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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 the IoDevice returned
       by file:open/2.

       For a description of the I/O protocols, see section The Erlang I/O Protocol in the  User's
       Guide.

   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).

       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}

       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)