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
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:
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 behaviour of the Tcl library with regard to them.