path: root/lib/stdlib/doc/src/io.xml

<?xml version="1.0" encoding="latin1" ?>
<!DOCTYPE erlref SYSTEM "erlref.dtd">

<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2012</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.

      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.

    </legalnotice>

    <title>io</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>io</module>
  <modulesummary>Standard I/O Server Interface Functions</modulesummary>
  <description>
    <p>This module provides an interface to standard Erlang I/O servers.
      The output functions all return <c>ok</c> if they are successful,
      or exit if they are not.</p>
    <p>In the following description, all functions have an optional
      parameter <c>IoDevice</c>. If included, it must be the pid of a
      process which handles the IO protocols. Normally, it is the
      <c>IoDevice</c> returned by
      <seealso marker="kernel:file#open/2">file:open/2</seealso>.</p>
    <p>For a description of the IO protocols refer to the STDLIB Users Guide.</p>
    <warning>

    <p>As of R13A, data supplied to the <seealso
    marker="#put_chars/2">put_chars</seealso> function should be in the
    <seealso marker="unicode#type-chardata"><c>unicode:chardata()</c></seealso> format. This means that programs
    supplying binaries to this function need to convert them to UTF-8
    before trying to output the data on an IO device.</p>

    <p>If an IO device is set in binary mode, the functions <seealso
    marker="#get_chars/3">get_chars</seealso> and <seealso
    marker="#get_line/2">get_line</seealso> may return binaries
    instead of lists. The binaries will, as of R13A, be encoded in
    UTF-8.</p>

    <p>To work with binaries in ISO-latin-1 encoding, use the <seealso
    marker="kernel:file">file</seealso> module instead.</p>

    <p>For conversion functions between character encodings, see the <seealso
    marker="stdlib:unicode">unicode</seealso> module.</p>

    </warning>

  </description>

  <datatypes>
    <datatype>
      <name name="device"/>
      <desc>
        <p>An IO device. Either <c>standard_io</c>, <c>standard_error</c>, a
          registered name, or a pid handling IO protocols (returned from
          <seealso marker="kernel:file#open/2">file:open/2</seealso>).</p>
      </desc>
    </datatype>
    <datatype>
      <name name="opt_pair"/>
    </datatype>
    <datatype>
      <name name="expand_fun"/>
    </datatype>
    <datatype>
      <name name="encoding"/>
    </datatype>
    <datatype>
      <name name="setopt"/>
    </datatype>
    <datatype>
      <name name="format"/>
    </datatype>
    <datatype>
      <name name="line"/>
    </datatype>
    <datatype>
      <name name="prompt"/>
    </datatype>
    <datatype>
      <name name="request_error"/>
    </datatype>
    <datatype>
      <name name="error_description"/>
      <desc><p>Whatever the I/O-server sends.</p></desc>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="columns" arity="0"/>
      <name name="columns" arity="1"/>
      <fsummary>Get the number of columns of an IO device</fsummary>
      <desc>
          <p>Retrieves the number of columns of the 
          <c><anno>IoDevice</anno></c> (i.e. the width of a terminal). The function
          only succeeds for terminal devices, for all other IO devices
          the function returns <c>{error, enotsup}</c></p>
      </desc>
    </func>
    <func>
      <name name="put_chars" arity="1"/>
      <name name="put_chars" arity="2"/>
      <fsummary>Write a list of characters</fsummary>
      <desc>
        <p>Writes the characters of <c><anno>CharData</anno></c> to the I/O server
          (<c><anno>IoDevice</anno></c>).</p>
      </desc>
    </func>
    <func>
      <name name="nl" arity="0"/>
      <name name="nl" arity="1"/>
      <fsummary>Write a newline</fsummary>
      <desc>
        <p>Writes new line to the standard output (<c><anno>IoDevice</anno></c>).</p>
      </desc>
    </func>
    <func>
      <name name="get_chars" arity="2"/>
      <name name="get_chars" arity="3"/>
      <fsummary>Read a specified number of characters</fsummary>
      <desc>
        <p>Reads <c><anno>Count</anno></c> characters from standard input
          (<c><anno>IoDevice</anno></c>), prompting it with <c><anno>Prompt</anno></c>. It
          returns:</p>
        <taglist>
          <tag><c><anno>Data</anno></c></tag>
          <item>
            <p>The input characters. If the IO device supports Unicode,
            the data may represent codepoints larger than 255 (the
            latin1 range). If the I/O server is set to deliver
            binaries, they will be encoded in UTF-8 (regardless of if
            the IO device actually supports Unicode or not).</p>
          </item>
          <tag><c>eof</c></tag>
          <item>
            <p>End of file was encountered.</p>
          </item>
	  <tag><c>{error,<anno>Reason</anno>}</c></tag>
	  <item>
	    <p>Other (rare) error condition, for instance <c>{error,estale}</c>
	    if reading from an NFS file system.</p>
	  </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="get_line" arity="1"/>
      <name name="get_line" arity="2"/>
      <fsummary>Read a line</fsummary>
      <desc>
        <p>Reads a line from the standard input (<c><anno>IoDevice</anno></c>),
          prompting it with <c><anno>Prompt</anno></c>. It returns:</p>
        <taglist>
          <tag><c><anno>Data</anno></c></tag>
          <item>
            <p>The characters in the line terminated by a LF (or end of
              file). If the IO device supports Unicode,
            the data  may represent codepoints larger than 255 (the
            latin1 range). If the I/O server is set to deliver
            binaries, they will be encoded in UTF-8 (regardless of if
            the IO device actually supports Unicode or not).</p>
          </item>
          <tag><c>eof</c></tag>
          <item>
            <p>End of file was encountered.</p>
          </item>
	  <tag><c>{error,<anno>Reason</anno>}</c></tag>
	  <item>
	    <p>Other (rare) error condition, for instance <c>{error,estale}</c>
	    if reading from an NFS file system.</p>
	  </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="getopts" arity="0"/>
      <name name="getopts" arity="1"/>
      <fsummary>Get the supported options and values from an I/O-server</fsummary>
      <desc>
      <p>This function requests all available options and their current values for a specific IO device. Example:</p>
<pre>
1> <input>{ok,F} = file:open("/dev/null",[read]).</input>
{ok,&lt;0.42.0&gt;}
2> <input>io:getopts(F).</input>
[{binary,false},{encoding,latin1}]</pre>
      <p>Here the file I/O-server returns all available options for a file,
      which are the expected ones, <c>encoding</c> and <c>binary</c>. The standard shell however has some more options:</p>
<pre>
3> io:getopts().
[{expand_fun,#Fun&lt;group.0.120017273&gt;},
 {echo,true},
 {binary,false},
 {encoding,unicode}]</pre>
      <p>This example is, as can be seen, run in an environment where the terminal supports Unicode input and output.</p>
      </desc>
    </func>
    <func>
      <name name="setopts" arity="1"/>
      <name name="setopts" arity="2"/>
      <fsummary>Set options</fsummary>
      <desc>
        <p>Set options for the standard IO device (<c><anno>IoDevice</anno></c>).</p>

        <p>Possible options and values vary depending on the actual
        IO device. For a list of supported options and their current values
        on a specific IO device, use the <seealso
        marker="#getopts/1">getopts/1</seealso> function.</p>

	<p>The options and values supported by the current OTP IO devices are:</p>
        <taglist>
          <tag><c>binary, list or {binary, boolean()}</c></tag>
          <item>
            <p>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 (see the I/O protocol description in STDLIB User's Guide for details). The immediate effect is that <c>get_chars/2,3</c> and <c>get_line/1,2</c> return UTF-8 binaries instead of lists of chars for the affected IO device.</p>
	    <p>By default, all IO devices in OTP are set in list mode, but the io functions can handle any of these modes and so should other, user written, modules behaving as clients to I/O-servers.</p>
	    <p>This option is supported by the standard shell (group.erl), the 'oldshell' (user.erl) and the file I/O servers.</p>
          </item>
          <tag><c>{echo, boolean()}</c></tag>
	  <item>
	  <p>Denotes if the terminal should echo input. Only supported for the standard shell I/O-server (group.erl)</p>
	  </item>
          <tag><c>{expand_fun, expand_fun()}</c></tag>
          <item>
            <p>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
              <c>get_line/1,2</c>.</p>
            <p>The function is called with the current line, upto
              the cursor, as a reversed string. It should return a
              three-tuple: <c>{yes|no, string(), [string(), ...]}</c>. The
              first element gives a beep if <c>no</c>, 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.</p>
            <p>Trivial example (beep on anything except empty line, which
              is expanded to "quit"):</p>
            <code type="none">
 fun("") -> {yes, "quit", []};
    (_) -> {no, "", ["quit"]} end</code>
            <p>This option is supported by the standard shell only (group.erl).</p>
          </item>
          <tag><c>{encoding, latin1 | unicode}</c></tag>
	  <item>
	  <p>Specifies how characters are input or output from or to the actual IO device, implying that i.e. a terminal is set to handle Unicode input and output or a file is set to handle UTF-8 data encoding.</p>
	  <p>The option <em>does not</em> affect how data is returned from the io-functions or how it is sent in the I/O-protocol, it only affects how the IO device is to handle Unicode characters towards the &quot;physical&quot; device.</p>
	  <p>The standard shell will be set for either unicode or latin1 encoding when the system is started. The actual 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. The bottom line is that the user can input Unicode characters and the IO device will be in {encoding, unicode} mode if the IO device supports it. The mode can be changed, if the assumption of the runtime system is wrong, by setting this option.</p>
	  <p>The IO device used when Erlang is started with the "-oldshell" or "-noshell" flags is by default set to latin1 encoding, meaning that any characters beyond codepoint 255 will be 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 will be in UTF-8 (regardless of operating system).</p>
	  <p>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.</p>
	  <p>{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.</p>
 	  </item>
          <tag><c>{encoding, utf8 | utf16 | utf32 | {utf16,big} | {utf16,little} | {utf32,big} | {utf32,little}}</c></tag>
	  <item>
	  <p>For disk files, the encoding can be set to various UTF variants. This will have the effect that data is expected to be read as the specified encoding from the file and the data will be written in the specified encoding to the disk file.</p>
	  <p>{encoding, utf8} will have the same effect as {encoding,unicode} on files.</p>
	  <p>The extended encodings are only supported on disk files (opened by the <seealso marker="kernel:file#open/2">file:open/2</seealso> function)</p>
 	  </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="write" arity="1"/>
      <name name="write" arity="2"/>
      <fsummary>Write a term</fsummary>
      <desc>
        <p>Writes the term <c><anno>Term</anno></c> to the standard output
          (<c><anno>IoDevice</anno></c>).</p>
      </desc>
    </func>
    <func>
      <name name="read" arity="1"/>
      <name name="read" arity="2"/>
      <fsummary>Read a term</fsummary>
      <desc>
        <p>Reads a term <c><anno>Term</anno></c> from the standard input
          (<c><anno>IoDevice</anno></c>), prompting it with <c><anno>Prompt</anno></c>. It
          returns:</p>
        <taglist>
          <tag><c>{ok, <anno>Term</anno>}</c></tag>
          <item>
            <p>The parsing was successful.</p>
          </item>
          <tag><c>eof</c></tag>
          <item>
            <p>End of file was encountered.</p>
          </item>
          <tag><c>{error, <anno>ErrorInfo</anno>}</c></tag>
          <item>
            <p>The parsing failed.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="read" arity="3"/>
      <name name="read" arity="4"/>
      <fsummary>Read a term</fsummary>
      <desc>
        <p>Reads a term <c><anno>Term</anno></c> from <c><anno>IoDevice</anno></c>, prompting it
          with <c><anno>Prompt</anno></c>. Reading starts at line number
          <c><anno>StartLine</anno></c>. The argument
          <c><anno>Options</anno></c> is passed on as the <c>Options</c>
          argument of the <c>erl_scan:tokens/4</c> function. It returns:</p>
        <taglist>
          <tag><c>{ok, Term, <anno>EndLine</anno>}</c></tag>
          <item>
            <p>The parsing was successful.</p>
          </item>
          <tag><c>{eof, <anno>EndLine</anno>}</c></tag>
          <item>
            <p>End of file was encountered.</p>
          </item>
          <tag><c>{error, <anno>ErrorInfo</anno>, <anno>ErrorLine</anno>}</c></tag>
          <item>
            <p>The parsing failed.</p>
          </item>
        </taglist>
      </desc>
    </func>
    <func>
      <name name="fwrite" arity="1"/>
      <name name="fwrite" arity="2"/>
      <name name="fwrite" arity="3"/>
      <name name="format" arity="1"/>
      <name name="format" arity="2"/>
      <name name="format" arity="3"/>
      <fsummary>Write formatted output</fsummary>
      <desc>
        <p>Writes the items in <c><anno>Data</anno></c> (<c>[]</c>) on the standard
          output (<c><anno>IoDevice</anno></c>) in accordance with <c><anno>Format</anno></c>.
          <c><anno>Format</anno></c> contains plain characters which are copied to
          the output device, and control sequences for formatting, see
          below. If <c><anno>Format</anno></c> is an atom or a binary, it is first
          converted to a list with the aid of <c>atom_to_list/1</c>
          or <c>binary_to_list/1</c>.</p>
        <pre>
1> <input>io:fwrite("Hello world!~n", []).</input>
Hello world!
ok</pre>
        <p>The general format of a control sequence is <c>~F.P.PadModC</c>.
          The character <c>C</c> determines the type of control sequence
          to be used, <c>F</c> and <c>P</c> are optional numeric
          arguments. If <c>F</c>, <c>P</c>, or <c>Pad</c> is <c>*</c>,
          the next argument in <c>Data</c> is used as the numeric value
          of <c>F</c> or <c>P</c>.</p>
        <p><c>F</c> is the <c>field width</c> 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 <c>*</c> characters.</p>
        <p><c>P</c> is the <c>precision</c> 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 <c>within</c> is used
          to determine print width.</p>
        <p><c>Pad</c> 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 <c>' '</c> (space).</p>
        <p><c>Mod</c> is the control sequence modifier. It is either a
	single character (currently only <c>t</c>, for Unicode translation,
	is supported) that changes the interpretation of Data.</p>

         <p>The following control sequences are available:</p>
        <taglist>
          <tag><c>~</c></tag>
          <item>
            <p>The character <c>~</c> is written.</p>
          </item>
          <tag><c>c</c></tag>
          <item>
            <p>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:</p>
            <pre>
2> <input>io:fwrite("|~10.5c|~-10.5c|~5c|~n", [$a, $b, $c]).</input>
|     aaaaa|bbbbb     |ccccc|
ok</pre>
            <p>If the Unicode translation modifier (<c>t</c>) is in effect,
	    the integer argument can be any number representing a
	    valid Unicode codepoint, otherwise it should be an integer
	    less than or equal to 255, otherwise it is masked with 16#FF:</p>
<pre>
1> <input>io:fwrite("~tc~n",[1024]).</input>
\x{400}
ok
2> <input>io:fwrite("~c~n",[1024]).</input>
^@
ok</pre>

          </item>
          <tag><c>f</c></tag>
          <item>
            <p>The argument is a float which is written as
              <c>[-]ddd.ddd</c>, where the precision is the number of
              digits after the decimal point. The default precision is 6
              and it cannot be less than 1.</p>
          </item>
          <tag><c>e</c></tag>
          <item>
            <p>The argument is a float which is written as
              <c>[-]d.ddde+-ddd</c>, where the precision is the number
              of digits written. The default precision is 6 and it
              cannot be less than 2.</p>
          </item>
          <tag><c>g</c></tag>
          <item>
            <p>The argument is a float which is written as <c>f</c>, if
              it is &gt;= 0.1 and &lt; 10000.0. Otherwise, it is written
              in the <c>e</c> 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 <c>f</c> format with the
              desired number of significant digits, it is also written
              in the <c>e</c> format.</p>
          </item>
          <tag><c>s</c></tag>
          <item>
            <p>Prints the argument with the <c>string</c> syntax. The
              argument is, if no Unicode translation modifier is present, an 
              iolist(), a binary, or an atom. If the Unicode translation modifier (<c>t</c>) 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 given precision and then padded and justified
	      to the given field width. The default precision is the field width.</p>
            <p>This format can be used for printing any object and
              truncating the output so it fits a specified field:</p>
            <pre>
3> <input>io:fwrite("|~10w|~n", [{hey, hey, hey}]).</input>
|**********|
ok
4> <input>io:fwrite("|~10s|~n", [io_lib:write({hey, hey, hey})]).</input>
|{hey,hey,h|
5> <input>io:fwrite("|~-10.8s|~n", [io_lib:write({hey, hey, hey})]).</input>
|{hey,hey  |
ok</pre>
          <p>A list with integers larger than 255 is considered an error if the Unicode translation modifier is not given:</p>
<pre>
1> <input>io:fwrite("~ts~n",[[1024]]).</input>
\x{400}
ok
2> io:fwrite("~s~n",[[1024]]).
** exception exit: {badarg,[{io,format,[&lt;0.26.0&gt;,"~s~n",[[1024]]]},
   ...</pre>
          </item>
          <tag><c>w</c></tag>
          <item>
            <p>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>
          </item>
          <tag><c>p</c></tag>
          <item>
            <p>Writes the data with standard syntax in the same way as
              <c>~w</c>, 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. The
              Unicode translation modifier is used for determining
              what characters are printable. For example:</p>
            <pre>
5> <input>T = [{attributes,[[{id,age,1.50000},{mode,explicit},</input>
<input>{typename,"INTEGER"}], [{id,cho},{mode,explicit},{typename,'Cho'}]]},</input>
<input>{typename,'Person'},{tag,{'PRIVATE',3}},{mode,implicit}].</input>
...
6> <input>io:fwrite("~w~n", [T]).</input>
[{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> <input>io:fwrite("~62p~n", [T]).</input>
[{attributes,[[{id,age,1.5},
               {mode,explicit},
               {typename,"INTEGER"}],
              [{id,cho},{mode,explicit},{typename,'Cho'}]]},
 {typename,'Person'},
 {tag,{'PRIVATE',3}},
 {mode,implicit}]
ok</pre>
            <p>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 <c>same</c> call to
              <c>io:fwrite</c> or <c>io:format</c>. For example, using
              <c>T</c> above:</p>
            <pre>
8> <input>io:fwrite("Here T = ~62p~n", [T]).</input>
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</pre>
            <p>Binaries that look like UTF-8 encoded strings will be
              output with the string syntax if the Unicode translation
              modifier is given:</p>
            <pre>
9> <input>io:fwrite("~p~n",[[1024]]).</input>
[1024]
10> <input>io:fwrite("~tp~n",[[1024]]).</input>
"\x{400}"
11> <input>io:fwrite("~tp~n", [&lt;&lt;128,128&gt;&gt;]).</input>
&lt;&lt;128,128&gt;&gt;
12> <input>io:fwrite("~tp~n", [&lt;&lt;208,128&gt;&gt;]).</input>
&lt;&lt;"\x{400}"/utf8&gt;&gt;
ok</pre>
          </item>
          <tag><c>W</c></tag>
          <item>
            <p>Writes data in the same way as <c>~w</c>, but takes an
              extra argument which is the maximum depth to which terms
              are printed. Anything below this depth is replaced with
              <c>...</c>. For example, using <c>T</c> above:</p>
            <pre>
9> <input>io:fwrite("~W~n", [T,9]).</input>
[{attributes,[[{id,age,1.5},{mode,explicit},{typename,...}],
[{id,cho},{mode,...},{...}]]},{typename,'Person'},
{tag,{'PRIVATE',3}},{mode,implicit}]
ok</pre>
            <p>If the maximum depth has been reached, then it is
              impossible to read in the resultant output. Also, the
              <c>,...</c> form in a tuple denotes that there are more
              elements in the tuple but these are below the print depth.</p>
          </item>
          <tag><c>P</c></tag>
          <item>
            <p>Writes data in the same way as <c>~p</c>, but takes an
              extra argument which is the maximum depth to which terms
              are printed. Anything below this depth is replaced with
              <c>...</c>. For example:</p>
            <pre>
10> <input>io:fwrite("~62P~n", [T,9]).</input>
[{attributes,[[{id,age,1.5},{mode,explicit},{typename,...}],
              [{id,cho},{mode,...},{...}]]},
 {typename,'Person'},
 {tag,{'PRIVATE',3}},
 {mode,implicit}]
ok</pre>
          </item>
          <tag><c>B</c></tag>
          <item>
            <p>Writes an integer in base 2..36, the default base is
              10. A leading dash is printed for negative integers.</p>
            <p>The precision field selects base. For example:</p>
            <pre>
11> <input>io:fwrite("~.16B~n", [31]).</input>
1F
ok
12> <input>io:fwrite("~.2B~n", [-19]).</input>
-10011
ok
13> <input>io:fwrite("~.36B~n", [5*36+35]).</input>
5Z
ok</pre>
          </item>
          <tag><c>X</c></tag>
          <item>
            <p>Like <c>B</c>, but takes an extra argument that is a
              prefix to insert before the number, but after the leading
              dash, if any.</p>
            <p>The prefix can be a possibly deep list of characters or
              an atom.</p>
            <pre>
14> <input>io:fwrite("~X~n", [31,"10#"]).</input>
10#31
ok
15> <input>io:fwrite("~.16X~n", [-31,"0x"]).</input>
-0x1F
ok</pre>
          </item>
          <tag><c>#</c></tag>
          <item>
            <p>Like <c>B</c>, but prints the number with an Erlang style
              <c>#</c>-separated base prefix.</p>
            <pre>
16> <input>io:fwrite("~.10#~n", [31]).</input>
10#31
ok
17> <input>io:fwrite("~.16#~n", [-31]).</input>
-16#1F
ok</pre>
          </item>
          <tag><c>b</c></tag>
          <item>
            <p>Like <c>B</c>, but prints lowercase letters.</p>
          </item>
          <tag><c>x</c></tag>
          <item>
            <p>Like <c>X</c>, but prints lowercase letters.</p>
          </item>
          <tag><c>+</c></tag>
          <item>
            <p>Like <c>#</c>, but prints lowercase letters.</p>
          </item>
          <tag><c>n</c></tag>
          <item>
            <p>Writes a new line.</p>
          </item>
          <tag><c>i</c></tag>
          <item>
            <p>Ignores the next term.</p>
          </item>
        </taglist>
        <p>Returns:</p>
        <taglist>
          <tag><c>ok</c></tag>
          <item>
            <p>The formatting succeeded.</p>
          </item>
        </taglist>
        <p>If an error occurs, there is no output. For example:</p>
        <pre>
18> <input>io:fwrite("~s ~w ~i ~w ~c ~n",['abc def', 'abc def', {foo, 1},{foo, 1}, 65]).</input>
abc def 'abc def'  {foo,1} A
ok
19> <input>io:fwrite("~s", [65]).</input>
** exception exit: {badarg,[{io,format,[&lt;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</pre>
        <p>In this example, an attempt was made to output the single
          character 65 with the aid of the string formatting directive
          "~s".</p>
      </desc>
    </func>
    <func>
      <name name="fread" arity="2"/>
      <name name="fread" arity="3"/>
      <fsummary>Read formatted input</fsummary>
      <desc>
        <p>Reads characters from the standard input (<c><anno>IoDevice</anno></c>),
          prompting it with <c><anno>Prompt</anno></c>. Interprets the characters in
          accordance with <c><anno>Format</anno></c>. <c><anno>Format</anno></c> contains control
          sequences which directs the interpretation of the input.</p>
        <p><c><anno>Format</anno></c> may contain:</p>
        <list type="bulleted">
          <item>
            <p>White space characters (SPACE, TAB and NEWLINE) which
              cause input to be read to the next non-white space
              character.</p>
          </item>
          <item>
            <p>Ordinary characters which must match the next input
              character.</p>
          </item>
          <item>

            <p>Control sequences, which have the general format
              <c>~*FMC</c>. The character <c>*</c> is an optional
              return suppression character. It provides a method to
              specify a field which is to be omitted. <c>F</c> is the
              <c>field width</c> of the input field, <c>M</c> is an optional
              translation modifier (of which <c>t</c> is the only currently
              supported, meaning Unicode translation) and <c>C</c>
              determines the type of control sequence.</p>

            <p>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:</p>
            <taglist>
              <tag><c>~</c></tag>
              <item>
                <p>A single <c>~</c> is expected in the input.</p>
              </item>
              <tag><c>d</c></tag>
              <item>
                <p>A decimal integer is expected.</p>
              </item>
              <tag><c>u</c></tag>
              <item>
                <p>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.</p>
              </item>
              <tag><c>-</c></tag>
              <item>
                <p>An optional sign character is expected. A sign
                  character <c>-</c> gives the return value <c>-1</c>. Sign
                  character <c>+</c> or none gives <c>1</c>. The field width
                  parameter is ignored. Leading white-space characters
                  are not skipped.</p>
              </item>
              <tag><c>#</c></tag>
              <item>
                <p>An integer in base 2..36 with Erlang-style base
                  prefix (for example <c>"16#ffff"</c>) is expected.</p>
              </item>
              <tag><c>f</c></tag>
              <item>
                <p>A floating point number is expected. It must follow
                  the Erlang floating point number syntax.</p>
              </item>
              <tag><c>s</c></tag>
              <item>
                <p>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.</p>

		  <p>If Unicode translation is in effect (~ts),
		  characters larger than 255 are accepted, otherwise
		  not. With the translation modifier, the list
		  returned may as a consequence also contain
		  integers larger than 255:</p>

<pre>
1> <input>io:fread("Prompt> ","~s").</input>
Prompt> <input>&lt;Characters beyond latin1 range not printable in this medium&gt;</input>
{error,{fread,string}}
2> <input>io:fread("Prompt> ","~ts").</input>
Prompt> <input>&lt;Characters beyond latin1 range not printable in this medium&gt;</input>
{ok,[[1091,1085,1080,1094,1086,1076,1077]]}</pre>

              </item>
              <tag><c>a</c></tag>
              <item>
                <p>Similar to <c>s</c>, but the resulting string is
                  converted into an atom.</p>
		  <p>The Unicode translation modifier is not allowed (atoms can not contain characters beyond the latin1 range).</p>
              </item>
              <tag><c>c</c></tag>
              <item>
                <p>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 <c>s</c>. All
                  characters are returned.</p>
		  <p>The Unicode translation modifier works as with <c>s</c>:</p>
<pre>
1> <input>io:fread("Prompt> ","~c").</input>
Prompt> <input>&lt;Character beyond latin1 range not printable in this medium&gt;</input>
{error,{fread,string}}
2> <input>io:fread("Prompt> ","~tc").</input>
Prompt> <input>&lt;Character beyond latin1 range not printable in this medium&gt;</input>
{ok,[[1091]]}</pre>

              </item>
              <tag><c>l</c></tag>
              <item>
                <p>Returns the number of characters which have been
                  scanned up to that point, including white-space
                  characters.</p>
              </item>
            </taglist>
            <p>It returns:</p>
            <taglist>
              <tag><c>{ok, <anno>Terms</anno>}</c></tag>
              <item>
                <p>The read was successful and <c><anno>Terms</anno></c> is the list
                  of successfully matched and read items.</p>
              </item>
              <tag><c>eof</c></tag>
              <item>
                <p>End of file was encountered.</p>
              </item>
              <tag><c>{error, <anno>What</anno>}</c></tag>
              <item>
                <p>The read operation failed and the parameter
                  <c><anno>What</anno></c> gives a hint about the error.</p>
              </item>
            </taglist>
          </item>
        </list>
        <p>Examples:</p>
        <pre>
20> <input>io:fread('enter>', "~f~f~f").</input>
enter><input>1.9 35.5e3 15.0</input>
{ok,[1.9,3.55e4,15.0]}
21> <input>io:fread('enter>', "~10f~d").</input>
enter>     <input>5.67899</input>
{ok,[5.678,99]}
22> <input>io:fread('enter>', ":~10s:~10c:").</input>
enter><input>:</input>   <input>alan</input>   <input>:</input>   <input>joe</input>    <input>:</input>
{ok, ["alan", "   joe    "]}</pre>
      </desc>
    </func>
    <func>
      <name name="rows" arity="0"/>
      <name name="rows" arity="1"/>
      <fsummary>Get the number of rows of an IO device</fsummary>
      <desc>
          <p>Retrieves the number of rows of the 
          <c><anno>IoDevice</anno></c> (i.e. the height of a terminal). The function
          only succeeds for terminal devices, for all other IO devices
          the function returns <c>{error, enotsup}</c></p>
      </desc>
    </func>
    <func>
      <name name="scan_erl_exprs" arity="1"/>
      <name name="scan_erl_exprs" arity="2"/>
      <name name="scan_erl_exprs" arity="3"/>
      <name name="scan_erl_exprs" arity="4"/>
      <fsummary>Read and tokenize Erlang expressions</fsummary>
      <desc>
        <p>Reads data from the standard input (<c>IoDevice</c>),
          prompting it with <c>Prompt</c>. Reading starts at line number
          <c>StartLine</c> (1). The argument <c><anno>Options</anno></c>
          is passed on as the <c>Options</c> argument of the
          <c>erl_scan:tokens/4</c> function. The data is tokenized as if
          it were a
          sequence of Erlang expressions until a final dot (<c>.</c>) is
          reached. This token is also returned. It returns:</p>
        <taglist>
          <tag><c>{ok, Tokens, EndLine}</c></tag>
          <item>
            <p>The tokenization succeeded.</p>
          </item>
          <tag><c>{eof, EndLine}</c></tag>
          <item>
            <p>End of file was encountered.</p>
          </item>
          <tag><c>{error, ErrorInfo, ErrorLine}</c></tag>
          <item>
            <p>An error occurred.</p>
          </item>
        </taglist>
        <p>Example:</p>
        <pre>
23> <input>io:scan_erl_exprs('enter>').</input>
enter><input>abc(), "hey".</input>
{ok,[{atom,1,abc},{'(',1},{')',1},{',',1},{string,1,"hey"},{dot,1}],2}
24> <input>io:scan_erl_exprs('enter>').</input>
enter><input>1.0er.</input>
{error,{1,erl_scan,{illegal,float}},2}</pre>
      </desc>
    </func>
    <func>
      <name name="scan_erl_form" arity="1"/>
      <name name="scan_erl_form" arity="2"/>
      <name name="scan_erl_form" arity="3"/>
      <name name="scan_erl_form" arity="4"/>
      <fsummary>Read and tokenize an Erlang form</fsummary>
      <desc>
        <p>Reads data from the standard input (<c><anno>IoDevice</anno></c>),
          prompting it with <c><anno>Prompt</anno></c>. Starts reading
          at line number <c><anno>StartLine</anno></c> (1). The
          argument <c><anno>Options</anno></c> is passed on as the
          <c>Options</c> argument of the <c>erl_scan:tokens/4</c>
          function. 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 dot (<c>.</c>) is reached.
          This last token is also returned. The return values are the
          same as for <c>scan_erl_exprs/1,2,3</c> above.</p>
      </desc>
    </func>
    <func>
      <name name="parse_erl_exprs" arity="1"/>
      <name name="parse_erl_exprs" arity="2"/>
      <name name="parse_erl_exprs" arity="3"/>
      <name name="parse_erl_exprs" arity="4"/>
      <type name="parse_ret"/>
      <fsummary>Read, tokenize and parse Erlang expressions</fsummary>
      <desc>
        <p>Reads data from the standard input
          (<c><anno>IoDevice</anno></c>), prompting it with
          <c><anno>Prompt</anno></c>. Starts reading at line number
          <c><anno>StartLine</anno></c> (1). The argument
          <c><anno>Options</anno></c> is passed on as the
          <c>Options</c> argument of the <c>erl_scan:tokens/4</c>
          function. The data is tokenized and parsed as if it were a
          sequence of Erlang expressions until a final dot (<c>.</c>) is reached.
          It returns:</p>
        <taglist>
          <tag><c>{ok, ExprList, EndLine}</c></tag>
          <item>
            <p>The parsing was successful.</p>
          </item>
          <tag><c>{eof, EndLine}</c></tag>
          <item>
            <p>End of file was encountered.</p>
          </item>
          <tag><c>{error, ErrorInfo, ErrorLine}</c></tag>
          <item>
            <p>An error occurred.</p>
          </item>
        </taglist>
        <p>Example:</p>
        <pre>
25> <input>io:parse_erl_exprs('enter>').</input>
enter><input>abc(), "hey".</input>
{ok, [{call,1,{atom,1,abc},[]},{string,1,"hey"}],2}
26> <input>io:parse_erl_exprs ('enter>').</input>
enter><input>abc("hey".</input>
{error,{1,erl_parse,["syntax error before: ",["'.'"]]},2}</pre>
      </desc>
    </func>
    <func>
      <name name="parse_erl_form" arity="1"/>
      <name name="parse_erl_form" arity="2"/>
      <name name="parse_erl_form" arity="3"/>
      <name name="parse_erl_form" arity="4"/>
      <type name="parse_form_ret"/>
      <fsummary>Read, tokenize and parse an Erlang form</fsummary>
      <desc>
        <p>Reads data from the standard input (<c><anno>IoDevice</anno></c>),
          prompting it with <c><anno>Prompt</anno></c>. Starts reading at
          line number <c><anno>StartLine</anno></c> (1). The argument
          <c><anno>Options</anno></c> is passed on as the
          <c>Options</c> argument of the <c>erl_scan:tokens/4</c>
          function. 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 dot (<c>.</c>) is reached. It
          returns:</p>
        <taglist>
          <tag><c>{ok, AbsForm, EndLine}</c></tag>
          <item>
            <p>The parsing was successful.</p>
          </item>
          <tag><c>{eof, EndLine}</c></tag>
          <item>
            <p>End of file was encountered.</p>
          </item>
          <tag><c>{error, ErrorInfo, ErrorLine}</c></tag>
          <item>
            <p>An error occurred.</p>
          </item>
        </taglist>
      </desc>
    </func>
  </funcs>

  <section>
    <title>Standard Input/Output</title>
    <p>All Erlang processes have a default standard IO device. This
      device is used when no <c>IoDevice</c> argument is specified in
      the above function calls. However, it is sometimes desirable to
      use an explicit <c>IoDevice</c> 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
      <c>standard_io</c> has this special meaning. The following example
      illustrates this:</p>
    <pre>
27> <input>io:read('enter>').</input>
enter><input>foo.</input>
{ok,foo}
28> <input>io:read(standard_io, 'enter>').</input>
enter><input>bar.</input>
{ok,bar}</pre>
    <p>There is always a process registered under the name of
      <c>user</c>. This can be used for sending output to the user.</p>
  </section>
 <section>
    <title>Standard Error</title>
    <p>In certain situations, especially when the standard output is redirected, access to an I/O-server specific for error messages might be convenient. The IO device <c>standard_error</c> can be used to direct output to whatever the current operating system considers a suitable IO device for error output. Example on a Unix-like operating system:</p>
<pre>
$ <input>erl -noshell -noinput -eval 'io:format(standard_error,"Error: ~s~n",["error 11"]),'\</input>
<input>'init:stop().' > /dev/null</input>
Error: error 11</pre>



  </section>

  <section>
    <title>Error Information</title>
    <p>The <c>ErrorInfo</c> mentioned above is the standard
      <c>ErrorInfo</c> structure which is returned from all IO modules.
      It has the format:</p>
    <code type="none">
{ErrorLine, Module, ErrorDescriptor}</code>
    <p>A string which describes the error is obtained with the following
      call:</p>
    <code type="none">
Module:format_error(ErrorDescriptor)</code>
  </section>
</erlref>