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CentOS 7.0 - man page for tcl_standardchannels (centos section 3)

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


       Tcl_StandardChannels - How the Tcl library deals with the standard channels

       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  application.  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 standard channels to  the
       script level.

       The  public  API  procedures dealing directly with standard channels are Tcl_GetStdChannel
       and  Tcl_SetStdChannel.	Additional  public  APIs  to  consider	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  implic-
       itly 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

	      (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 immediately  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 registered 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_Creat-
       eChannel 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 initial-
       ized  before.  It is this behavior which enables an application to employ method 1 of ini-
       tialization, 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 imple-
       mentation) uses method 2 to initialize the standard channels.

       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_InitConsoleChan-
       nels)  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),

       standard channels

Tcl					       7.5			     Standard Channels(3)
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