Tcl_StandardChannels

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



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



Tcl                                   7.5              Standard Channels(3tcl)