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Tcl_GetStdChannel(3)	     Tcl Library Procedures	    Tcl_GetStdChannel(3)

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NAME
     Tcl_GetStdChannel,  Tcl_SetStdChannel  -  procedures for retrieving and re-
     placing 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  argu-
     ment.   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 cor-
     rected 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 han-
     dle.  If Tcl_SetStdChannel is called before Tcl_GetStdChannel, then the de-
     fault channel will not be created.

     If one of the standard channels is set to NULL, either by calling	Tcl_Set-
     StdChannel  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 behavior of the Tcl library with regard to them.

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

KEYWORDS
     standard channel, standard input, standard output, standard error

Tcl				       7.5		    Tcl_GetStdChannel(3)

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