NAME

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

SYNOPSIS

       #include <tcl.h>

       Tcl_Channel
       Tcl_GetStdChannel(type)

       Tcl_SetStdChannel(channel, type)

ARGUMENTS

       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  specified
                                         standard channel.
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DESCRIPTION

       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 channel 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.

       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 Tcl_SetStdChannel.

       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  channel with the newly created
       channel.  If more than one standard 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 standard channels and the  behaviour
       of the Tcl library with regard to them.

SEE ALSO

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

KEYWORDS

       standard channel, standard input, standard output, standard error