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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.

       In the following description, all  functions  have  an  optional  parameter  IoDevice.  If
       included,  it must be the pid of a process which handles the IO protocols. Normally, it is
       the IoDevice returned by file:open/2.

       For a description of the IO protocols refer to the STDLIB User's Guide.

   Warning:
       As of R13A, data supplied to the put_chars function should be  in  the  unicode:chardata()
       format.  This means that programs supplying binaries to this function need to convert them
       to UTF-8 before trying to output the data on an IO device.

       If an IO device is set in binary mode, the functions get_chars  and  get_line  may  return
       binaries instead of lists. The binaries will, as of R13A, be 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  IO  device.  Either  standard_io,  standard_error,  a registered name, or a pid
              handling IO 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_scan: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 (i.e. the width of a terminal). The
              function  only succeeds for terminal devices, for all other IO devices the function
              returns {error, enotsup}

       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).

       nl() -> ok

       nl(IoDevice) -> ok

              Types:

                 IoDevice = device()

              Writes new line to the standard output (IoDevice).

       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. It
              returns:

                Data:
                  The input characters. If the IO device supports Unicode, the data may represent
                  codepoints larger than 255 (the latin1 range). If the  I/O  server  is  set  to
                  deliver binaries, they will be encoded in UTF-8 (regardless of if the IO device
                  actually supports Unicode or not).

                eof:
                  End of file was encountered.

                {error, ErrorDescription}:
                  Other (rare) error condition, for instance {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. It
              returns:

                Data:
                  The characters in the line terminated by a LF (or  end  of  file).  If  the  IO
                  device supports Unicode, the data may represent codepoints larger than 255 (the
                  latin1 range). If the I/O server is set  to  deliver  binaries,  they  will  be
                  encoded  in  UTF-8 (regardless of if the IO device actually supports Unicode or
                  not).

                eof:
                  End of file was encountered.

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

       getopts() -> [opt_pair()]

       getopts(IoDevice) -> [opt_pair()]

              Types:

                 IoDevice = device()

              This  function  requests  all  available  options  and  their  current values for a
              specific IO device. 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.  The  standard  shell however 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.

       printable_range() -> unicode | latin1

              Return 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.

              Currently  the  only  valid values for <range> are latin1 and unicode. latin1 means
              that only code points below 256 (with the exception of control characters etc) will
              be considered printable. unicode means that all printable characters in all unicode
              character ranges are considered printable by the io functions.

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

              The  simplest  way to utilize the setting is to call io_lib:printable_list/1, which
              will use the return value of this function to decide if  a  list  is  a  string  of
              printable characters or not.

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

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

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

              Types:

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

              Set options for the standard IO device (IoDevice).

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

              The options and values supported by the current OTP IO devices are:

                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 (see the I/O protocol description in STDLIB User's Guide for
                  details).  The  immediate  effect is that get_chars/2,3 and get_line/1,2 return
                  UTF-8 binaries instead of lists of chars for the affected IO device.

                  By default, all IO devices in OTP are set in list mode, but 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  should  echo input. Only supported for the standard
                  shell I/O-server (group.erl)

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

                  The function is called with the current line, upto the cursor,  as  a  reversed
                  string.  It  should  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, and the third
                  is a list of possible expansions. If this list is non-empty, the list  will  be
                  printed and the current input line will be written once again.

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

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

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

                {encoding, latin1 | unicode}:
                  Specifies how characters are input or output from or to the actual  IO  device,
                  implying  that  i.e.  a terminal is set to handle Unicode input and output or a
                  file is set to handle UTF-8 data encoding.

                  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 IO device is to handle
                  Unicode characters towards the "physical" device.

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

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

                  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(R)), 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 will have
                  the  effect that data is expected to be read as the specified encoding from the
                  file and the data will be written in the specified encoding to the disk file.

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

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

       write(Term) -> ok

       write(IoDevice, Term) -> ok

              Types:

                 IoDevice = device()
                 Term = term()

              Writes the term Term to the standard output (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. It
              returns:

                {ok, Term}:
                  The parsing was successful.

                eof:
                  End of file was encountered.

                {error, ErrorInfo}:
                  The parsing failed.

                {error, ErrorDescription}:
                  Other (rare) error condition, for instance {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. The argument Options is passed on as the  Options  argument
              of the erl_scan:tokens/4 function. It 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, for instance {error, estale} if reading from an
                  NFS file system.

       fwrite(Format) -> ok

       fwrite(Format, Data) -> ok

       fwrite(IoDevice, Format, Data) -> ok

       format(Format) -> ok

       format(Format, Data) -> ok

       format(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 which 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.

              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, F and P  are  optional  numeric
              arguments.  If  F, P, or Pad is *, the next argument in Data is used as the numeric
              value of F or P.

              F is the field width of the printed argument.  A  negative  value  means  that  the
              argument  will  be  left  justified  within  the  field, otherwise it will be right
              justified. If no field width is specified, the required print width will  be  used.
              If the field width specified is too small, then the whole field will be filled with
              * 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, the 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. It is either a  single  character  (currently
              only  t,  for  Unicode  translation,  and  l,  for  stopping p and P from detecting
              printable characters, are supported) that changes the interpretation of Data.

              The following control sequences are available:

                ~:
                  The character ~ is written.

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

                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 should 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 which 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 less than 1.

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

                g:
                  The  argument  is a float which 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 should not be less than 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  given  precision
                  and  then  padded and justified to the given 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  larger  than 255 is considered an error if the Unicode
                  translation modifier is not given:

                4> io:fwrite("~ts~n",[[1024]]).
                \x{400}
                ok
                5> io:fwrite("~s~n",[[1024]]).
                ** exception exit: {badarg,[{io,format,[<0.26.0>,"~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, and 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.  It  also  tries  to  detect  lists of printable
                  characters and to output these as strings. The Unicode translation modifier  is
                  used for determining what characters are printable. For example:

                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 io:fwrite or
                  io:format. 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

                  When the modifier l is given no detection of  printable  character  lists  will
                  take place. For example:

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

                  Binaries  that  look  like UTF-8 encoded strings will be output with the string
                  syntax if the Unicode translation modifier is given:

                9> io:fwrite("~p~n",[[1024]]).
                [1024]
                10> io:fwrite("~tp~n",[[1024]]).
                "\x{400}"
                11> io:fwrite("~tp~n", [<<128,128>>]).
                <<128,128>>
                12> io:fwrite("~tp~n", [<<208,128>>]).
                <<"\x{400}"/utf8>>
                ok

                W:
                  Writes data in the same way as ~w, but takes an extra  argument  which  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 has been reached, then it is impossible  to  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  which  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.

                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.

                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.

              Returns:

                ok:
                  The formatting succeeded.

              If an error occurs, there is no output. For 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 exit: {badarg,[{io,format,[<0.22.0>,"~s","A"]},
                                          {erl_eval,do_apply,5},
                                          {shell,exprs,6},
                                          {shell,eval_exprs,6},
                                          {shell,eval_loop,3}]}
                   in function  io:o_request/2

              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 which directs the interpretation of the input.

              Format may contain:

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

                * Ordinary characters which must match the next input character.

                * Control sequences, which have the general format ~*FMC. The character *  is  an
                  optional  return suppression character. It provides a method to specify a field
                  which 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 currently supported,
                  meaning Unicode translation) and C determines the type of control sequence.

                  Unless otherwise specified, leading white-space  is  ignored  for  all  control
                  sequences.  An  input  field  cannot  be more than one line wide. The following
                  control sequences are available:

                  ~:
                    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 white-space characters are not skipped.

                  -:
                    An  optional  sign character is expected. A sign character - gives the return
                    value -1. Sign character + or none gives 1.  The  field  width  parameter  is
                    ignored. Leading white-space 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-white-space  characters is read. If a field width has been
                    specified, this number of characters are read and  all  trailing  white-space
                    characters are stripped. An Erlang string (list of characters) is returned.

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

                  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.

                    The  Unicode  translation  modifier  is  not  allowed  (atoms can not contain
                    characters beyond the latin1 range).

                  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 white-space
                    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 which have been scanned up  to  that  point,
                    including white-space characters.

                  It 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 the 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    "]}

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

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

              Types:

                 IoDevice = device()

              Retrieves  the  number of rows of the IoDevice (i.e. the height of a terminal). The
              function only succeeds for terminal devices, for all other IO 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). The argument Options  is  passed  on  as  the
              Options  argument of the erl_scan:tokens/4 function. The data is tokenized as if it
              were a sequence of Erlang expressions until a final dot (.) is reached. This  token
              is also returned. It 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, for instance {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). The argument Options is passed on as the
              Options argument of the erl_scan:tokens/4 function. The data is tokenized as if  it
              were  an Erlang form - one of the valid Erlang expressions in an Erlang source file
              - until a final dot (.) is reached. This last token is also  returned.  The  return
              values are the same as for scan_erl_exprs/1,2,3 above.

       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). The argument Options is  passed  on  as  the
              Options  argument  of  the  erl_scan:tokens/4  function.  The data is tokenized and
              parsed as if it were a sequence of Erlang expressions until  a  final  dot  (.)  is
              reached. It 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, for instance {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). The argument Options is passed on as the
              Options argument of the erl_scan:tokens/4  function.  The  data  is  tokenized  and
              parsed  as  if  it  were an Erlang form - one of the valid Erlang expressions in an
              Erlang source file - until a final dot (.) is reached. It 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, for instance {error, estale} if reading  from  an
                  NFS file system.

STANDARD INPUT/OUTPUT

       All  Erlang  processes  have  a  default  standard  IO device. This device is used when no
       IoDevice argument is specified in the above  function  calls.  However,  it  is  sometimes
       desirable to use an explicit IoDevice argument which refers to the default IO device. This
       is the case with functions that can access either a file or the  default  IO  device.  The
       atom 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 might be convenient. The IO device standard_error
       can be used to direct output to whatever the current operating system considers a suitable
       IO device for error output. Example on a Unix-like operating system:

       $ 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 above is the standard ErrorInfo structure which is returned from
       all IO modules. It has the format:

       {ErrorLocation, Module, ErrorDescriptor}

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

       Module:format_error(ErrorDescriptor)