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 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.








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 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_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







  

    7.5
    Tcl