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

__________________________________________________________________________________________________________________________________________________

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 appli-
       cation. 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  chan-
       nels  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 stan-
       dard 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)