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_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 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)