Tcl_GetStdChannel(3) Tcl Library Procedures  Tcl_GetStdChannel(3)



     Tcl_GetStdChannel,  Tcl_SetStdChannel   -   procedures   for
     retrieving and replacing the standard channels


     #include <tcl.h>


     Tcl_SetStdChannel(channel, type)


     int type (in)                     The  identifier  for   the
                                       standard     channel    to
                                       retrieve or modify.   Must
                                       be   one   of   TCL_STDIN,
                                       TCL_STDOUT, or TCL_STDERR.

     Tcl_Channel channel (in)          The channel to use as  the
                                       new  value  for the speci-
                                       fied standard channel.


     Tcl defines three special channels that are used by  various
     I/O  related  commands  if  no other channels are specified.
     The standard input channel has a channel name of  stdin  and
     is  used  by  read and gets.  The standard output channel is
     named stdout and is used by puts.  The standard error  chan-
     nel  is  named  stderr and is used for reporting errors.  In
     addition, the standard channels are inherited by  any  child
     processes  created  using exec or open in the absence of any
     other redirections.

     The standard channels are actually aliases for other  normal
     channels.   The  current  channel associated with a standard
     channel can be retrieved by calling  Tcl_GetStdChannel  with
     one  of  TCL_STDIN,  TCL_STDOUT,  or TCL_STDERR as the type.
     The return value will be a valid channel, or NULL.

     A new channel can be set for the standard channel  specified
     by  type  by calling Tcl_SetStdChannel with a new channel or
     NULL in the channel argument.  If the specified  channel  is
     closed  by a later call to Tcl_Close, then the corresponding
     standard channel will automatically be set to NULL.

Tcl                     Last change: 7.5                        1

Tcl_GetStdChannel(3) Tcl Library Procedures  Tcl_GetStdChannel(3)

     If   a   non-NULL   value   for   channel   is   passed   to
     Tcl_SetStdChannel,  then that same value should be passed to
     Tcl_RegisterChannel, like so:
          Tcl_RegisterChannel(NULL, channel);
     This is a workaround for a misfeature  in  Tcl_SetStdChannel
     that  it  fails  to do some reference counting housekeeping.
     This misfeature cannot be  corrected  without  contradicting
     the   assumptions   of   some   existing   code  that  calls

     If Tcl_GetStdChannel is called before Tcl_SetStdChannel, Tcl
     will  construct  a  new  channel  to  wrap  the  appropriate
     platform-specific     standard     file     handle.       If
     Tcl_SetStdChannel  is  called before Tcl_GetStdChannel, then
     the default channel will not be created.

     If one of the standard channels is set to  NULL,  either  by
     calling  Tcl_SetStdChannel  with a NULL channel argument, or
     by calling Tcl_Close on the channel, then the next  call  to
     Tcl_CreateChannel  will automatically set the standard chan-
     nel with the newly created channel.  If more than one  stan-
     dard  channel  is  NULL,  then the standard channels will be
     assigned starting with standard input, followed by  standard
     output, with standard error being last.

     See Tcl_StandardChannels for a general treatise about  stan-
     dard  channels  and  the  behaviour  of the Tcl library with
     regard to them.


     Tcl_Close(3), Tcl_CreateChannel(3), Tcl_Main(3), tclsh(1)


     standard channel, standard input, standard output,  standard

Tcl                     Last change: 7.5                        2

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