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 can-
       not 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  stan-
       dard 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 out-
       put, 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)