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<H1>io</H1>
</CENTER>

<H3>MODULE</H3>
<DIV CLASS=REFBODY>
io
</DIV>

<H3>MODULE SUMMARY</H3>
<DIV CLASS=REFBODY>
Standard IO Server Interface Functions
</DIV>

<H3>DESCRIPTION</H3>
<DIV CLASS=REFBODY>

<P>This module provides an interface to standard Erlang IO servers.
The output functions all return <CODE>ok</CODE> if they are successful,
or exit if they are not.
<P>In the following description, all functions have an optional
parameter <CODE>IoDevice</CODE>. If included, it must be the pid of a
process which handles the IO protocols. Normally, it is the
<CODE>IoDevice</CODE> returned by
<A HREF="javascript:erlhref('../../../../', 'kernel', 'file.html#open/2');">file:open/2</A>.
<P>For a description of the IO protocols refer to Armstrong,
Virding and Williams, 'Concurrent Programming in Erlang', Chapter
13, unfortunately now very outdated, but the general principles
still apply.
</DIV>

<H3>DATA TYPES</H3>
<DIV CLASS=REFBODY>

<PRE>
io_device()
  as returned by file:open/2, a process handling IO protocols
    
</PRE>

</DIV>

<H3>EXPORTS</H3>

<P><A NAME="put_chars/2"><STRONG><CODE>put_chars([IoDevice,] IoData) -&#62; ok</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>IoDevice = io_device()</CODE></STRONG><BR>
<STRONG><CODE>IoData = iodata() -- see erlang(3)</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>Writes the characters of <CODE>IoData</CODE> to the standard output
(<CODE>IoDevice</CODE>).
</DIV>

<P><A NAME="nl/1"><STRONG><CODE>nl([IoDevice]) -&#62; ok</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>IoDevice = io_device()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>Writes new line to the standard output (<CODE>IoDevice</CODE>).
</DIV>

<P><A NAME="get_chars/3"><STRONG><CODE>get_chars([IoDevice,] Prompt, Count) -&#62; string() | eof
</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>IoDevice = io_device()</CODE></STRONG><BR>
<STRONG><CODE>Prompt = atom() | string()</CODE></STRONG><BR>
<STRONG><CODE>Count = int()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>Reads <CODE>Count</CODE> characters from standard input
         (<CODE>IoDevice</CODE>), prompting it with <CODE>Prompt</CODE>. It
         returns:
<P>
<DL>

<DT>
<CODE>String</CODE>
</DT>

<DD>
         The input characters.<BR>

         
</DD>

<DT>
<CODE>eof</CODE>
</DT>

<DD>
         End of file was encountered.<BR>

         
</DD>

</DL>

</DIV>

<P><A NAME="get_line/2"><STRONG><CODE>get_line([IoDevice,] Prompt) -&#62; string() | eof</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>IoDevice = io_device()</CODE></STRONG><BR>
<STRONG><CODE>Prompt = atom() | string()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>Reads a line from the standard input (<CODE>IoDevice</CODE>),
prompting it with <CODE>Prompt</CODE>. It returns:
<P>
<DL>

<DT>
<CODE>String</CODE>
</DT>

<DD>
         The characters in the line terminated by a LF (or end of
         file).<BR>

         
</DD>

<DT>
<CODE>eof</CODE>
</DT>

<DD>
         End of file was encountered.<BR>

         
</DD>

</DL>

</DIV>

<P><A NAME="setopts/2"><STRONG><CODE>setopts([IoDevice,] Opts) -&#62; ok | {error, Reason}</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>IoDevice = io_device()</CODE></STRONG><BR>
<STRONG><CODE>Opts = [Opt]</CODE></STRONG><BR>
<STRONG><CODE>Opt = binary | list</CODE></STRONG><BR>
<STRONG><CODE>Reason = term()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>Set options for standard input/output (<CODE>IoDevice</CODE>).
         Possible options are:
        
<P>
<DL>

<DT>
<CODE>binary</CODE>
</DT>

<DD>
         Makes <CODE>get_chars/2,3</CODE> and <CODE>get_line/1,2</CODE> return
         binaries instead of lists of chars.<BR>

         
</DD>

<DT>
<CODE>list</CODE>
</DT>

<DD>
         Makes <CODE>get_chars/2,3</CODE> and <CODE>get_line/1,2</CODE> return
         lists of chars, which is the default.<BR>

         
</DD>

<DT>
<CODE>expand_fun</CODE>
</DT>

<DD>
         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
         <CODE>get_line/1,2</CODE>.<BR>

         The function is called with the current line, upto
         the cursor, as a reversed string. It should return a
         three-tuple: <CODE>{yes|no, string(), [string(), ...]}</CODE>. The
         first element gives a beep if <CODE>no</CODE>, 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.<BR>

         Trivial example (beep on anything except empty line, which
         is expanded to &#34;quit&#34;):
         <BR>

<PRE>
 fun(&#34;&#34;) -&#62; {yes, &#34;quit&#34;, []};
    (_) -&#62; {no, &#34;&#34;, [&#34;quit&#34;]} end
              
</PRE>

         
</DD>

</DL>

<P>
<TABLE CELLPADDING=4>
  <TR>
    <TD VALIGN=TOP><IMG ALT="Note!" SRC="note.gif"></TD>
    <TD>

<P>The <CODE>binary</CODE> option does not work against IO servers
         on remote nodes running an older version of Erlang/OTP than
         R9C.    </TD>
  </TR>
</TABLE>

</DIV>

<P><A NAME="write/2"><STRONG><CODE>write([IoDevice,] Term) -&#62; ok</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>IoDevice = io_device()</CODE></STRONG><BR>
<STRONG><CODE>Term = term()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>Writes the term <CODE>Term</CODE> to the standard output
         (<CODE>IoDevice</CODE>).
</DIV>

<P><A NAME="read/2"><STRONG><CODE>read([IoDevice,] Prompt) -&#62; Result</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>IoDevice = io_device()</CODE></STRONG><BR>
<STRONG><CODE>Prompt = atom() | string()</CODE></STRONG><BR>
<STRONG><CODE>Result = {ok, Term} | eof | {error, ErrorInfo}</CODE></STRONG><BR>
<STRONG><CODE>Term = term()</CODE></STRONG><BR>
<STRONG><CODE>ErrorInfo -- see section Error Information below</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>Reads a term <CODE>Term</CODE> from the standard input
         (<CODE>IoDevice</CODE>), prompting it with <CODE>Prompt</CODE>. It
         returns:
<P>
<DL>

<DT>
<CODE>{ok, Term}</CODE>
</DT>

<DD>
         The parsing was successful.<BR>

         
</DD>

<DT>
<CODE>eof</CODE>
</DT>

<DD>
         End of file was encountered.<BR>

         
</DD>

<DT>
<CODE>{error, ErrorInfo}</CODE>
</DT>

<DD>
         The parsing failed.<BR>

         
</DD>

</DL>

</DIV>

<P><A NAME="read/3"><STRONG><CODE>read(IoDevice, Prompt, StartLine) -&#62; Result</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>IoDevice = io_device()</CODE></STRONG><BR>
<STRONG><CODE>Prompt = atom() | string()</CODE></STRONG><BR>
<STRONG><CODE>StartLine = int()</CODE></STRONG><BR>
<STRONG><CODE>Result = {ok, Term, EndLine} | {eof, EndLine} | 
{error, ErrorInfo, EndLine}</CODE></STRONG><BR>
<STRONG><CODE>Term = term()</CODE></STRONG><BR>
<STRONG><CODE>EndLine = int()</CODE></STRONG><BR>
<STRONG><CODE>ErrorInfo -- see section Error Information below</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>Reads a term <CODE>Term</CODE> from <CODE>IoDevice</CODE>, prompting it
         with <CODE>Prompt</CODE>. Reading starts at line number
         <CODE>StartLine</CODE>. It returns:
<P>
<DL>

<DT>
<CODE>{ok, Term, EndLine}</CODE>
</DT>

<DD>
         The parsing was successful.<BR>

         
</DD>

<DT>
<CODE>{eof, EndLine}</CODE>
</DT>

<DD>
         End of file was encountered.<BR>

         
</DD>

<DT>
<CODE>{error, ErrorInfo, EndLine}</CODE>
</DT>

<DD>
         The parsing failed.<BR>

         
</DD>

</DL>

</DIV>

<P><A NAME="fwrite/1"><STRONG><CODE>fwrite(Format) -&#62;</CODE></STRONG></A><BR>
<A NAME="fwrite/3"><STRONG><CODE>fwrite([IoDevice,] Format, Data) -&#62; ok</CODE></STRONG></A><BR>
<A NAME="format/1"><STRONG><CODE>format(Format) -&#62;</CODE></STRONG></A><BR>
<A NAME="format/3"><STRONG><CODE>format([IoDevice,] Format, Data) -&#62; ok</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>IoDevice = io_device()</CODE></STRONG><BR>
<STRONG><CODE>Format = atom() | string()</CODE></STRONG><BR>
<STRONG><CODE>Data = [term()]</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>Writes the items in <CODE>Data</CODE> (<CODE>[]</CODE>) on the standard
         output (<CODE>IoDevice</CODE>) in accordance with <CODE>Format</CODE>.
         <CODE>Format</CODE> contains plain characters which are copied to
         the output device, and control sequences for formatting, see
         below. If <CODE>Format</CODE> is an atom, it is first converted to
         a list with the aid of <CODE>atom_to_list/1</CODE>.
<PRE>
1&#62; <STRONG>io:fwrite(&#34;Hello world!~n&#34;, []).</STRONG>
Hello world!
ok
        
</PRE>

<P>The general format of a control sequence is <CODE>~F.P.PadC</CODE>.
         The character <CODE>C</CODE> determines the type of control sequence
         to be used, <CODE>F</CODE> and <CODE>P</CODE> are optional numeric
         arguments. If <CODE>F</CODE>, <CODE>P</CODE>, or <CODE>Pad</CODE> is <CODE>*</CODE>,
         the next argument in <CODE>Data</CODE> is used as the numeric value
         of <CODE>F</CODE> or <CODE>P</CODE>.
<P><CODE>F</CODE> is the <CODE>field width</CODE> 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 <CODE>*</CODE> characters.
<P><CODE>P</CODE> is the <CODE>precision</CODE> 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 <CODE>within</CODE> is used
         to determine print width.
<P><CODE>Pad</CODE> 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 <CODE>' '</CODE> (space).
<P>The following control sequences are available:
<P>
<DL>

<DT>
<CODE>~</CODE>
</DT>

<DD>
         The character <CODE>~</CODE> is written.<BR>

         
</DD>

<DT>
<CODE>c</CODE>
</DT>

<DD>
 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:<BR>

         
<PRE>
2&#62; <STRONG>io:fwrite(&#34;|~10.5c|~-10.5c|~5c|~n&#34;, [$a, $b, $c]).</STRONG>
|     aaaaa|aaaaa     |ccccc|
ok
            
</PRE>


</DD>

<DT>
<CODE>f</CODE>
</DT>

<DD>
 The argument is a float which is written as
         <CODE>[-]ddd.ddd</CODE>, where the precision is the number of
digits after the decimal point. The default precision is 6
         and it cannot be less than 1.<BR>

         
</DD>

<DT>
<CODE>e</CODE>
</DT>

<DD>
 The argument is a float which is written as
         <CODE>[-]d.ddde+-ddd</CODE>, where the precision is the number
of digits written. The default precision is 6 and it
         cannot be less than 2.<BR>

         
</DD>

<DT>
<CODE>g</CODE>
</DT>

<DD>
 The argument is a float which is written as <CODE>f</CODE>, if
         it is &#62;= 0.1 and &#60; 10000.0. Otherwise, it is written
         in the <CODE>e</CODE> 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 <CODE>f</CODE> format with the
         desired number of significant digits, it is also written
         in the <CODE>e</CODE> format.<BR>

         
</DD>

<DT>
<CODE>s</CODE>
</DT>

<DD>
 Prints the argument with the <CODE>string</CODE> syntax. The
         argument is an <A HREF="javascript:erlhref('../../../../', 'kernel', 'erlang.html#iolist_definition');">I/O
         list</A>, a binary, or an atom. The characters
         are printed without quotes. In this format, the printed
         argument is truncated to the given precision and field
         width.<BR>


         This format can be used for printing any object and
         truncating the output so it fits a specified field:<BR>

         
<PRE>
3&#62; <STRONG>io:fwrite(&#34;|~10w|~n&#34;, [{hey, hey, hey}]).</STRONG>
|**********|
ok
4&#62; <STRONG>io:fwrite(&#34;|~10s|~n&#34;, [io_lib:write({hey, hey, hey})]).</STRONG>
|{hey,hey,h|
ok
            
</PRE>


</DD>

<DT>
<CODE>w</CODE>
</DT>

<DD>
 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 in the default <CODE>g</CODE> format.<BR>

         
</DD>

<DT>
<CODE>p</CODE>
</DT>

<DD>
 Writes the data with standard syntax in the same way as
<CODE>~w</CODE>, but breaks terms whose printed representation
is longer than one line into many lines and indents each
line sensibly. It also tries to detect lists of printable
         characters and to output these as strings. For example:
         <BR>

         
<PRE>
5&#62; <STRONG>T = [{attributes,[[{id,age,1.50000},{mode,explicit},</STRONG>
<STRONG>{typename,&#34;INTEGER&#34;}], [{id,cho},{mode,explicit},{typename,'Cho'}]]},</STRONG>
<STRONG>{typename,'Person'},{tag,{'PRIVATE',3}},{mode,implicit}].</STRONG>
...
6&#62; <STRONG>io:fwrite(&#34;~w~n&#34;, [T]).</STRONG>
[{attributes,[[{id,age,1.50000},{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&#62; <STRONG>io:fwrite(&#34;~p~n&#34;, [T]).</STRONG>
[{attributes,[[{id,age,1.50000},
               {mode,explicit},
               {typename,&#34;INTEGER&#34;}],
              [{id,cho},{mode,explicit},{typename,'Cho'}]]},
 {typename,'Person'},
 {tag,{'PRIVATE',3}},
 {mode,implicit}]
ok
            
</PRE>


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 <CODE>same</CODE> call to
         <CODE>io:fwrite</CODE> or <CODE>io:format</CODE>. For example, using
         <CODE>T</CODE> above:<BR>

         
<PRE>
8&#62; <STRONG>io:fwrite(&#34;Here T = ~p~n&#34;, [T]).</STRONG>
Here T = [{attributes,[[{id,age,1.50000},
                        {mode,explicit},
                        {typename,&#34;INTEGER&#34;}],
                       [{id,cho},{mode,explicit},
                        {typename,'Cho'}]]},
          {typename,'Person'},
          {tag,{'PRIVATE',3}},
          {mode,implicit}]
ok
            
</PRE>


</DD>

<DT>
<CODE>W</CODE>
</DT>

<DD>
 Writes data in the same way as <CODE>~w</CODE>, but takes an
         extra argument which is the maximum depth to which terms
         are printed. Anything below this depth is replaced with
         <CODE>...</CODE>. For example, using <CODE>T</CODE> above:<BR>

         
<PRE>
9&#62; <STRONG>io:fwrite(&#34;~W~n&#34;, [T,9]).</STRONG>
[{attributes,[[{id,age,1.50000},{mode,explicit},{typename|
...}],[{id,cho},{mode|...},{...}]]},{typename,'Person'},{t
ag,{'PRIVATE',3}},{mode,implicit}]
ok
            
</PRE>


If the maximum depth has been reached, then it is
         impossible to read in the resultant output. Also, the
<CODE>|...</CODE> form in a tuple denotes that there are more
elements in the tuple but these are below the print depth.
         <BR>

         
</DD>

<DT>
<CODE>P</CODE>
</DT>

<DD>
 Writes data in the same way as <CODE>~p</CODE>, but takes an
         extra argument which is the maximum depth to which terms
         are printed. Anything below this depth is replaced with
         <CODE>...</CODE>. For example:
         <BR>

         
<PRE>
10&#62; <STRONG>io:fwrite(&#34;~P~n&#34;, [T,9]).</STRONG>
[{attributes,[[{id,age,1.50000},{mode,explicit},
               {typename|...}],
              [{id,cho},{mode|...},{...}]]},
 {typename,'Person'},
 {tag,{'PRIVATE',3}},
 {mode,implicit}]
ok
            
</PRE>


</DD>

<DT>
<CODE>B</CODE>
</DT>

<DD>
         Writes an integer in base 2..36, the default base is
         10. A leading dash is printed for negative integers.
         <BR>
The precision field selects base. For example:<BR>

         
<PRE>
11&#62; <STRONG>io:format(&#34;~.16B~n&#34;, [31]).</STRONG>
1F
ok
12&#62; <STRONG>io:format(&#34;~.2B~n&#34;, [-19]).</STRONG>
-10011
ok
13&#62; <STRONG>io:format(&#34;~.36B~n&#34;, [5*36+35]).</STRONG>
5Z
ok
            
</PRE>

         
</DD>

<DT>
<CODE>X</CODE>
</DT>

<DD>
         Like <CODE>B</CODE>, but takes an extra argument that is a
         prefix to insert before the number, but after the leading
         dash, if any.<BR>

         The prefix can be a possibly deep list of characters or
         an atom.<BR>

         
<PRE>
14&#62; <STRONG>io:format(&#34;~X~n&#34;, [31,&#34;10#&#34;]).</STRONG>
10#31
ok
15&#62; <STRONG>io:format(&#34;~.16X~n&#34;, [-31,&#34;0x&#34;]).</STRONG>
-0x1F
ok
            
</PRE>

         
</DD>

<DT>
<CODE>#</CODE>
</DT>

<DD>
         Like <CODE>B</CODE>, but prints the number with an Erlang style
         '#'-separated base prefix.<BR>

         
<PRE>
16&#62; <STRONG>io:format(&#34;~.10#~n&#34;, [31]).</STRONG>
10#31
ok
17&#62; <STRONG>io:format(&#34;~.16#~n&#34;, [-31]).</STRONG>
-16#1F
ok
            
</PRE>

         
</DD>

<DT>
<CODE>b</CODE>
</DT>

<DD>
         Like <CODE>B</CODE>, but prints lowercase letters.<BR>

         
</DD>

<DT>
<CODE>x</CODE>
</DT>

<DD>
         Like <CODE>X</CODE>, but prints lowercase letters.<BR>

         
</DD>

<DT>
<CODE>+</CODE>
</DT>

<DD>
         Like <CODE>#</CODE>, but prints lowercase letters.<BR>

         
</DD>

<DT>
<CODE>n</CODE>
</DT>

<DD>
         Writes a new line.<BR>


</DD>

<DT>
<CODE>i</CODE>
</DT>

<DD>
         Ignores the next term.<BR>

         
</DD>

</DL>

<P>Returns:
<P>
<DL>

<DT>
<CODE>ok</CODE>
</DT>

<DD>
The formatting succeeded.<BR>

         
</DD>

</DL>

<P>If an error occurs, there is no output. For example:
<PRE>
18&#62; <STRONG>io:fwrite(&#34;~s ~w ~i ~w ~c ~n&#34;,['abc def', 'abc def', {foo, 1},{foo, 1}, 65]).</STRONG>
abc def 'abc def'  {foo, 1} A
ok
19&#62; <STRONG>io:fwrite(&#34;~s&#34;, [65]).</STRONG>
** exited: {badarg,[{io,format,[&#60;0.22.0&#62;,&#34;~s&#34;,&#34;A&#34;]},
                    {erl_eval,do_apply,5},
                    {shell,exprs,6},
                    {shell,eval_loop,2}]} **
        
</PRE>

<P>In this example, an attempt was made to output the single
         character '65' with the aid of the string formatting directive
         &#34;~s&#34;.
<P>The two functions <CODE>fwrite</CODE> and <CODE>format</CODE> are
identical. The old name <CODE>format</CODE> has been retained for
backwards compatibility, while the new name <CODE>fwrite</CODE> has
been added as a logical complement to <CODE>fread</CODE>.
</DIV>

<P><A NAME="fread/3"><STRONG><CODE>fread([IoDevice,] Prompt, Format) -&#62; Result</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>IoDevice = io_device()</CODE></STRONG><BR>
<STRONG><CODE>Prompt = atom() | string()</CODE></STRONG><BR>
<STRONG><CODE>Format = string()</CODE></STRONG><BR>
<STRONG><CODE>Result = {ok, Terms} | eof | {error, What}</CODE></STRONG><BR>
<STRONG><CODE>Terms = [term()]</CODE></STRONG><BR>
<STRONG><CODE>What = term()</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>Reads characters from the standard input (<CODE>IoDevice</CODE>),
prompting it with <CODE>Prompt</CODE>. Interprets the characters in
accordance with <CODE>Format</CODE>. <CODE>Format</CODE> contains control
         sequences which directs the interpretation of the input.
<P><CODE>Format</CODE> may contain:
<P>
<UL>

<LI>
 White space characters (SPACE, TAB and NEWLINE) which
cause input to be read to the next non-white space
character.<BR>

         
</LI>


<LI>
 Ordinary characters which must match the next input
         character.<BR>

         
</LI>


<LI>
 Control sequences, which have the general format
         <CODE>~*FC</CODE>. The character <CODE>*</CODE> is an optional return
         suppression character. It provides a method to specify a
         field which is to be omitted. <CODE>F</CODE> is the <CODE>field
                width</CODE> of the input field and <CODE>C</CODE> determines the
         type of control sequence.<BR>


         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:<BR>

         
<DL>

<DT>
<CODE>~</CODE>
</DT>

<DD>
                A single <CODE>~</CODE> is expected in the input.<BR>

         
</DD>

<DT>
<CODE>d</CODE>
</DT>

<DD>
                A decimal integer is expected.<BR>

         
</DD>

<DT>
<CODE>u</CODE>
</DT>

<DD>
                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.<BR>

         
</DD>

<DT>
<CODE>-</CODE>
</DT>

<DD>
                An optional sign character is expected. A sign
                 character '-' gives the return value <CODE>-1</CODE>. Sign
                 character '+' or none gives <CODE>1</CODE>. The field width
                 parameter is ignored. Leading white-space characters
                 are not skipped.<BR>

         
</DD>

<DT>
<CODE>#</CODE>
</DT>

<DD>
                An integer in base 2..36 with Erlang-style base
                 prefix (for example <CODE>&#34;16#ffff&#34;</CODE>) is expected.<BR>

         
</DD>

<DT>
<CODE>f</CODE>
</DT>

<DD>
 A floating point number is expected. It must follow
the Erlang floating point number syntax.<BR>

         
</DD>

<DT>
<CODE>s</CODE>
</DT>

<DD>
 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.<BR>

         
</DD>

<DT>
<CODE>a</CODE>
</DT>

<DD>
 Similar to <CODE>s</CODE>, but the resulting string is
converted into an atom.<BR>

         
</DD>

<DT>
<CODE>c</CODE>
</DT>

<DD>
 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 <CODE>s</CODE>. All
                 characters are returned.<BR>

         
</DD>

<DT>
<CODE>l</CODE>
</DT>

<DD>
 Returns the number of characters which have been
                 scanned up to that point, including white-space
                 characters.<BR>

         
</DD>

</DL>


It returns:<BR>

         
<DL>

<DT>
<CODE>{ok, Terms}</CODE>
</DT>

<DD>
 The read was successful and <CODE>Terms</CODE> is the list
                 of successfully matched and read items.<BR>

         
</DD>

<DT>
<CODE>eof</CODE>
</DT>

<DD>
                End of file was encountered.<BR>

         
</DD>

<DT>
<CODE>{error, What}</CODE>
</DT>

<DD>
 The read operation failed and the parameter
                 <CODE>What</CODE> gives a hint about the error.<BR>

         
</DD>

</DL>

         
</LI>


</UL>

<P>Examples:
<PRE>
20&#62; <STRONG>io:fread('enter&#62;', &#34;~f~f~f&#34;).</STRONG>
enter&#62;<STRONG>1.9 35.5e3 15.0</STRONG>
{ok,[1.90000,3.55000e+4,15.0000]}
21&#62; <STRONG>io:fread('enter&#62;', &#34;~10f~d&#34;).</STRONG>
enter&#62;     <STRONG>5.67899</STRONG>
{ok, [5.67800, 99]}
22&#62; <STRONG>io:fread('enter&#62;', &#34;:~10s:~10c:&#34;).</STRONG>
enter&#62;<STRONG>:</STRONG>   <STRONG>alan</STRONG>   <STRONG>:</STRONG>   <STRONG>joe</STRONG>    <STRONG>:</STRONG>
{ok, [&#34;alan&#34;, &#34;   joe    &#34;]}
        
</PRE>

</DIV>

<P><A NAME="scan_erl_exprs/1"><STRONG><CODE>scan_erl_exprs(Prompt) -&#62;</CODE></STRONG></A><BR>
<A NAME="scan_erl_exprs/3"><STRONG><CODE>scan_erl_exprs([IoDevice,] Prompt, StartLine) -&#62; Result
</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>IoDevice = io_device()</CODE></STRONG><BR>
<STRONG><CODE>Prompt = atom() | string()</CODE></STRONG><BR>
<STRONG><CODE>StartLine = int()</CODE></STRONG><BR>
<STRONG><CODE>Result = {ok, Tokens, EndLine} | {eof, EndLine} |
         {error, ErrorInfo, EndLine}</CODE></STRONG><BR>
<STRONG><CODE>Tokens -- see erl_scan(3)</CODE></STRONG><BR>
<STRONG><CODE>EndLine = int()</CODE></STRONG><BR>
<STRONG><CODE>ErrorInfo -- see section Error Information below</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>Reads data from the standard input (<CODE>IoDevice</CODE>),
prompting it with <CODE>Prompt</CODE>. Reading starts at line number
<CODE>StartLine</CODE> (1). The data is tokenized as if it were a
         sequence of Erlang expressions until a final <CODE>'.'</CODE> is
         reached. This token is also returned. It returns:
<P>
<DL>

<DT>
<CODE>{ok, Tokens, EndLine}</CODE>
</DT>

<DD>
         The tokenization succeeded.<BR>

         
</DD>

<DT>
<CODE>{eof, EndLine}</CODE>
</DT>

<DD>
         End of file was encountered.<BR>

         
</DD>

<DT>
<CODE>{error, ErrorInfo, EndLine}</CODE>
</DT>

<DD>
         An error occurred.<BR>

         
</DD>

</DL>

<P>Example:
<PRE>
23&#62; <STRONG>io:scan_erl_exprs('enter&#62;').</STRONG>
enter&#62;<STRONG>abc(), &#34;hey&#34;.</STRONG>
{ok,[{atom,1,abc},{'(',1},{')',1},{',',1},{string,1,&#34;hey&#34;},{dot,1}],2}
24&#62; <STRONG>io:scan_erl_exprs('enter&#62;').</STRONG>
enter&#62;<STRONG>1.0er.</STRONG>
{error,{1,erl_scan,{illegal,float}},2}
        
</PRE>

</DIV>

<P><A NAME="scan_erl_form/1"><STRONG><CODE>scan_erl_form(Prompt) -&#62;</CODE></STRONG></A><BR>
<A NAME="scan_erl_form/3"><STRONG><CODE>scan_erl_form([IoDevice,] Prompt, StartLine) -&#62; Result
</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>IoDevice = io_device()</CODE></STRONG><BR>
<STRONG><CODE>Prompt = atom() | string()</CODE></STRONG><BR>
<STRONG><CODE>StartLine = int()</CODE></STRONG><BR>
<STRONG><CODE>Result = {ok, Tokens, EndLine} | {eof, EndLine} |
         {error, ErrorInfo, EndLine}</CODE></STRONG><BR>
<STRONG><CODE>Tokens -- see erl_scan(3)</CODE></STRONG><BR>
<STRONG><CODE>EndLine = int()</CODE></STRONG><BR>
<STRONG><CODE>ErrorInfo -- see section Error Information below</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>Reads data from the standard input (<CODE>IoDevice</CODE>),
prompting it with <CODE>Prompt</CODE>. Starts reading at line number
<CODE>StartLine</CODE> (1). The data is tokenized as if it were an
         Erlang form - one of the valid Erlang expressions in an
         Erlang source file - until a final <CODE>'.'</CODE> is reached.
         This last token is also returned. The return values are the
         same as for <CODE>scan_erl_exprs/1,2,3</CODE> above.
</DIV>

<P><A NAME="parse_erl_exprs/1"><STRONG><CODE>parse_erl_exprs(Prompt) -&#62;</CODE></STRONG></A><BR>
<A NAME="parse_erl_exprs/3"><STRONG><CODE>parse_erl_exprs([IoDevice,] Prompt, StartLine) -&#62; Result
</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>IoDevice = io_device()</CODE></STRONG><BR>
<STRONG><CODE>Prompt = atom() | string()</CODE></STRONG><BR>
<STRONG><CODE>StartLine = int()</CODE></STRONG><BR>
<STRONG><CODE>Result = {ok, Expr_list, EndLine} | {eof, EndLine} |
         {error, ErrorInfo, EndLine}</CODE></STRONG><BR>
<STRONG><CODE>Expr_list -- see erl_parse(3)</CODE></STRONG><BR>
<STRONG><CODE>EndLine = int()</CODE></STRONG><BR>
<STRONG><CODE>ErrorInfo -- see section Error Information below</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>Reads data from the standard input (<CODE>IoDevice</CODE>),
         prompting it with <CODE>Prompt</CODE>. Starts reading at line number
<CODE>StartLine</CODE> (1). The data is tokenized and parsed as if
         it were a sequence of Erlang expressions until a final '.' is
         reached. It returns:
<P>
<DL>

<DT>
<CODE>{ok, Expr_list, EndLine}</CODE>
</DT>

<DD>
         The parsing was successful.<BR>

         
</DD>

<DT>
<CODE>{eof, EndLine}</CODE>
</DT>

<DD>
         End of file was encountered.<BR>

         
</DD>

<DT>
<CODE>{error, ErrorInfo, EndLine}</CODE>
</DT>

<DD>
         An error occurred.<BR>

         
</DD>

</DL>

<P>Example:
<PRE>
25&#62; <STRONG>io:parse_erl_exprs('enter&#62;').</STRONG>
enter&#62;<STRONG>abc(), &#34;hey&#34;.</STRONG>
{ok, [{call,1,{atom,1,abc},[]},{string,1,&#34;hey&#34;}],2}
26&#62; <STRONG>io:parse_erl_exprs ('enter&#62;').</STRONG>
enter&#62;<STRONG>abc(&#34;hey&#34;.</STRONG>
{error,{1,erl_parse,[&#34;syntax error before: &#34;,[&#34;'.'&#34;]]},2}
        
</PRE>

</DIV>

<P><A NAME="parse_erl_form/1"><STRONG><CODE>parse_erl_form(Prompt) -&#62;</CODE></STRONG></A><BR>
<A NAME="parse_erl_form/3"><STRONG><CODE>parse_erl_form([IoDevice,] Prompt, StartLine) -&#62; Result
</CODE></STRONG></A><BR>

<DIV CLASS=REFBODY><P>Types:
  <DIV CLASS=REFTYPES>
<P>
<STRONG><CODE>IoDevice = io_device()</CODE></STRONG><BR>
<STRONG><CODE>Prompt = atom() | string()</CODE></STRONG><BR>
<STRONG><CODE>StartLine = int()</CODE></STRONG><BR>
<STRONG><CODE>Result = {ok, AbsForm, EndLine} | {eof, EndLine} |
         {error, ErrorInfo, EndLine}</CODE></STRONG><BR>
<STRONG><CODE>AbsForm -- see erl_parse(3)</CODE></STRONG><BR>
<STRONG><CODE>EndLine = int()</CODE></STRONG><BR>
<STRONG><CODE>ErrorInfo -- see section Error Information below</CODE></STRONG><BR>

  </DIV>
</DIV>

<DIV CLASS=REFBODY>

<P>Reads data from the standard input (<CODE>IoDevice</CODE>),
prompting it with <CODE>Prompt</CODE>. Starts reading at line number
<CODE>StartLine</CODE> (1). The data is tokenized and parsed as if
         it were an Erlang form - one of the valid Erlang expressions
         in an Erlang source file - until a final '.' is reached. It
         returns:
<P>
<DL>

<DT>
<CODE>{ok, AbsForm, EndLine}</CODE>
</DT>

<DD>
         The parsing was successful.<BR>

         
</DD>

<DT>
<CODE>{eof, EndLine}</CODE>
</DT>

<DD>
         End of file was encountered.<BR>

         
</DD>

<DT>
<CODE>{error, ErrorInfo, EndLine}</CODE>
</DT>

<DD>
         An error occurred.<BR>

         
</DD>

</DL>

</DIV>

<H3>Standard Input/Output</H3>
<DIV CLASS=REFBODY>

<P>All Erlang processes have a default standard IO device. This
device is used when no <CODE>IoDevice</CODE> argument is specified in
the above function calls. However, it is sometimes desirable to
use an explicit <CODE>IoDevice</CODE> 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
<CODE>standard_io</CODE> has this special meaning. The following example
illustrates this:
<PRE>
27&#62; <STRONG>io:read('enter&#62;').</STRONG>
enter&#62;<STRONG>foo.</STRONG>
{ok,foo}
28&#62; <STRONG>io:read(standard_io, 'enter&#62;').</STRONG>
enter&#62;<STRONG>bar.</STRONG>
{ok,bar}
    
</PRE>

<P>There is always a process registered under the name of
<CODE>user</CODE>. This can be used for sending output to the user.
</DIV>

<H3>Error Information</H3>
<DIV CLASS=REFBODY>

<P>The <CODE>ErrorInfo</CODE> mentioned above is the standard
<CODE>ErrorInfo</CODE> structure which is returned from all IO modules.
It has the format:
<PRE>
{ErrorLine, Module, ErrorDescriptor}
    
</PRE>

<P>A string which describes the error is obtained with the following
call:
<PRE>
apply(Module, format_error, ErrorDescriptor)
    
</PRE>

</DIV>

<H3>AUTHORS</H3>
<DIV CLASS=REFBODY>
Robert Virding - support@erlang.ericsson.se<BR>

</DIV>
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<A HREF="http://www.erlang.se">Ericsson AB</A><BR>
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