source: downloads/tcl8.5.2/doc/GetStdChan.3 @ 43

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.so man.macros
.TH Tcl_GetStdChannel 3 7.5 Tcl "Tcl Library Procedures"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
Tcl_GetStdChannel, Tcl_SetStdChannel \- procedures for retrieving and replacing the standard channels
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
Tcl_Channel
\fBTcl_GetStdChannel\fR(\fItype\fR)
.sp
\fBTcl_SetStdChannel\fR(\fIchannel, type\fR)
.sp
.SH ARGUMENTS
.AS Tcl_Channel channel
.AP int type in
The identifier for the standard channel to retrieve or modify.  Must be one of
\fBTCL_STDIN\fR, \fBTCL_STDOUT\fR, or \fBTCL_STDERR\fR.
.AP Tcl_Channel channel in
The channel to use as the new value for the specified standard channel.
.BE

.SH DESCRIPTION
.PP
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 \fBstdin\fR and is used by \fBread\fR and \fBgets\fR.
The standard output channel is named \fBstdout\fR and is used by
\fBputs\fR.  The standard error channel is named \fBstderr\fR and is used for 
reporting errors.  In addition, the standard channels are inherited by any
child processes created using \fBexec\fR or \fBopen\fR in the absence of any
other redirections.
.PP
The standard channels are actually aliases for other normal channels.  The
current channel associated with a standard channel can be retrieved by calling
\fBTcl_GetStdChannel\fR with one of 
\fBTCL_STDIN\fR, \fBTCL_STDOUT\fR, or \fBTCL_STDERR\fR as the \fItype\fR.  The
return value will be a valid channel, or NULL.
.PP
A new channel can be set for the standard channel specified by \fItype\fR 
by calling \fBTcl_SetStdChannel\fR with a new channel or NULL in the
\fIchannel\fR argument.  If the specified channel is closed by a later call to
\fBTcl_Close\fR, then the corresponding standard channel will automatically be
set to NULL.
.PP
If a non-NULL value for \fIchannel\fR is passed to \fBTcl_SetStdChannel\fR,
then that same value should be passed to \fBTcl_RegisterChannel\fR, like so:
.CS
Tcl_RegisterChannel(NULL, channel);
.CE
This is a workaround for a misfeature in \fBTcl_SetStdChannel\fR that it
fails to do some reference counting housekeeping.  This misfeature cannot
be corrected without contradicting the assumptions of some existing
code that calls \fBTcl_SetStdChannel\fR.
.PP
If \fBTcl_GetStdChannel\fR is called before \fBTcl_SetStdChannel\fR, Tcl will
construct a new channel to wrap the appropriate platform-specific standard 
file handle.  If \fBTcl_SetStdChannel\fR is called before
\fBTcl_GetStdChannel\fR, then the default channel will not be created.
.PP
If one of the standard channels is set to NULL, either by calling
\fBTcl_SetStdChannel\fR with a NULL \fIchannel\fR argument, or by calling
\fBTcl_Close\fR on the channel, then the next call to \fBTcl_CreateChannel\fR
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.
.PP
See \fBTcl_StandardChannels\fR for a general treatise about standard
channels and the behaviour of the Tcl library with regard to them.

.SH "SEE ALSO"
Tcl_Close(3), Tcl_CreateChannel(3), Tcl_Main(3), tclsh(1)

.SH KEYWORDS
standard channel, standard input, standard output, standard error