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NAME

       io - Standard I/O server interface functions.

DESCRIPTION

       This module provides an interface to standard Erlang I/O servers. The output functions all
       return ok if they are successful, or exit if they are not.

       All functions in this module have an optional parameter IoDevice. If included, it must  be
       the  pid  of a process that handles the I/O protocols. Normally, it is a IoDevice returned
       by file:open/2. If no IoDevice is given, standard_io is used.

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

   Warning:
       As  from  Erlang/OTP  R13A,  data  supplied  to  function  put_chars/2  is  to  be  in the
       unicode:chardata() format. This means that programs supplying binaries  to  this  function
       must convert them to UTF-8 before trying to output the data on an I/O device.

       If  an  I/O  device  is  set  in binary mode, functions get_chars/2,3 and get_line/1,2 can
       return binaries instead of lists. The binaries are, as from Erlang/OTP  R13A,  encoded  in
       UTF-8.

       To work with binaries in ISO Latin-1 encoding, use the file module instead.

       For conversion functions between character encodings, see the unicode module.

DATA TYPES

       device() = atom() | pid()

              An  I/O  device,  either  standard_io,  standard_error, a registered name, or a pid
              handling I/O protocols (returned from file:open/2).

              For more information about the  built-in  devices  see  Standard  Input/Output  and
              Standard Error.

       opt_pair() =
           {binary, boolean()} |
           {echo, boolean()} |
           {expand_fun, expand_fun()} |
           {encoding, encoding()}

       expand_fun() =
           fun((term()) -> {yes | no, string(), [string(), ...]})

       encoding() =
           latin1 | unicode | utf8 | utf16 | utf32 |
           {utf16, big | little} |
           {utf32, big | little}

       setopt() = binary | list | opt_pair()

       format() = atom() | string() | binary()

       location() = erl_anno:location()

       prompt() = atom() | unicode:chardata()

       server_no_data() = {error, ErrorDescription :: term()} | eof

              What the I/O server sends when there is no data.

EXPORTS

       columns() -> {ok, integer() >= 1} | {error, enotsup}

       columns(IoDevice) -> {ok, integer() >= 1} | {error, enotsup}

              Types:

                 IoDevice = device()

              Retrieves the number of columns of the IoDevice (that is, the width of a terminal).
              The function succeeds for terminal devices and returns  {error,  enotsup}  for  all
              other I/O devices.

       format(Format) -> ok

       format(Format, Data) -> ok

       format(IoDevice, Format, Data) -> ok

       fwrite(Format) -> ok

       fwrite(Format, Data) -> ok

       fwrite(IoDevice, Format, Data) -> ok

              Types:

                 IoDevice = device()
                 Format = format()
                 Data = [term()]

              Writes  the items in Data ([]) on the standard output (IoDevice) in accordance with
              Format. Format contains plain characters that are copied to the output device,  and
              control  sequences  for formatting, see below. If Format is an atom or a binary, it
              is first converted to a list with the aid of  atom_to_list/1  or  binary_to_list/1.
              Example:

              1> io:fwrite("Hello world!~n", []).
              Hello world!
              ok

              The general format of a control sequence is ~F.P.PadModC.

              The  character C determines the type of control sequence to be used. It is the only
              required field. All of F, P, Pad, and Mod are optional. For example, to use a # for
              Pad but use the default values for F and P, you can write ~..#C.

                * F  is  the field width of the printed argument. A negative value means that the
                  argument is left-justified within the field, otherwise right-justified.  If  no
                  field  width is specified, the required print width is used. If the field width
                  specified is too small, the whole field is filled with * characters.

                * P is the precision of the printed argument. A  default  value  is  used  if  no
                  precision  is specified. The interpretation of precision depends on the control
                  sequences. Unless otherwise specified, argument within  is  used  to  determine
                  print width.

                * Pad  is  the  padding  character. This is the character used to pad the printed
                  representation of the argument so that it conforms to the specified field width
                  and  precision.  Only  one  padding  character  can  be specified and, whenever
                  applicable, it is used for both the field  width  and  precision.  The  default
                  padding character is ' ' (space).

                * Mod  is  the  control  sequence  modifier.  This is one or more characters that
                  change the interpretation of Data. The current modifiers  are  t,  for  Unicode
                  translation, and l, for stopping p and P from detecting printable characters.

              If  F,  P, or Pad is a * character, the next argument in Data is used as the value.
              For example:

              1> io:fwrite("~*.*.0f~n",[9, 5, 3.14159265]).
              003.14159
              ok

              To use a literal * character as Pad, it must be passed as an argument:

              2> io:fwrite("~*.*.*f~n",[9, 5, $*, 3.14159265]).
              **3.14159
              ok

              Available control sequences:

                ~:
                  Character ~ is written.

                c:
                  The argument is a number that is interpreted as an ASCII code. The precision is
                  the  number  of times the character is printed and defaults to the field width,
                  which in turn defaults to 1. Example:

                1> io:fwrite("|~10.5c|~-10.5c|~5c|~n", [$a, $b, $c]).
                |     aaaaa|bbbbb     |ccccc|
                ok

                  If the Unicode translation modifier (t) is in effect, the integer argument  can
                  be  any number representing a valid Unicode codepoint, otherwise it is to be an
                  integer less than or equal to 255, otherwise it is masked with 16#FF:

                2> io:fwrite("~tc~n",[1024]).
                \x{400}
                ok
                3> io:fwrite("~c~n",[1024]).
                ^@
                ok

                f:
                  The argument is a float that is written as [-]ddd.ddd, where the  precision  is
                  the number of digits after the decimal point. The default precision is 6 and it
                  cannot be < 1.

                e:
                  The argument is a float that is written as [-]d.ddde+-ddd, where the  precision
                  is  the number of digits written. The default precision is 6 and it cannot be <
                  2.

                g:
                  The argument is a float that is written as f, if it is >= 0.1  and  <  10000.0.
                  Otherwise,  it  is  written  in  the  e  format. The precision is the number of
                  significant digits. It defaults to 6 and is not to be  <  2.  If  the  absolute
                  value  of  the  float  does not allow it to be written in the f format with the
                  desired number of significant digits, it is also written in the e format.

                s:
                  Prints the argument with the string syntax. The  argument  is,  if  no  Unicode
                  translation  modifier is present, an iolist(), a binary(), or an atom(). If the
                  Unicode  translation   modifier   (t)   is   in   effect,   the   argument   is
                  unicode:chardata(),  meaning  that  binaries  are  in UTF-8. The characters are
                  printed without  quotes.  The  string  is  first  truncated  by  the  specified
                  precision  and  then  padded  and  justified  to the specified field width. The
                  default precision is the field width.

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

                1> io:fwrite("|~10w|~n", [{hey, hey, hey}]).
                |**********|
                ok
                2> io:fwrite("|~10s|~n", [io_lib:write({hey, hey, hey})]).
                |{hey,hey,h|
                3> io:fwrite("|~-10.8s|~n", [io_lib:write({hey, hey, hey})]).
                |{hey,hey  |
                ok

                  A  list  with  integers > 255 is considered an error if the Unicode translation
                  modifier is not specified:

                4> io:fwrite("~ts~n",[[1024]]).
                \x{400}
                ok
                5> io:fwrite("~s~n",[[1024]]).
                ** exception error: bad argument
                     in function  io:format/3
                        called as io:format(<0.53.0>,"~s~n",[[1024]])

                w:
                  Writes data with the standard syntax. This is  used  to  output  Erlang  terms.
                  Atoms  are  printed  within  quotes  if  they  contain  embedded  non-printable
                  characters. Atom characters > 255 are escaped unless  the  Unicode  translation
                  modifier  (t) is used. Floats are printed accurately as the shortest, correctly
                  rounded string.

                p:
                  Writes the data with standard syntax in the same way as ~w,  but  breaks  terms
                  whose  printed  representation  is  longer  than  one  line into many lines and
                  indents each line sensibly. Left-justification is not supported. It also  tries
                  to  detect  flat lists of printable characters and output these as strings. For
                  example:

                1> T = [{attributes,[[{id,age,1.50000},{mode,explicit},
                {typename,"INTEGER"}], [{id,cho},{mode,explicit},{typename,'Cho'}]]},
                {typename,'Person'},{tag,{'PRIVATE',3}},{mode,implicit}].
                ...
                2> io:fwrite("~w~n", [T]).
                [{attributes,[[{id,age,1.5},{mode,explicit},{typename,
                [73,78,84,69,71,69,82]}],[{id,cho},{mode,explicit},{typena
                me,'Cho'}]]},{typename,'Person'},{tag,{'PRIVATE',3}},{mode
                ,implicit}]
                ok
                3> io:fwrite("~62p~n", [T]).
                [{attributes,[[{id,age,1.5},
                               {mode,explicit},
                               {typename,"INTEGER"}],
                              [{id,cho},{mode,explicit},{typename,'Cho'}]]},
                 {typename,'Person'},
                 {tag,{'PRIVATE',3}},
                 {mode,implicit}]
                ok

                  The field width specifies the maximum line  length.  It  defaults  to  80.  The
                  precision  specifies  the  initial  indentation of the term. It defaults to the
                  number of characters printed on this line  in  the  same  call  to  write/1  or
                  format/1,2,3. For example, using T above:

                4> io:fwrite("Here T = ~62p~n", [T]).
                Here T = [{attributes,[[{id,age,1.5},
                                        {mode,explicit},
                                        {typename,"INTEGER"}],
                                       [{id,cho},
                                        {mode,explicit},
                                        {typename,'Cho'}]]},
                          {typename,'Person'},
                          {tag,{'PRIVATE',3}},
                          {mode,implicit}]
                ok

                  As  from  Erlang/OTP  21.0, a field width of value 0 can be used for specifying
                  that a line is infinitely long, which means that no line breaks  are  inserted.
                  For example:

                5> io:fwrite("~0p~n", [lists:seq(1, 30)]).
                [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30]
                ok

                  When  the  modifier  l  is specified, no detection of printable character lists
                  takes place, for example:

                6> S = [{a,"a"}, {b, "b"}], io:fwrite("~15p~n", [S]).
                [{a,"a"},
                 {b,"b"}]
                ok
                7> io:fwrite("~15lp~n", [S]).
                [{a,[97]},
                 {b,[98]}]
                ok

                  The Unicode translation modifier t specifies how to  treat  characters  outside
                  the  Latin-1 range of codepoints, in atoms, strings, and binaries. For example,
                  printing an atom containing a character > 255:

                8> io:fwrite("~p~n",[list_to_atom([1024])]).
                ok
                9> io:fwrite("~tp~n",[list_to_atom([1024])]).
                ok

                  By default, Erlang only detects lists of characters in  the  Latin-1  range  as
                  strings,   but   the  +pc  unicode  flag  can  be  used  to  change  this  (see
                  printable_range/0 for details). For example:

                10> io:fwrite("~p~n",[[214]]).
                "Ö"
                ok
                11> io:fwrite("~p~n",[[1024]]).
                [1024]
                ok
                12> io:fwrite("~tp~n",[[1024]]).
                [1024]
                ok

                  but if Erlang was started with +pc unicode:

                13> io:fwrite("~p~n",[[1024]]).
                [1024]
                ok
                14> io:fwrite("~tp~n",[[1024]]).
                "Ѐ"
                ok

                  Similarly, binaries that look like UTF-8 encoded strings are  output  with  the
                  binary string syntax if the t modifier is specified:

                15> io:fwrite("~p~n", [<<208,128>>]).
                <<208,128>>
                ok
                16> io:fwrite("~tp~n", [<<208,128>>]).
                <<"Ѐ"/utf8>>
                ok
                17> io:fwrite("~tp~n", [<<128,128>>]).
                <<128,128>>
                ok

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

                8> io:fwrite("~W~n", [T,9]).
                [{attributes,[[{id,age,1.5},{mode,explicit},{typename,...}],
                [{id,cho},{mode,...},{...}]]},{typename,'Person'},
                {tag,{'PRIVATE',3}},{mode,implicit}]
                ok

                  If  the  maximum  depth  is reached, it cannot be read in the resultant output.
                  Also, the ,... form in a tuple denotes that there  are  more  elements  in  the
                  tuple but these are below the print depth.

                P:
                  Writes  data  in  the  same  way as ~p, but takes an extra argument that is the
                  maximum depth to which terms are printed. Anything below this depth is replaced
                  with ..., for example:

                9> io:fwrite("~62P~n", [T,9]).
                [{attributes,[[{id,age,1.5},{mode,explicit},{typename,...}],
                              [{id,cho},{mode,...},{...}]]},
                 {typename,'Person'},
                 {tag,{'PRIVATE',3}},
                 {mode,implicit}]
                ok

                B:
                  Writes  an  integer  in  base  2-36,  the default base is 10. A leading dash is
                  printed for negative integers.

                  The precision field selects base, for example:

                1> io:fwrite("~.16B~n", [31]).
                1F
                ok
                2> io:fwrite("~.2B~n", [-19]).
                -10011
                ok
                3> io:fwrite("~.36B~n", [5*36+35]).
                5Z
                ok

                X:
                  Like B, but takes an extra argument that is  a  prefix  to  insert  before  the
                  number, but after the leading dash, if any.

                  The prefix can be a possibly deep list of characters or an atom. Example:

                1> io:fwrite("~X~n", [31,"10#"]).
                10#31
                ok
                2> io:fwrite("~.16X~n", [-31,"0x"]).
                -0x1F
                ok

                #:
                  Like  B,  but  prints  the number with an Erlang style #-separated base prefix.
                  Example:

                1> io:fwrite("~.10#~n", [31]).
                10#31
                ok
                2> io:fwrite("~.16#~n", [-31]).
                -16#1F
                ok

                b:
                  Like B, but prints lowercase letters.

                x:
                  Like X, but prints lowercase letters.

                +:
                  Like #, but prints lowercase letters.

                n:
                  Writes a new line.

                i:
                  Ignores the next term.

              The function returns:

                ok:
                  The formatting succeeded.

              If an error occurs, there is no output. Example:

              1> io:fwrite("~s ~w ~i ~w ~c ~n",['abc def', 'abc def', {foo, 1},{foo, 1}, 65]).
              abc def 'abc def'  {foo,1} A
              ok
              2> io:fwrite("~s", [65]).
              ** exception error: bad argument
                   in function  io:format/3
                      called as io:format(<0.53.0>,"~s","A")

              In this example, an attempt was made to output the single character 65 with the aid
              of the string formatting directive "~s".

       fread(Prompt, Format) -> Result

       fread(IoDevice, Prompt, Format) -> Result

              Types:

                 IoDevice = device()
                 Prompt = prompt()
                 Format = format()
                 Result =
                     {ok, Terms :: [term()]} |
                     {error, {fread, FreadError :: io_lib:fread_error()}} |
                     server_no_data()
                 server_no_data() = {error, ErrorDescription :: term()} | eof

              Reads  characters  from  the  standard  input (IoDevice), prompting it with Prompt.
              Interprets the characters  in  accordance  with  Format.  Format  contains  control
              sequences that directs the interpretation of the input.

              Format can contain the following:

                * Whitespace  characters (Space, Tab, and Newline) that cause input to be read to
                  the next non-whitespace character.

                * Ordinary characters that must match the next input character.

                * Control sequences, which have the general format ~*FMC, where:

                  * Character * is an optional return suppression character. It provides a method
                    to specify a field that is to be omitted.

                  * F is the field width of the input field.

                  * M  is  an  optional  translation  modifier (of which t is the only supported,
                    meaning Unicode translation).

                  * C determines the type of control sequence.

                  Unless otherwise specified, leading  whitespace  is  ignored  for  all  control
                  sequences. An input field cannot be more than one line wide.

                  Available control sequences:

                  ~:
                    A single ~ is expected in the input.

                  d:
                    A decimal integer is expected.

                  u:
                    An  unsigned  integer  in base 2-36 is expected. The field width parameter is
                    used to specify base. Leading whitespace characters are not skipped.

                  -:
                    An optional sign character is expected. A sign character - gives return value
                    -1.  Sign  character + or none gives 1. The field width parameter is ignored.
                    Leading whitespace characters are not skipped.

                  #:
                    An  integer  in  base  2-36  with  Erlang-style  base  prefix  (for  example,
                    "16#ffff") is expected.

                  f:
                    A floating point number is expected. It must follow the Erlang floating point
                    number syntax.

                  s:
                    A string of non-whitespace characters is read. If  a  field  width  has  been
                    specified,  this  number  of  characters are read and all trailing whitespace
                    characters are stripped. An Erlang string (list of characters) is returned.

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

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

                  a:
                    Similar to s, but the resulting string is converted into an atom.

                  c:
                    The number of characters equal to the field width are read (default is 1) and
                    returned  as  an  Erlang  string.  However,  leading  and trailing whitespace
                    characters are not omitted as they are with s. All characters are returned.

                    The Unicode translation modifier works as with s:

                  1> io:fread("Prompt> ","~c").
                  Prompt> <Character beyond latin1 range not printable in this medium>
                  {error,{fread,string}}
                  2> io:fread("Prompt> ","~tc").
                  Prompt> <Character beyond latin1 range not printable in this medium>
                  {ok,[[1091]]}

                  l:
                    Returns the number of characters that have been scanned  up  to  that  point,
                    including whitespace characters.

                  The function returns:

                  {ok, Terms}:
                    The  read  was  successful  and Terms is the list of successfully matched and
                    read items.

                  eof:
                    End of file was encountered.

                  {error, FreadError}:
                    The reading failed and FreadError gives a hint about the error.

                  {error, ErrorDescription}:
                    The read operation failed and parameter ErrorDescription gives a  hint  about
                    the error.

              Examples:

              20> io:fread('enter>', "~f~f~f").
              enter>1.9 35.5e3 15.0
              {ok,[1.9,3.55e4,15.0]}
              21> io:fread('enter>', "~10f~d").
              enter>     5.67899
              {ok,[5.678,99]}
              22> io:fread('enter>', ":~10s:~10c:").
              enter>:   alan   :   joe    :
              {ok, ["alan", "   joe    "]}

       get_chars(Prompt, Count) -> Data | server_no_data()

       get_chars(IoDevice, Prompt, Count) -> Data | server_no_data()

              Types:

                 IoDevice = device()
                 Prompt = prompt()
                 Count = integer() >= 0
                 Data = string() | unicode:unicode_binary()
                 server_no_data() = {error, ErrorDescription :: term()} | eof

              Reads Count characters from standard input (IoDevice), prompting it with Prompt.

              The function returns:

                Data:
                  The  input  characters.  If  the  I/O  device  supports  Unicode,  the data can
                  represent codepoints > 255 (the latin1 range). If the  I/O  server  is  set  to
                  deliver  binaries,  they  are  encoded  in UTF-8 (regardless of whether the I/O
                  device supports Unicode).

                eof:
                  End of file was encountered.

                {error, ErrorDescription}:
                  Other (rare) error condition, such as {error, estale} if reading  from  an  NFS
                  file system.

       get_line(Prompt) -> Data | server_no_data()

       get_line(IoDevice, Prompt) -> Data | server_no_data()

              Types:

                 IoDevice = device()
                 Prompt = prompt()
                 Data = string() | unicode:unicode_binary()
                 server_no_data() = {error, ErrorDescription :: term()} | eof

              Reads a line from the standard input (IoDevice), prompting it with Prompt.

              The function returns:

                Data:
                  The  characters  in the line terminated by a line feed (or end of file). If the
                  I/O device supports Unicode, the data  can  represent  codepoints  >  255  (the
                  latin1  range).  If the I/O server is set to deliver binaries, they are encoded
                  in UTF-8 (regardless of if the I/O device supports Unicode).

                eof:
                  End of file was encountered.

                {error, ErrorDescription}:
                  Other (rare) error condition, such as {error, estale} if reading  from  an  NFS
                  file system.

       getopts() -> [opt_pair()] | {error, Reason}

       getopts(IoDevice) -> [opt_pair()] | {error, Reason}

              Types:

                 IoDevice = device()
                 Reason = term()

              Requests  all available options and their current values for a specific I/O device,
              for example:

              1> {ok,F} = file:open("/dev/null",[read]).
              {ok,<0.42.0>}
              2> io:getopts(F).
              [{binary,false},{encoding,latin1}]

              Here the file I/O server returns all available options for a file,  which  are  the
              expected  ones,  encoding  and  binary.  However,  the standard shell has some more
              options:

              3> io:getopts().
              [{expand_fun,#Fun<group.0.120017273>},
               {echo,true},
               {binary,false},
               {encoding,unicode}]

              This example is, as can be seen, run in an environment where the terminal  supports
              Unicode input and output.

       nl() -> ok

       nl(IoDevice) -> ok

              Types:

                 IoDevice = device()

              Writes new line to the standard output (IoDevice).

       parse_erl_exprs(Prompt) -> Result

       parse_erl_exprs(IoDevice, Prompt) -> Result

       parse_erl_exprs(IoDevice, Prompt, StartLocation) -> Result

       parse_erl_exprs(IoDevice, Prompt, StartLocation, Options) ->
                          Result

              Types:

                 IoDevice = device()
                 Prompt = prompt()
                 StartLocation = location()
                 Options = erl_scan:options()
                 Result = parse_ret()
                 parse_ret() =
                     {ok,
                      ExprList :: [erl_parse:abstract_expr()],
                      EndLocation :: location()} |
                     {eof, EndLocation :: location()} |
                     {error,
                      ErrorInfo :: erl_scan:error_info() | erl_parse:error_info(),
                      ErrorLocation :: location()} |
                     server_no_data()
                 server_no_data() = {error, ErrorDescription :: term()} | eof

              Reads  data  from  the  standard input (IoDevice), prompting it with Prompt. Starts
              reading at location StartLocation (1). Argument Options is passed  on  as  argument
              Options  of  function  erl_scan:tokens/4. The data is tokenized and parsed as if it
              was a sequence of Erlang expressions until a final dot (.) is reached.

              The function returns:

                {ok, ExprList, EndLocation}:
                  The parsing was successful.

                {eof, EndLocation}:
                  End of file was encountered by the tokenizer.

                eof:
                  End of file was encountered by the I/O server.

                {error, ErrorInfo, ErrorLocation}:
                  An error occurred while tokenizing or parsing.

                {error, ErrorDescription}:
                  Other (rare) error condition, such as {error, estale} if reading  from  an  NFS
                  file system.

              Example:

              25> io:parse_erl_exprs('enter>').
              enter>abc(), "hey".
              {ok, [{call,1,{atom,1,abc},[]},{string,1,"hey"}],2}
              26> io:parse_erl_exprs ('enter>').
              enter>abc("hey".
              {error,{1,erl_parse,["syntax error before: ",["'.'"]]},2}

       parse_erl_form(Prompt) -> Result

       parse_erl_form(IoDevice, Prompt) -> Result

       parse_erl_form(IoDevice, Prompt, StartLocation) -> Result

       parse_erl_form(IoDevice, Prompt, StartLocation, Options) -> Result

              Types:

                 IoDevice = device()
                 Prompt = prompt()
                 StartLocation = location()
                 Options = erl_scan:options()
                 Result = parse_form_ret()
                 parse_form_ret() =
                     {ok,
                      AbsForm :: erl_parse:abstract_form(),
                      EndLocation :: location()} |
                     {eof, EndLocation :: location()} |
                     {error,
                      ErrorInfo :: erl_scan:error_info() | erl_parse:error_info(),
                      ErrorLocation :: location()} |
                     server_no_data()
                 server_no_data() = {error, ErrorDescription :: term()} | eof

              Reads  data  from  the  standard input (IoDevice), prompting it with Prompt. Starts
              reading at location StartLocation (1). Argument Options is passed  on  as  argument
              Options  of  function  erl_scan:tokens/4. The data is tokenized and parsed as if it
              was an Erlang form (one of the valid Erlang expressions in an Erlang  source  file)
              until a final dot (.) is reached.

              The function returns:

                {ok, AbsForm, EndLocation}:
                  The parsing was successful.

                {eof, EndLocation}:
                  End of file was encountered by the tokenizer.

                eof:
                  End of file was encountered by the I/O server.

                {error, ErrorInfo, ErrorLocation}:
                  An error occurred while tokenizing or parsing.

                {error, ErrorDescription}:
                  Other  (rare)  error  condition, such as {error, estale} if reading from an NFS
                  file system.

       printable_range() -> unicode | latin1

              Returns the user-requested range of printable Unicode characters.

              The user can request a range of characters that are to be considered  printable  in
              heuristic  detection  of strings by the shell and by the formatting functions. This
              is done by supplying +pc <range> when starting Erlang.

              The only valid values for <range> are latin1 and unicode. latin1  means  that  only
              code  points < 256 (except control characters, and so on) are considered printable.
              unicode means that all printable characters in all  Unicode  character  ranges  are
              considered printable by the I/O functions.

              By  default, Erlang is started so that only the latin1 range of characters indicate
              that a list of integers is a string.

              The simplest way to use the setting is to call io_lib:printable_list/1, which  uses
              the  return  value  of  this  function to decide if a list is a string of printable
              characters.

          Note:
              In a future release, this function may return more  values  and  ranges.  To  avoid
              compatibility problems, it is recommended to use function io_lib:printable_list/1.

       put_chars(CharData) -> ok

       put_chars(IoDevice, CharData) -> ok

              Types:

                 IoDevice = device()
                 CharData = unicode:chardata()

              Writes the characters of CharData to the I/O server (IoDevice).

       read(Prompt) -> Result

       read(IoDevice, Prompt) -> Result

              Types:

                 IoDevice = device()
                 Prompt = prompt()
                 Result =
                     {ok, Term :: term()} | server_no_data() | {error, ErrorInfo}
                 ErrorInfo = erl_scan:error_info() | erl_parse:error_info()
                 server_no_data() = {error, ErrorDescription :: term()} | eof

              Reads a term Term from the standard input (IoDevice), prompting it with Prompt.

              The function returns:

                {ok, Term}:
                  The parsing was successful.

                eof:
                  End of file was encountered.

                {error, ErrorInfo}:
                  The parsing failed.

                {error, ErrorDescription}:
                  Other  (rare)  error  condition, such as {error, estale} if reading from an NFS
                  file system.

       read(IoDevice, Prompt, StartLocation) -> Result

       read(IoDevice, Prompt, StartLocation, Options) -> Result

              Types:

                 IoDevice = device()
                 Prompt = prompt()
                 StartLocation = location()
                 Options = erl_scan:options()
                 Result =
                     {ok, Term :: term(), EndLocation :: location()} |
                     {eof, EndLocation :: location()} |
                     server_no_data() |
                     {error, ErrorInfo, ErrorLocation :: location()}
                 ErrorInfo = erl_scan:error_info() | erl_parse:error_info()
                 server_no_data() = {error, ErrorDescription :: term()} | eof

              Reads a term Term from IoDevice,  prompting  it  with  Prompt.  Reading  starts  at
              location  StartLocation.  Argument  Options  is  passed  on  as argument Options of
              function erl_scan:tokens/4.

              The function returns:

                {ok, Term, EndLocation}:
                  The parsing was successful.

                {eof, EndLocation}:
                  End of file was encountered.

                {error, ErrorInfo, ErrorLocation}:
                  The parsing failed.

                {error, ErrorDescription}:
                  Other (rare) error condition, such as {error, estale} if reading  from  an  NFS
                  file system.

       rows() -> {ok, integer() >= 1} | {error, enotsup}

       rows(IoDevice) -> {ok, integer() >= 1} | {error, enotsup}

              Types:

                 IoDevice = device()

              Retrieves  the  number of rows of IoDevice (that is, the height of a terminal). The
              function only succeeds for terminal devices, for all other I/O devices the function
              returns {error, enotsup}.

       scan_erl_exprs(Prompt) -> Result

       scan_erl_exprs(Device, Prompt) -> Result

       scan_erl_exprs(Device, Prompt, StartLocation) -> Result

       scan_erl_exprs(Device, Prompt, StartLocation, Options) -> Result

              Types:

                 Device = device()
                 Prompt = prompt()
                 StartLocation = location()
                 Options = erl_scan:options()
                 Result = erl_scan:tokens_result() | server_no_data()
                 server_no_data() = {error, ErrorDescription :: term()} | eof

              Reads  data  from  the standard input (IoDevice), prompting it with Prompt. Reading
              starts at location StartLocation (1). Argument Options is  passed  on  as  argument
              Options  of  function  erl_scan:tokens/4.  The  data  is  tokenized as if it were a
              sequence of Erlang expressions until a final dot (.) is reached. This token is also
              returned.

              The function returns:

                {ok, Tokens, EndLocation}:
                  The tokenization succeeded.

                {eof, EndLocation}:
                  End of file was encountered by the tokenizer.

                eof:
                  End of file was encountered by the I/O server.

                {error, ErrorInfo, ErrorLocation}:
                  An error occurred while tokenizing.

                {error, ErrorDescription}:
                  Other  (rare)  error  condition, such as {error, estale} if reading from an NFS
                  file system.

              Example:

              23> io:scan_erl_exprs('enter>').
              enter>abc(), "hey".
              {ok,[{atom,1,abc},{'(',1},{')',1},{',',1},{string,1,"hey"},{dot,1}],2}
              24> io:scan_erl_exprs('enter>').
              enter>1.0er.
              {error,{1,erl_scan,{illegal,float}},2}

       scan_erl_form(Prompt) -> Result

       scan_erl_form(IoDevice, Prompt) -> Result

       scan_erl_form(IoDevice, Prompt, StartLocation) -> Result

       scan_erl_form(IoDevice, Prompt, StartLocation, Options) -> Result

              Types:

                 IoDevice = device()
                 Prompt = prompt()
                 StartLocation = location()
                 Options = erl_scan:options()
                 Result = erl_scan:tokens_result() | server_no_data()
                 server_no_data() = {error, ErrorDescription :: term()} | eof

              Reads data from the standard input (IoDevice), prompting  it  with  Prompt.  Starts
              reading  at  location  StartLocation (1). Argument Options is passed on as argument
              Options of function erl_scan:tokens/4. The data is tokenized as if it was an Erlang
              form  (one  of the valid Erlang expressions in an Erlang source file) until a final
              dot (.) is reached. This last token is also returned.

              The return values are the same as for scan_erl_exprs/1,2,3,4.

       setopts(Opts) -> ok | {error, Reason}

       setopts(IoDevice, Opts) -> ok | {error, Reason}

              Types:

                 IoDevice = device()
                 Opts = [setopt()]
                 Reason = term()

              Set options for the standard I/O device (IoDevice).

              Possible options and values vary depending  on  the  I/O  device.  For  a  list  of
              supported  options  and their current values on a specific I/O device, use function
              getopts/1.

              The options and values supported by the OTP I/O devices are as follows:

                binary, list, or {binary, boolean()}:
                  If set in binary mode (binary or {binary, true}), the I/O server  sends  binary
                  data  (encoded  in  UTF-8)  as  answers  to  the  get_line,  get_chars, and, if
                  possible, get_until requests (for details, see section The Erlang I/O Protocol)
                  in   the  User's  Guide).  The  immediate  effect  is  that  get_chars/2,3  and
                  get_line/1,2 return UTF-8 binaries instead  of  lists  of  characters  for  the
                  affected I/O device.

                  By  default,  all  I/O  devices  in  OTP are set in list mode. However, the I/O
                  functions can handle any of these modes  and  so  should  other,  user-written,
                  modules behaving as clients to I/O servers.

                  This  option  is  supported  by  the standard shell (group.erl), the 'oldshell'
                  (user.erl), and the file I/O servers.

                {echo, boolean()}:
                  Denotes if the terminal is to echo input. Only supported for the standard shell
                  I/O server (group.erl)

                {expand_fun, expand_fun()}:
                  Provides  a function for tab-completion (expansion) like the Erlang shell. This
                  function is called when the user presses the Tab key. The expansion  is  active
                  when calling line-reading functions, such as get_line/1,2.

                  The  function  is called with the current line, up to the cursor, as a reversed
                  string. It is to return a three-tuple: {yes|no, string(), [string(), ...]}. The
                  first element gives a beep if no, otherwise the expansion is silent; the second
                  is a string that will be entered at the cursor position; the third is a list of
                  possible  expansions.  If this list is not empty, it is printed and the current
                  input line is written once again.

                  Trivial example (beep on anything except  empty  line,  which  is  expanded  to
                  "quit"):

                fun("") -> {yes, "quit", []};
                   (_) -> {no, "", ["quit"]} end

                  This option is only supported by the standard shell (group.erl).

                {encoding, latin1 | unicode}:
                  Specifies  how  characters  are  input  or  output  from  or to the I/O device,
                  implying that, for example, a terminal is  set  to  handle  Unicode  input  and
                  output or a file is set to handle UTF-8 data encoding.

                  The  option  does not affect how data is returned from the I/O functions or how
                  it is sent in the I/O protocol, it only affects how the I/O device is to handle
                  Unicode characters to the "physical" device.

                  The  standard  shell  is  set for unicode or latin1 encoding when the system is
                  started. The encoding is set with the help of the LANG or LC_CTYPE  environment
                  variables  on Unix-like system or by other means on other systems. So, the user
                  can input Unicode characters and the I/O device is in {encoding, unicode}  mode
                  if  the  I/O  device supports it. The mode can be changed, if the assumption of
                  the runtime system is wrong, by setting this option.

                  The I/O device used when Erlang is started with the "-oldshell"  or  "-noshell"
                  flags  is  by  default  set  to  latin1 encoding, meaning that any characters >
                  codepoint 255 are escaped and that input is expected  to  be  plain  8-bit  ISO
                  Latin-1.  If  the  encoding  is  changed  to Unicode, input and output from the
                  standard file descriptors are in UTF-8 (regardless of operating system).

                  Files can also be set in {encoding, unicode}, meaning that data is written  and
                  read as UTF-8. More encodings are possible for files, see below.

                  {encoding, unicode | latin1} is supported by both the standard shell (group.erl
                  including werl on  Windows),  the  'oldshell'  (user.erl),  and  the  file  I/O
                  servers.

                {encoding,  utf8  |  utf16 | utf32 | {utf16,big} | {utf16,little} | {utf32,big} |
                {utf32,little}}:
                  For disk files, the encoding can be set to various UTF variants. This  has  the
                  effect  that  data  is  expected  to be read as the specified encoding from the
                  file, and the data is written in the specified encoding to the disk file.

                  {encoding, utf8} has the same effect as {encoding, unicode} on files.

                  The extended encodings are only supported on disk  files  (opened  by  function
                  file:open/2).

       write(Term) -> ok

       write(IoDevice, Term) -> ok

              Types:

                 IoDevice = device()
                 Term = term()

              Writes term Term to the standard output (IoDevice).

STANDARD INPUT/OUTPUT

       All  Erlang  processes  have  a  default  standard I/O device. This device is used when no
       IoDevice argument is specified in the function  calls  in  this  module.  However,  it  is
       sometimes  desirable  to  use an explicit IoDevice argument that refers to the default I/O
       device. This is the case with functions that can access either a file or the  default  I/O
       device.  The  atom standard_io has this special meaning. The following example illustrates
       this:

       27> io:read('enter>').
       enter>foo.
       {ok,foo}
       28> io:read(standard_io, 'enter>').
       enter>bar.
       {ok,bar}

       standard_io is an alias for  group_leader/0, so in  order  to  change  where  the  default
       input/output  requests  are  sent  you can change the group leader for the current process
       using  group_leader(NewGroupLeader, self()).

       There is always a process registered under the name of user. This can be used for  sending
       output to the user.

STANDARD ERROR

       In certain situations, especially when the standard output is redirected, access to an I/O
       server specific for error messages can be convenient. The I/O device standard_error can be
       used  to  direct  output to whatever the current operating system considers a suitable I/O
       device for error output. Example on a Unix-like operating system:

       $ erl -noshell -noinput -eval 'io:format(standard_error,"Error: ~s~n",["error 11"]),'\
       'init:stop().' > /dev/null
       Error: error 11

ERROR INFORMATION

       The ErrorInfo mentioned in this  module  is  the  standard  ErrorInfo  structure  that  is
       returned from all I/O modules. It has the following format:

       {ErrorLocation, Module, ErrorDescriptor}

       A string that describes the error is obtained with the following call:

       Module:format_error(ErrorDescriptor)