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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 protocols. Normally, it is a IoDevice returned
by file:open/2. If no IoDevice is given, standard_io is used.
For a description of the I/O protocols, see section The Erlang I/O Protocol 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 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.
* 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 specified, 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
applicable, 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 current 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 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
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 modifier (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 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]])
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. Atom characters > 255 are escaped unless the Unicode translation
modifier (t) is used. 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. 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 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
W:
Writes data in the same way as ~w, but takes an extra argument 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 argument 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 #-separated 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 character 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 character.
* 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 whitespace 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 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]]}
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 successfully 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 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.
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 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.
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, ErrorInfo}
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. Reading 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-reading functions, such as get_line/1,2.
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).
{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 "-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 | utf16 | utf32 | {utf16,big} | {utf16,little} | {utf32,big} |
{utf32,little}}:
For disk files, the encoding can be set to various UTF variants. 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(NewGroupLeader, 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 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
ERROR INFORMATION
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)