tcl_getstdchannel(3)

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 chan
nel.

DESCRIPTION

Tcl defines three special channels that are used by vari
ous I/O related commands if no other channels are speci
fied. 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 nor
mal channels. The current channel associated with a stan
dard 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 speci
fied by type by calling Tcl_SetStdChannel with a new chan nel 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 Tcl_GetStdChannel is called before Tcl_SetStdChannel, Tcl will construct a new channel to wrap the appropriate
platform-specific standard file handle. If Tcl_SetStd Channel 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(3), Tcl_CreateChannel(3), Tcl_Main(3), tclsh(1)

KEYWORDS

standard channel, standard input, standard output, stan
dard error

Tcl 7.5 Tcl_GetStdChannel(3)