])]).
ok
By default, Erlang only detects lists of characters in the Latin-1 range as strings, but the +pc unicode flag can be used to change this (see printable_range/0 for details). For example:
10> io:fwrite("~p~n",[[214]]).
"Ö"
ok
11> io:fwrite("~p~n",[[1024]]).
[1024]
ok
12> io:fwrite("~tp~n",[[1024]]).
[1024]
ok
but if Erlang was started with +pc unicode:
13> io:fwrite("~p~n",[[1024]]).
[1024]
ok
14> io:fwrite("~tp~n",[[1024]]).
"Ѐ"
ok
Similarly, binaries that look like UTF-8 encoded strings are output with the binary string syntax if the t modifier is specified:
15> io:fwrite("~p~n", [<<208,128>>]).
<<208,128>>
ok
16> io:fwrite("~tp~n", [<<208,128>>]).
<<"Ѐ"/utf8>>
ok
17> io:fwrite("~tp~n", [<<128,128>>]).
<<128,128>>
ok
8> 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 is reached, it cannot be 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.
9> 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
The precision field selects base, for example:
1> io:fwrite("~.16B~n", [31]).
1F
ok
2> io:fwrite("~.2B~n", [-19]).
-10011
ok
3> io:fwrite("~.36B~n", [5*36+35]).
5Z
ok
The prefix can be a possibly deep list of characters or an atom. Example:
1> io:fwrite("~X~n", [31,"10#"]).
10#31
ok
2> io:fwrite("~.16X~n", [-31,"0x"]).
-0x1F
ok
1> io:fwrite("~.10#~n", [31]).
10#31
ok
2> io:fwrite("~.16#~n", [-31]).
-16#1F
ok
The function returns:
If an error occurs, there is no output. Example:
1> 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
2> io:fwrite("~s", [65]).
** exception error: bad argument
in function io:format/3
called as io:format(<0.53.0>,"~s","A")
In this example, an attempt was made to output the single character 65 with the aid of the string formatting directive "~s".
fread(Prompt, Format) -> Result
fread(IoDevice, Prompt, Format) -> Result
Types:
server_no_data() = {error, ErrorDescription :: term()} | eof
Reads characters from the standard input (IoDevice), prompting it with Prompt. Interprets the characters in accordance with Format. Format contains control sequences that directs the interpretation of the input.
Format can contain the following:
Unless otherwise specified, leading whitespace is ignored for all control sequences. An input field cannot be more than one line wide.
Available control sequences:
If Unicode translation is in effect (~ts), characters > 255 are accepted, otherwise not. With the translation modifier, the returned list can as a consequence also contain integers > 255:
1> io:fread("Prompt> ","~s").
Prompt> <Characters beyond latin1 range not printable in this medium>
{error,{fread,string}}
2> io:fread("Prompt> ","~ts").
Prompt> <Characters beyond latin1 range not printable in this medium>
{ok,[[1091,1085,1080,1094,1086,1076,1077]]}
The Unicode translation modifier works as with s:
1> io:fread("Prompt> ","~c").
Prompt> <Character beyond latin1 range not printable in this medium>
{error,{fread,string}}
2> io:fread("Prompt> ","~tc").
Prompt> <Character beyond latin1 range not printable in this medium>
{ok,[[1091]]}
The function returns:
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 "]}
get_chars(Prompt, Count) -> Data | server_no_data()
get_chars(IoDevice, Prompt, Count) -> Data | server_no_data()
Types:
server_no_data() = {error, ErrorDescription :: term()} | eof
Reads Count characters from standard input (IoDevice), prompting it with Prompt.
The function returns:
get_line(Prompt) -> Data | server_no_data()
get_line(IoDevice, Prompt) -> Data | server_no_data()
Types:
server_no_data() = {error, ErrorDescription :: term()} | eof
Reads a line from the standard input (IoDevice), prompting it with Prompt.
The function returns:
getopts() -> [opt_pair()] | {error, Reason}
getopts(IoDevice) -> [opt_pair()] | {error, Reason}
Types:
Requests all available options and their current values for a specific I/O device, for example:
1> {ok,F} = file:open("/dev/null",[read]).
{ok,<0.42.0>}
2> io:getopts(F).
[{binary,false},{encoding,latin1}]
Here the file I/O server returns all available options for a file, which are the expected ones, encoding and binary. However, the standard shell has some more options:
3> io:getopts().
[{expand_fun,#Fun<group.0.120017273>},
{echo,true},
{binary,false},
{encoding,unicode}]
This example is, as can be seen, run in an environment where the terminal supports Unicode input and output.
nl() -> ok
nl(IoDevice) -> ok
Types:
Writes new line to the standard output (IoDevice).
parse_erl_exprs(Prompt) -> Result
parse_erl_exprs(IoDevice, Prompt) -> Result
parse_erl_exprs(IoDevice, Prompt, StartLocation) -> Result
parse_erl_exprs(IoDevice, Prompt, StartLocation, Options) ->
Result
Types:
parse_ret() =
{ok,
ExprList :: [erl_parse:abstract_expr()],
EndLocation :: location()} |
{eof, EndLocation :: location()} |
{error,
ErrorInfo :: erl_scan:error_info() | erl_parse:error_info(),
ErrorLocation :: location()} |
server_no_data()
server_no_data() = {error, ErrorDescription :: term()} | eof
Reads data from the standard input (IoDevice), prompting it with Prompt. Starts reading at location StartLocation (1). Argument Options is passed on as argument Options of function erl_scan:tokens/4. The data is tokenized and parsed as if it was a sequence of Erlang expressions until a final dot (.) is reached.
The function returns:
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) -> Result
parse_erl_form(IoDevice, Prompt) -> Result
parse_erl_form(IoDevice, Prompt, StartLocation) -> Result
parse_erl_form(IoDevice, Prompt, StartLocation, Options) -> Result
Types:
parse_form_ret() =
{ok,
AbsForm :: erl_parse:abstract_form(),
EndLocation :: location()} |
{eof, EndLocation :: location()} |
{error,
ErrorInfo :: erl_scan:error_info() | erl_parse:error_info(),
ErrorLocation :: location()} |
server_no_data()
server_no_data() = {error, ErrorDescription :: term()} | eof
Reads data from the standard input (IoDevice), prompting it with Prompt. Starts reading at location StartLocation (1). Argument Options is passed on as argument Options of function erl_scan:tokens/4. The data is tokenized and parsed as if it was an Erlang form (one of the valid Erlang expressions in an Erlang source file) until a final dot (.) is reached.
The function returns:
printable_range() -> unicode | latin1
Returns the user-requested range of printable Unicode characters.
The user can request a range of characters that are to be considered printable in heuristic detection of strings by the shell and by the formatting functions. This is done by supplying +pc <range> when starting Erlang.
The only valid values for <range> are latin1 and unicode. latin1 means that only code points < 256 (except control characters, and so on) are considered printable. unicode means that all printable characters in all Unicode character ranges are considered printable by the I/O functions.
By default, Erlang is started so that only the latin1 range of characters indicate that a list of integers is a string.
The simplest way to use the setting is to call io_lib:printable_list/1, which uses the return value of this function to decide if a list is a string of printable characters.
put_chars(CharData) -> ok
put_chars(IoDevice, CharData) -> ok
Types:
Writes the characters of CharData to the I/O server (IoDevice).
read(Prompt) -> Result
read(IoDevice, Prompt) -> Result
Types:
server_no_data() = {error, ErrorDescription :: term()} | eof
Reads a term Term from the standard input (IoDevice), prompting it with Prompt.
The function returns:
read(IoDevice, Prompt, StartLocation) -> Result
read(IoDevice, Prompt, StartLocation, Options) -> Result
Types:
server_no_data() = {error, ErrorDescription :: term()} | eof
Reads a term Term from IoDevice, prompting it with Prompt. Reading starts at location StartLocation. Argument Options is passed on as argument Options of function erl_scan:tokens/4.
The function returns:
rows() -> {ok, integer() >= 1} | {error, enotsup}
rows(IoDevice) -> {ok, integer() >= 1} | {error, enotsup}
Types:
Retrieves the number of rows of IoDevice (that is, the height of a terminal). The function only succeeds for terminal devices, for all other I/O devices the function returns {error, enotsup}.
scan_erl_exprs(Prompt) -> Result
scan_erl_exprs(Device, Prompt) -> Result
scan_erl_exprs(Device, Prompt, StartLocation) -> Result
scan_erl_exprs(Device, Prompt, StartLocation, Options) -> Result
Types:
server_no_data() = {error, ErrorDescription :: term()} | eof
Reads data from the standard input (IoDevice), prompting it with Prompt. Reading starts at location StartLocation (1). Argument Options is passed on as argument Options of function erl_scan:tokens/4. The data is tokenized as if it were a sequence of Erlang expressions until a final dot (.) is reached. This token is also returned.
The function returns:
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) -> Result
scan_erl_form(IoDevice, Prompt) -> Result
scan_erl_form(IoDevice, Prompt, StartLocation) -> Result
scan_erl_form(IoDevice, Prompt, StartLocation, Options) -> Result
Types:
server_no_data() = {error, ErrorDescription :: term()} | eof
Reads data from the standard input (IoDevice), prompting it with Prompt. Starts reading at location StartLocation (1). Argument Options is passed on as argument Options of function erl_scan:tokens/4. The data is tokenized as if it was an Erlang form (one of the valid Erlang expressions in an Erlang source file) until a final dot (.) is reached. This last token is also returned.
The return values are the same as for scan_erl_exprs/1,2,3,4.
setopts(Opts) -> ok | {error, Reason}
setopts(IoDevice, Opts) -> ok | {error, Reason}
Types:
Set options for the standard I/O device (IoDevice).
Possible options and values vary depending on the I/O device. For a list of supported options and their current values on a specific I/O device, use function getopts/1.
The options and values supported by the OTP I/O devices are as follows:
By default, all I/O devices in OTP are set in list mode. However, the I/O functions can handle any of these modes and so should other, user-written, modules behaving as clients to I/O servers.
This option is supported by the standard shell (group.erl), the 'oldshell' (user.erl), and the file I/O servers.
The function is called with the current line, up to the cursor, as a reversed string. It is to 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; the third is a list of possible expansions. If this list is not empty, it is printed and the current input line is written once again.
Trivial example (beep on anything except empty line, which is expanded to "quit"):
fun("") -> {yes, "quit", []};
(_) -> {no, "", ["quit"]} end
This option is only supported by the standard shell (group.erl).
The option does not affect how data is returned from the I/O functions or how it is sent in the I/O protocol, it only affects how the I/O device is to handle Unicode characters to the "physical" device.
The standard shell is set for unicode or latin1 encoding when the system is started. The encoding is set with the help of the LANG or LC_CTYPE environment variables on Unix-like system or by other means on other systems. So, the user can input Unicode characters and the I/O device is in {encoding, unicode} mode if the I/O device supports it. The mode can be changed, if the assumption of the runtime system is wrong, by setting this option.
The I/O device used when Erlang is started with the "-oldshell" or "-noshell" flags is by default set to latin1 encoding, meaning that any characters > codepoint 255 are escaped and that input is expected to be plain 8-bit ISO Latin-1. If the encoding is changed to Unicode, input and output from the standard file descriptors are in UTF-8 (regardless of operating system).
Files can also be set in {encoding, unicode}, meaning that data is written and read as UTF-8. More encodings are possible for files, see below.
{encoding, unicode | latin1} is supported by both the standard shell (group.erl including werl on Windows), the 'oldshell' (user.erl), and the file I/O servers.
{encoding, utf8} has the same effect as {encoding, unicode} on files.
The extended encodings are only supported on disk files (opened by function file:open/2).
write(Term) -> ok
write(IoDevice, Term) -> ok
Types:
Writes term Term to the standard output (IoDevice).
All Erlang processes have a default standard I/O device. This device is used when no IoDevice argument is specified in the function calls in this module. However, it is sometimes desirable to use an explicit IoDevice argument that refers to the default I/O device. This is the case with functions that can access either a file or the default I/O 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.
In certain situations, especially when the standard output is redirected, access to an I/O server specific for error messages can be convenient. The I/O device standard_error can be used to direct output to whatever the current operating system considers a suitable I/O device for error output. Example on a Unix-like operating system:
$ erl -noshell -noinput -eval 'io:format(standard_error,"Error: ~s~n",["error 11"]),'\ 'init:stop().' > /dev/null Error: error 11
The ErrorInfo mentioned in this module is the standard ErrorInfo structure that is returned from all I/O modules. It has the following format:
{ErrorLocation, Module, ErrorDescriptor}
A string that describes the error is obtained with the following call:
Module:format_error(ErrorDescriptor)
| stdlib 3.7.1 | Ericsson AB |