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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 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_STD-OUT, TCL_STDOUT,
       OUT, 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  count-ing counting
       ing  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_GetSt-dChannel, Tcl_GetStdChannel,
       dChannel, 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 chan-nel channel
       nel  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(3), Tcl_CreateChannel(3), Tcl_Main(3), tclsh(1)


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



Tcl                                                  7.5                                Tcl_GetStdChannel(3)