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Standard Channels(3)	    Tcl	Library	Procedures	  Standard Channels(3)

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NAME
       Tcl_StandardChannels  -	How  the  Tcl  library deals with the standard
       channels
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DESCRIPTION
       This page explains the initialization and use of	standard  channels  in
       the Tcl library.

       The  term  standard  channels comes out of the Unix world and refers to
       the three channels automatically	opened by the OS for each new applica-
       tion.  They are stdin, stdout and stderr. The first is the standard in-
       put an application can read from, the other two refer to	writable chan-
       nels, one for regular output and	the other for error messages.

       Tcl  generalizes	this concept in	a cross-platform way and exposes stan-
       dard channels to	the script level.

   APPLICATION PROGRAMMING INTERFACES
       The public API procedures dealing directly with standard	 channels  are
       Tcl_GetStdChannel and Tcl_SetStdChannel.	Additional public APIs to con-
       sider are Tcl_RegisterChannel, Tcl_CreateChannel	and Tcl_GetChannel.

INITIALIZATION OF TCL STANDARD CHANNELS
       Standard	channels are initialized by the	Tcl library  in	 three	cases:
       when  explicitly	 requested,  when implicitly required before returning
       channel information, or when implicitly required	during registration of
       a new channel.

       These  cases  differ  in	how they handle	unavailable platform- specific
       standard	channels.  (A channel is not "available" if it	could  not  be
       successfully opened; for	example, in a Tcl application run as a Windows
       NT service.)

       1)     A	single standard	channel	is initialized when it	is  explicitly
	      specified	 in  a	call  to Tcl_SetStdChannel.  The states	of the
	      other standard channels are unaffected.

	      Missing platform-specific	standard channels do not matter	 here.
	      This approach is not available at	the script level.

       2)     All uninitialized	standard channels are initialized to platform-
	      specific default values:

	      (a)    when open channels	are  listed  with  Tcl_GetChannelNames
		     (or the file channels script command), or

	      (b)    when  information about any standard channel is requested
		     with a call to  Tcl_GetStdChannel,	 or  with  a  call  to
		     Tcl_GetChannel  which specifies one of the	standard names
		     (stdin, stdout and	stderr).

	      In case of missing platform-specific standard channels, the  Tcl
	      standard channels	are considered as initialized and then immedi-
	      ately closed. This means that the	first three Tcl	channels  then
	      opened  by  the  application  are	designated as the Tcl standard
	      channels.

       3)     All uninitialized	standard channels are initialized to platform-
	      specific	default	values when a user-requested channel is	regis-
	      tered with Tcl_RegisterChannel.

       In case of unavailable platform-specific	standard channels the  channel
       whose  creation	caused the initialization of the Tcl standard channels
       is made a normal	channel.  The next three Tcl channels  opened  by  the
       application  are	 designated  as	 the  Tcl standard channels.  In other
       words, of the first four	Tcl channels opened  by	 the  application  the
       second to fourth	are designated as the Tcl standard channels.

RE-INITIALIZATION OF TCL STANDARD CHANNELS
       Once  a	Tcl standard channel is	initialized through one	of the methods
       above, closing this Tcl standard	channel	will cause the	next  call  to
       Tcl_CreateChannel  to  make  the	 new channel the new standard channel,
       too. If more than one Tcl standard channel was closed Tcl_CreateChannel
       will fill the empty slots in the	order stdin, stdout and	stderr.

       Tcl_CreateChannel  will	not  try to reinitialize an empty slot if that
       slot was	not initialized	before.	It is this behavior which  enables  an
       application  to	employ	method 1 of initialization, i.e. to create and
       designate their own Tcl standard	channels.

SHELL-SPECIFIC DETAILS
   tclsh
       The Tcl shell (or rather	the function Tcl_Main, which forms the core of
       the  shell's  implementation)  uses method 2 to initialize the standard
       channels.

   wish
       The windowing shell (or rather the function Tk_MainEx, which forms  the
       core  of	 the  shell's  implementation) uses method 1 to	initialize the
       standard	channels (See Tk_InitConsoleChannels) on  non-Unix  platforms.
       On Unix platforms, Tk_MainEx implicitly uses method 2 to	initialize the
       standard	channels.

SEE ALSO
       Tcl_CreateChannel(3),	Tcl_RegisterChannel(3),	    Tcl_GetChannel(3),
       Tcl_GetStdChannel(3),  Tcl_SetStdChannel(3), Tk_InitConsoleChannels(3),
       tclsh(1), wish(1), Tcl_Main(3), Tk_MainEx(3)

KEYWORDS
       standard	channels

Tcl				      7.5		  Standard Channels(3)

NAME | DESCRIPTION | INITIALIZATION OF TCL STANDARD CHANNELS | RE-INITIALIZATION OF TCL STANDARD CHANNELS | SHELL-SPECIFIC DETAILS | SEE ALSO | KEYWORDS

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