NAME

       Tcl_StandardChannels - How the Tcl library deals with the standard channels
_________________________________________________________________________________________________

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  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.

   APPLICATION PROGRAMMING INTERFACES
       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.

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 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.

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(3tcl),         Tcl_RegisterChannel(3tcl),          Tcl_GetChannel(3tcl),
       Tcl_GetStdChannel(3tcl),  Tcl_SetStdChannel(3tcl),  Tk_InitConsoleChannels(3tk), tclsh(1),
       wish(1), Tcl_Main(3tcl), Tk_MainEx(3tk)

KEYWORDS

       standard channels