Tcl_StandardChannels man page on QNX

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


       Tcl_StandardChannels  -	How  the  Tcl  library deals with the standard

       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
       input an application can read from, the other  two  refer  to  writable
       channels, 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.

       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.

       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 state 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

       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	 chan‐
	      nels  opened  by the application the second to fourth are desig‐
	      nated as the 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.

       The  Tcl	 shell	(or  rather  Tcl_Main) uses method 2 to initialize the
       standard channels.

       The windowing shell (or rather Tk_MainEx) uses method 1	to  initialize
       the  standard  channels	(See Tk_InitConsoleChannels) on non-Unix plat‐
       forms.  On Unix platforms, Tk_MainEx implicitly uses method 2  to  ini‐
       tialize the standard channels.

       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)

       standard channels

Tcl				      7.5		  Standard Channels(3)
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                           \   /     \   /     \   /    
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