Tcl_StandardChannels man page on Manjaro

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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 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

       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.

       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 the function Tcl_Main, which forms the core of
       the  shell's  implementation)  uses method 2 to initialize the standard

       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.

       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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