io - Standard I/O server interface functions.
Please see following description for synopsis
io(3) Erlang Module Definition io(3)
NAME
io - Standard I/O server interface functions.
DESCRIPTION
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 proto-
cols. Normally, it is a IoDevice returned by file:open/2. If no IoDe-
vice is given, standard_io is used.
For a description of the I/O protocols, see section The Erlang I/O Pro-
tocol in the User's Guide.
Warning:
As from Erlang/OTP R13A, data supplied to function put_chars/2 is to be
in the unicode:chardata() format. This means that programs supplying
binaries to this function must convert them to UTF-8 before trying to
output the data on an I/O device.
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.
DATA TYPES
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).
For more information about the built-in devices see Standard
Input/Output and Standard Error.
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.
EXPORTS
columns() -> {ok, integer() >= 1} | {error, enotsup}
columns(IoDevice) -> {ok, integer() >= 1} | {error, enotsup}
Types:
IoDevice = device()
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:
IoDevice = device()
Format = format()
Data = [term()]
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 for-
matting, 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.
* F is the field width of the printed argument. A negative
value means that the argument is left-justified within the
field, otherwise right-justified. If no field width is spec-
ified, the required print width is used. If the field width
specified is too small, the whole field is 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, 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 appli-
cable, it is used for both the field width and precision.
The default padding character is ' ' (space).
* Mod is the control sequence modifier. This is one or more
characters that change the interpretation of Data. The cur-
rent modifiers are t, for Unicode translation, and l, for
stopping p and P from detecting printable characters.
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, $*, 3.14159265]).
**3.14159
ok
Available control sequences:
~:
Character ~ is written.
c:
The argument is a number that is interpreted as an ASCII
code. The precision is the number of times the character is
printed and defaults to the field width, which in turn
defaults to 1. Example:
1> io:fwrite("|~10.5c|~-10.5c|~5c|~n", [$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 Uni-
code 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
f:
The argument is a float that 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 < 1.
e:
The argument is a float that is written as [-]d.ddde+-ddd,
where the precision is the number of digits written. The
default precision is 6 and it cannot be < 2.
g:
The argument is a float that 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 is not to be < 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,
if no Unicode translation modifier is present, an iolist(),
a binary(), or an atom(). If the Unicode translation modi-
fier (t) is in effect, the argument is unicode:chardata(),
meaning that binaries are in UTF-8. The characters are
printed without quotes. The string is first truncated by the
specified precision and then padded and justified to the
specified field width. The default precision is the field
width.
This format can be used for printing any object and truncat-
ing 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]])
w:
Writes data with the standard syntax. This is used to output
Erlang terms. Atoms are printed within quotes if they con-
tain embedded non-printable characters. Atom characters >
255 are escaped unless the Unicode translation modifier (t)
is used. Floats are printed accurately as the shortest, cor-
rectly 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.
Left-justification is not supported. It also tries to detect
flat lists of printable characters and output these as
strings. For example:
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 indenta-
tion of the term. It defaults to the number of characters
printed on this line in the same call to write/1 or for-
mat/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]]).
""
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]]).
""
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>>]).
<<""/utf8>>
ok
17> io:fwrite("~tp~n", [<<128,128>>]).
<<128,128>>
ok
W:
Writes data in the same way as ~w, but takes an extra argu-
ment that is the maximum depth to which terms are printed.
Anything below this depth is replaced with .... For example,
using T above:
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.
P:
Writes data in the same way as ~p, but takes an extra argu-
ment that is the maximum depth to which terms are printed.
Anything below this depth is replaced with ..., for example:
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
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:
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
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. Example:
1> io:fwrite("~X~n", [31,"10#"]).
10#31
ok
2> io:fwrite("~.16X~n", [-31,"0x"]).
-0x1F
ok
#:
Like B, but prints the number with an Erlang style #-sepa-
rated base prefix. Example:
1> io:fwrite("~.10#~n", [31]).
10#31
ok
2> 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.
The function returns:
ok:
The formatting succeeded.
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 char-
acter 65 with the aid of the string formatting directive "~s".
fread(Prompt, Format) -> Result
fread(IoDevice, Prompt, Format) -> Result
Types:
IoDevice = device()
Prompt = prompt()
Format = format()
Result =
{ok, Terms :: [term()]} |
{error, {fread, FreadError :: io_lib:fread_error()}} |
server_no_data()
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:
* Whitespace characters (Space, Tab, and Newline) that cause
input to be read to the next non-whitespace character.
* Ordinary characters that must match the next input charac-
ter.
* Control sequences, which have the general format ~*FMC,
where:
* Character * is an optional return suppression character.
It provides a method to specify a field that is to be
omitted.
* F is the field width of the input field.
* M is an optional translation modifier (of which t is the
only supported, meaning Unicode translation).
* C determines the type of control sequence.
Unless otherwise specified, leading whitespace is ignored
for all control sequences. An input field cannot be more
than one line wide.
Available control sequences:
~:
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 return value -1. Sign character + or none gives 1.
The field width parameter is ignored. Leading whitespace
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-whitespace characters is read. If a field
width has been specified, this number of characters are
read and all trailing whitespace characters are stripped.
An Erlang string (list of characters) is returned.
If Unicode translation is in effect (~ts), characters >
255 are accepted, otherwise not. With the translation mod-
ifier, 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]]}
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 whitespace characters are not omitted
as they are with s. All characters are returned.
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]]}
l:
Returns the number of characters that have been scanned up
to that point, including whitespace characters.
The function returns:
{ok, Terms}:
The read was successful and Terms is the list of success-
fully matched and read items.
eof:
End of file was encountered.
{error, FreadError}:
The reading failed and FreadError gives a hint about the
error.
{error, ErrorDescription}:
The read operation failed and parameter ErrorDescription
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 "]}
get_chars(Prompt, Count) -> Data | server_no_data()
get_chars(IoDevice, Prompt, Count) -> Data | server_no_data()
Types:
IoDevice = device()
Prompt = prompt()
Count = integer() >= 0
Data = string() | unicode:unicode_binary()
server_no_data() = {error, ErrorDescription :: term()} | eof
Reads Count characters from standard input (IoDevice), prompting
it with Prompt.
The function returns:
Data:
The input characters. If the I/O device supports Unicode,
the data can represent codepoints > 255 (the latin1 range).
If the I/O server is set to deliver binaries, they are
encoded in UTF-8 (regardless of whether the I/O device sup-
ports Unicode).
eof:
End of file was encountered.
{error, ErrorDescription}:
Other (rare) error condition, such as {error, estale} if
reading from an NFS file system.
get_line(Prompt) -> Data | server_no_data()
get_line(IoDevice, Prompt) -> Data | server_no_data()
Types:
IoDevice = device()
Prompt = prompt()
Data = string() | unicode:unicode_binary()
server_no_data() = {error, ErrorDescription :: term()} | eof
Reads a line from the standard input (IoDevice), prompting it
with Prompt.
The function returns:
Data:
The characters in the line terminated by a line feed (or end
of file). If the I/O device supports Unicode, the data can
represent codepoints > 255 (the latin1 range). If the I/O
server is set to deliver binaries, they are encoded in UTF-8
(regardless of if the I/O device supports Unicode).
eof:
End of file was encountered.
{error, ErrorDescription}:
Other (rare) error condition, such as {error, estale} if
reading from an NFS file system.
getopts() -> [opt_pair()] | {error, Reason}
getopts(IoDevice) -> [opt_pair()] | {error, Reason}
Types:
IoDevice = device()
Reason = term()
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:
IoDevice = device()
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:
IoDevice = device()
Prompt = prompt()
StartLocation = location()
Options = erl_scan:options()
Result = parse_ret()
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:
{ok, ExprList, EndLocation}:
The parsing was successful.
{eof, EndLocation}:
End of file was encountered by the tokenizer.
eof:
End of file was encountered by the I/O server.
{error, ErrorInfo, ErrorLocation}:
An error occurred while tokenizing or parsing.
{error, ErrorDescription}:
Other (rare) error condition, such as {error, estale} if
reading from an NFS file system.
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:
IoDevice = device()
Prompt = prompt()
StartLocation = location()
Options = erl_scan:options()
Result = parse_form_ret()
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:
{ok, AbsForm, EndLocation}:
The parsing was successful.
{eof, EndLocation}:
End of file was encountered by the tokenizer.
eof:
End of file was encountered by the I/O server.
{error, ErrorInfo, ErrorLocation}:
An error occurred while tokenizing or parsing.
{error, ErrorDescription}:
Other (rare) error condition, such as {error, estale} if
reading from an NFS file system.
printable_range() -> unicode | latin1
Returns the user-requested range of printable Unicode charac-
ters.
The user can request a range of characters that are to be con-
sidered 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 consid-
ered 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:print-
able_list/1, which uses the return value of this function to
decide if a list is a string of printable characters.
Note:
In a future release, this function may return more values and
ranges. To avoid compatibility problems, it is recommended to
use function io_lib:printable_list/1.
put_chars(CharData) -> ok
put_chars(IoDevice, CharData) -> ok
Types:
IoDevice = device()
CharData = unicode:chardata()
Writes the characters of CharData to the I/O server (IoDevice).
read(Prompt) -> Result
read(IoDevice, Prompt) -> Result
Types:
IoDevice = device()
Prompt = prompt()
Result =
{ok, Term :: term()} | server_no_data() | {error, Error-
Info}
ErrorInfo = erl_scan:error_info() | erl_parse:error_info()
server_no_data() = {error, ErrorDescription :: term()} | eof
Reads a term Term from the standard input (IoDevice), prompting
it with Prompt.
The function returns:
{ok, Term}:
The parsing was successful.
eof:
End of file was encountered.
{error, ErrorInfo}:
The parsing failed.
{error, ErrorDescription}:
Other (rare) error condition, such as {error, estale} if
reading from an NFS file system.
read(IoDevice, Prompt, StartLocation) -> Result
read(IoDevice, Prompt, StartLocation, Options) -> Result
Types:
IoDevice = device()
Prompt = prompt()
StartLocation = location()
Options = erl_scan:options()
Result =
{ok, Term :: term(), EndLocation :: location()} |
{eof, EndLocation :: location()} |
server_no_data() |
{error, ErrorInfo, ErrorLocation :: location()}
ErrorInfo = erl_scan:error_info() | erl_parse:error_info()
server_no_data() = {error, ErrorDescription :: term()} | eof
Reads a term Term from IoDevice, prompting it with Prompt. Read-
ing starts at location StartLocation. Argument Options is passed
on as argument Options of function erl_scan:tokens/4.
The function returns:
{ok, Term, EndLocation}:
The parsing was successful.
{eof, EndLocation}:
End of file was encountered.
{error, ErrorInfo, ErrorLocation}:
The parsing failed.
{error, ErrorDescription}:
Other (rare) error condition, such as {error, estale} if
reading from an NFS file system.
rows() -> {ok, integer() >= 1} | {error, enotsup}
rows(IoDevice) -> {ok, integer() >= 1} | {error, enotsup}
Types:
IoDevice = device()
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:
Device = device()
Prompt = prompt()
StartLocation = location()
Options = erl_scan:options()
Result = erl_scan:tokens_result() | server_no_data()
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:
{ok, Tokens, EndLocation}:
The tokenization succeeded.
{eof, EndLocation}:
End of file was encountered by the tokenizer.
eof:
End of file was encountered by the I/O server.
{error, ErrorInfo, ErrorLocation}:
An error occurred while tokenizing.
{error, ErrorDescription}:
Other (rare) error condition, such as {error, estale} if
reading from an NFS file system.
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:
IoDevice = device()
Prompt = prompt()
StartLocation = location()
Options = erl_scan:options()
Result = erl_scan:tokens_result() | 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 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:
IoDevice = device()
Opts = [setopt()]
Reason = term()
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:
binary, list, or {binary, boolean()}:
If set in binary mode (binary or {binary, true}), the I/O
server sends binary data (encoded in UTF-8) as answers to
the get_line, get_chars, and, if possible, get_until
requests (for details, see section The Erlang I/O Protocol)
in the User's Guide). The immediate effect is that
get_chars/2,3 and get_line/1,2 return UTF-8 binaries instead
of lists of characters for the affected I/O device.
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.
{echo, boolean()}:
Denotes if the terminal is to echo input. Only supported for
the standard shell I/O server (group.erl)
{expand_fun, expand_fun()}:
Provides 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-read-
ing functions, such as get_line/1,2.
The function is called with the current line, up to the cur-
sor, 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).
{encoding, latin1 | unicode}:
Specifies how characters are input or output from or to the
I/O device, implying that, for example, a terminal is set to
handle Unicode input and output or a file is set to handle
UTF-8 data encoding.
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 "-old-
shell" 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 possi-
ble for files, see below.
{encoding, unicode | latin1} is supported by both the stan-
dard shell (group.erl including werl on Windows), the 'old-
shell' (user.erl), and the file I/O servers.
{encoding, utf8 | utf16 | utf32 | {utf16,big} | {utf16,little}
| {utf32,big} | {utf32,little}}:
For disk files, the encoding can be set to various UTF vari-
ants. This has the effect that data is expected to be read
as the specified encoding from the file, and the data is
written in the specified encoding to the disk file.
{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:
IoDevice = device()
Term = term()
Writes term Term to the standard output (IoDevice).
STANDARD INPUT/OUTPUT
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}
standard_io is an alias for group_leader/0, so in order to change
where the default input/output requests are sent you can change the
group leader for the current process using group_leader(NewGrou-
pLeader, self()).
There is always a process registered under the name of user. This can
be used for sending output to the user.
STANDARD ERROR
In certain situations, especially when the standard output is redi-
rected, access to an I/O server specific for error messages can be con-
venient. 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
ERROR INFORMATION
The ErrorInfo mentioned in this module is the standard ErrorInfo struc-
ture that is returned from all I/O modules. It has the following for-
mat:
{ErrorLocation, Module, ErrorDescriptor}
A string that describes the error is obtained with the following call:
Module:format_error(ErrorDescriptor)
Ericsson AB stdlib 3.17 io(3)