Tcl_StandardChannels(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 applica-
       tion.  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 stan-
       dard channels to the script level.

APIs

       The public API procedures dealing directly with standard  channels  are
       Tcl_GetStdChannel and Tcl_SetStdChannel. Additional public APIs to con-
       sider 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 state 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 immedi-
              ately 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 regis-
              tered 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  chan-
              nels  opened  by the application the second to fourth are desig-
              nated 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.

tclsh

       The  Tcl  shell  (or  rather  Tcl_Main) uses method 2 to initialize the
       standard channels.

wish

       The windowing shell (or rather Tk_MainEx) uses method 1  to  initialize
       the  standard  channels  (See Tk_InitConsoleChannels) on non-Unix plat-
       forms.  On Unix platforms, Tk_MainEx implicitly uses method 2  to  ini-
       tialize 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)