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REF CHARIO                                          John Gibson Jul 1993

       COPYRIGHT University of Sussex 1993. All Rights Reserved.

<<<<<<<<<<<<<<<<<<<<<                             >>>>>>>>>>>>>>>>>>>>>>
<<<<<<<<<<<<<<<<<<<<<      CHARACTER STREAM       >>>>>>>>>>>>>>>>>>>>>>
<<<<<<<<<<<<<<<<<<<<<      INPUT AND OUTPUT       >>>>>>>>>>>>>>>>>>>>>>
<<<<<<<<<<<<<<<<<<<<<                             >>>>>>>>>>>>>>>>>>>>>>

Character stream input and output is the simplest form of I/O in Poplog.
This REF  file details  the procedures,  etc, associated  with  standard
character repeaters and consumers, and the creation of new repeaters and

         CONTENTS - (Use <ENTER> g to access required sections)

  1   Introduction

  2   Standard Character Repeaters

  3   Standard Character Consumers

  4   Raw Mode Repeaters/Consumers

  5   Creating New Repeaters/Consumers

1  Introduction

Character streams are the simplest form of input/output in Poplog.  They
are used by many parts of the system, including the Pop-11 compiler  and
printing  utilities.  For  more  complex  I/O  facilities  using  device
records, see REF * SYSIO.

    Character stream  I/O  is  performed using  character  repeater  and
consumer procedures. A character repeater is a procedure which each time
it is  called  returns  the  next  character  from  an  input  stream; a
character consumer takes  a character as  argument and writes  it to  an
output stream.  Character streams  are terminated  by the  special  item
<termin> (the value of the  constant termin); a character repeater  will
return <termin> when an input  stream is exhausted, and giving  <termin>
to a character consumer will close its output stream.

    See also REF * ITEMISE, which describes the procedure incharitem for
turning  character  repeaters  into  item  repeaters.   REF * POPCOMPILE
explains how a text item stream  is created from a character stream  for
the Pop11 compilation process.

2  Standard Character Repeaters

charin() -> char                                             [procedure]
        This is  a character  repeater for  the current  standard  input
        channel,   i.e.   it   reads   characters   from   the    device
        pop_charin_device (normally in interactive mode this is a system
        device that inputs from the user's terminal/window etc, but when
        in Ved  interactive mode,  it is  a pseudo  `user' device  which
        reads input from the current Ved input buffer).

        The prompt string  output by charin  is determined by  popprompt
        (see REF * popprompt).

        If the character read in  is `\^G` (CTRL-G) then charin  invokes
        setpop (This mechanism  may be withdrawn,  as it is  a relic  of
        earlier versions of Pop-2).

        If  the  last  character  read   in  was  a  newline   character
        (poplastchar == `\n`), and the  next character read in is  char,
        charin  checks   to   see   whether  char   is   in   the   list
        pop_charin_escapes, in which  case it will  spawn a DCL  process
        (in VMS) or  Shell process (in  Unix) and hand  the rest of  the
        line as a command to be run by that process, using sysobey.  The
        character char is given to sysobey as its second argument.  When
        sysobey returns, charin assigns the newline character to char.

        Before finally  returning, charin  assigns char  to  poplastchar
        then checks whether poplogfile is a procedure, in which case  it
        is applied to char. This provides a simple logging mechanism.

        REF * ITEMISE explains  how an  "item-repeater" can  be  created
        from a  character  repeater like  charin,  and  REF * POPCOMPILE
        explains how program  text read  in from the  standard input  is
        compiled. HELP * IM explains how charin is made to read from the
        Ved buffer.

        The character returned by charin is normally an ASCII  character
        code, but  if the  "end  of file"  character  was typed  at  the
        terminal, or if the  device has been  "emptied" then it  returns
        the special  object  <termin>,  as explained  in  the  Overview,

        (Note that before  reading a  character from  pop_charin_device,
        charin first calls the  trap procedure pop_chario_trap with  the
        device and true as arguments, i.e.

            pop_chario_trap(pop_charin_device, true)

        See pop_chario_trap below.)

pop_charin_escapes -> list                                    [variable]
list -> pop_charin_escapes
        This list is examined by  charin as explained above,  whenever a
        character is read in immediately  after a newline character.  If
        the new character is a member of pop_charin_escapes then  charin
        uses sysobey to spawn a sub-process given the rest of the  input
        line as a command.

        In VMS the list pop_charin_escapes defaults to [`$`] and on Unix
        to [ `%`  `$` `!`],  i.e. the characters  that can  be given  as
        second  argument  to  sysobey.  The  procedure  vedsetpop   sets
        pop_charin_escapes locally to [], so that the mechanism will not
        be  triggered  accidentially  in  Ved  immediate  in  mode  (See
        HELP * IM) or in processes that compile from the Ved buffer (See
        HELP * LMR). (Ved allows an  alternative mechanism for  spawning
        sub-processes from  the  command  line,  as  described  in  HELP

poplastchar -> char                                           [variable]
char -> poplastchar
        Always contains the last character read by charin.

poplinenum -> int                                   [protected variable]
        This  integer  variable  is   incremented  whenever  a   newline
        character is read by charin  or a character repeater created  by
        discin.  poplinenum  is  local  to  pop11_compile  where  it  is
        initialised to  1:  hence  it  records  the  number  of  newline
        characters  read  in  since  the   beginning  of  the  call   of
        pop11_compile (and  is  used  by  * sys_pr_message  in  printing
        mishap and warning messages).

poplogfile -> char_cons_or_false                              [variable]
false_or_char_cons -> poplogfile
        This variable  may contain  a  character consumer  procedure  or
        false. In the  former case  every character read  by charin,  or
        output  by  charout  is  also  given  to  poplogfile  (including
        prompts). This can be used to record terminal interactions  (for
        an example of which, see LIB * RECORD). (Default value false)

termin -> <termin>                                            [constant]
termin_key -> key                                             [constant]
        The value of  termin is  the unique  item <termin>,  used as  an
        end-of-file marker for character/item  stream I/O (and also  for
        dynamic list streams, see REF * LISTS). termin_key is a constant
        holding the  key  structure of  the  unique item  <termin>  (see
        REF * KEYS).

pop_chario_trap(dev, is_input)            [protected procedure variable]
        The procedure in this variable is called by charin, charout  and
        charerr before  doing  anything;  it  is  redefined  in  various
        contexts, e.g. in Ved, and is intended for system use only.

        (If a  new trap  procedure trap_p  is to  be added,  it must  be
        concatenated onto the existing value, i.e.

            trap_p <> pop_chario_trap -> pop_chario_trap;

        where trap_p is defined as

            trap_p(dev, is_input) -> (dev, is_input);

        that is,  passes  on the  arguments  given  to it  to  the  next
        procedure in the chain.)

3  Standard Character Consumers

charout(char)                                                [procedure]
charerr(char)                                                [procedure]
        These are character  consumers for the  current standard  output
        and error  output  channels respectively,  i.e.  characters  are
        output via the devices pop_charout_device and pop_charerr_device
        respectively (normally  in  interactive mode  these  are  system
        devices that output to the user's terminal/window etc, but  when
        in Ved interactive  mode, they are  pseudo `user' devices  which
        output to the current Ved output buffer).

        (Note that before writing a character to pop_charX_device, these
        procedures first call  the trap  procedure pop_chario_trap  with
        the device and false as arguments, i.e.

            pop_chario_trap(pop_charX_device, false)

        See pop_chario_trap above.)

pop_charout_col -> int                                        [variable]
int -> pop_charout_col
pop_charerr_col -> int                                        [variable]
int -> pop_charerr_col
        These variables  contain  integers representing  the  number  of
        columns filled  by charout  and  charerr respectively  in  their
        current lines,  where  tab  characters  count  as  vedindentstep
        columns if in Ved or 8 otherwise, control characters (i.e. ASCII
        value < 32) count as none, and all other characters count as  1.
        These values are reset to 0 on outputting a newline character.

poplinewidth -> int                                           [variable]
int -> poplinewidth
poplinemax -> int                                             [variable]
int -> poplinemax
        When poplinewidth is  an integer, these  two variables  together
        control the breaking of long lines by charout and charerr.

        If on  outputting  a  character  with  charX,  pop_charX_col  is
        greater than or  equal to poplinewidth,  and either the  current
        character for output is a space  or tab, OR the column count  is
        already greater than or equal  to poplinemax, then an  automatic
        linebreak is inserted before printing the character.

        (In other  words,  lines  are  broken  at  the  next  whitespace
        character after  poplinewidth  columns, or  failing  that  after
        poplinemax columns. Note that if a line is broken at a space  or
        tab, that character is NOT output.)

        A  linebreak   consists   of   a   newline   followed   by   any
        continuation-line prefix characters specified by * poplineprefix

        If poplinewidth is not an integer, linebreaks are not  inserted.
        poplinewidth defaults to 70, and poplinemax to 78.

poplineprefix -> pair_or_false                                [variable]
pair_or_false -> poplineprefix
        This variable enables  automatic prefixing of  lines output  via
        charout and charerr. It may be  false, or a pair whose front  or
        back are each either a string or false.

        If poplineprefix is a pair whose front is a string, that  string
        is output at the beginning of every line (that is, when charX is
        called with pop_charX_col = 0).

        If poplineprefix is a pair whose  back is a string, that  string
        is output instead at the  beginning of every continuation  line,
        i.e. after  an automatic  newline inserted  by the  poplinewidth

        If poplineprefix is  not a pair,  or the back  is not a  string,
        automatic newlines  are  followed  by  any  'every-line'  prefix
        specified, plus a single tab character.

cucharout(char)                                     [procedure variable]
cucharerr(char)                                     [procedure variable]
        These  procedure   variables  contain   the  current   character
        consumers for  standard output  and error  output. All  printing
        utilities in Poplog  use cucharout  to produce  output, and  all
        error printing is  done via  cucharerr. The  standard values  of
        these variables are charout and charerr respectively.

cuchartrace -> char_cons                                      [variable]
char_cons -> cuchartrace
        If this variable  is false  then cucharout is  used for  tracing
        output. Otherwise the character consumer cuchartrace contains is
        used instead. Assigning  a character consumer  to this  variable
        enables trace printing to be separated from normal printout.

pop_buffer_charout -> bool                                    [variable]
bool -> pop_buffer_charout
        This boolean  variable  controls whether  normal-mode  character
        output to terminals is buffered within each line; if false, each
        character is output  immediately (it  therefore affects  charout
        and charerr when these reference terminals). See REF * SYSIO.

4  Raw Mode Repeaters/Consumers

rawcharin() -> char                                          [procedure]
        This repeater  reads  characters  from  the  device  poprawdevin
        (giving 'raw mode' input from the terminal). See REF * SYSIO.

rawcharout(char_or_string)                                   [procedure]
        This consumer  writes  characters  to  the  device  poprawdevout
        (giving buffered 'raw mode' output to the terminal). It can take
        either a single character or a byte vector. See REF * SYSIO.

rawsubstringout(string, index, num);                         [procedure]
        Just like rawcharout,  but outputs num  characters, starting  at
        index, from string. See REF * SYSIO.

rawoutflush()                                                [procedure]
        Flushes poprawdevout, i.e. writes out any outstanding characters
        in the buffer. Same as sysflush(poprawdevout).

charin_timeout(hsecs) -> char                                [procedure]
        Returns  the  first  character  read  from  the  terminal   with
        rawcharin during  the time-span  hsecs, or  false if  none  were
        available. hsecs  is measured  in hundredths  of a  second  (but
        rounded  to  the  nearest  whole  second).  (This  procedure  is
        implemented in terms  of pop_timeout_secs  and pop_timeout,  see
        REF * SYSIO).

5  Creating New Repeaters/Consumers

bits_out(filename, n, signed) -> int_consumer                [procedure]
bits_in(filename, n, signed) -> int_repeater                 [procedure]
        The bits_in/bits_out mechanism facilitates compact storage  of a
        series of integers.  For example,  the data for  a binary  array
        could be stored using 1/8 the space used by simply printing  the
        numbers to disk.

        For both procedures, the  argument filename may  be a file  name
        string, or a device.

        The n argument  specifies the  bitsize of the  integers; if  the
        boolean argument  signed  is  true,  the  integers  are  signed,
        otherwise unsigned.

        bits_out returns  an 'integer  consumer' procedure  int_consumer
        for the specified file.  Successive calls of int_consumer  write
        n-bit integers to the file, i.e.


        bits_in returns  an  'integer repeater'  procedure  int_repeater
        linked  to  the  specified  file  (normally,  one  created  with
        bits_out). Successive calls of int_repeater read n-bit  integers
        from the file, i.e.

                int_repeater() -> int_or_termin

discin(filename) -> char_rep                                 [procedure]
        Returns a character  repeater for the  filename filename,  which
        may be a string  or word -- if  a word then pop_default_type  is
        appended to the name (see below).

        Note that, despite the name  of this procedure, filename is  not
        restricted to  being  a  disk  file;  it  may  be  any  suitable
        operating system device. (The device is opened using

            sysopen(filename, 0, false, `N`) -> dev;

        i.e. a  disk  file  is  assumed  to be  a  text  file.  See  the
        description of the third argument to sysopen in  REF * SYSIO.) A
        mishap will result if the specified file cannot be opened.

        filename may also be  a device record  open for reading,  e.g. a
        device created by sysopen or syspipe (in Unix) or sysmailbox (in
        VMS). In  that  case a  character  repeater for  the  device  is
        returned. This technique  can be used  for 'binary' disk  files,

            sysopen(filename, 0, true, `N`) -> dev;
            discin(dev) -> char_rep;

discout(filename) -> char_cons                               [procedure]
        Returns a character consumer for the filename filename (which is
        as for discin). If filename is a  disk file, then a new file  is
        created, otherwise the  file is  simply opened.  (The device  is
        created using

            syscreate(filename, 1, false) -> dev;

        i.e. a  disk  file  is  assumed  to be  a  text  file.  See  the
        description of the third argument to syscreate in  REF * SYSIO.)
        A mishap will result if the specified file cannot be created  or

        filename may also be a device record open for writing, in  which
        case a character consumer for  that device is returned. As  with
        discin, this technique can be used for 'binary' disk files, e.g.

            syscreate(filename, 1, true) -> dev;
            discout(dev) -> char_cons;

discappend(filename) -> char_cons                            [procedure]
        Opens  an  existing  file,  and  returns  a  character  consumer
        char_cons that will  append characters  to the  file (i.e.  will
        actually update  an  existing disk  file).  filename is  as  for

discin_device(char_rep) -> dev                               [procedure]
discin_device(char_rep, mishap) -> dev_or_false
        Given a  character  repeater  procedure  char_rep,  returns  the
        device record  dev  associated  with it.  If  char_rep  was  not
        created by discin, a mishap  occurs unless the optional  boolean
        argument mishap is supplied and is false (in which case false is

discout_device(char_cons) -> dev                             [procedure]
        Returns the  device record  dev  associated with  the  character
        consumer procedure char_cons. An error is signalled if char_cons
        was not created by discout.

pop_default_type -> string                                    [variable]
string -> pop_default_type
        The string in this variable (default value '.p') is used as  the
        default file type/extension by  discin, discout and  discappend,
        and  is  concatenated  onto  a  word  argument  given  to  these
        procedures (but not onto a string argument).

sysisprogfile(filename) -> bool                              [procedure]
        Where filename is  a string,  returns true if  filename has  the
        file type/extension given by pop_default_type, false otherwise.

stringin(string) -> char_rep                                 [procedure]
        Returns a  character  repeater  for the  string  string,  i.e. a
        procedure which  each  time  it  is  called  produces  the  next
        character from  the  string, and  <termin>  when the  string  is

isclosed(item, mishap) -> bool                               [procedure]
isclosed(item) -> bool
        For item a device, this procedure returns true if the device  is
        closed, false if it is still open.

        For item a character repeater produced by discin or stringin, it
        returns true if  the repeater  is exhausted  (i.e. would  return
        <termin> if  called),  or  false  if  it  still  has  characters

        For a character consumer produced by discout, it returns  true
        if <termin> has been given to the consumer.

        The mishap argument  is a  boolean controlling  what happens  if
        item is not one  of the above: if  false, the procedure  returns
        "undef", otherwise a mishap occurs. The mishap argument defaults
        to true if omitted.

--- C.all/ref/chario
--- Copyright University of Sussex 1993. All rights reserved.