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Tcl_GetStdChannel(3tcl) Tcl Library Procedures Tcl_GetStdChannel(3tcl) 





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.
    
    

    

  








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(3tcl), Tcl_CreateChannel(3tcl), Tcl_Main(3tcl),
  tclsh(1)








KEYWORDS


standard channel, standard input, standard output, standard
  error







  

    7.5
    Tcl