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

NAME
       TclStandardChannels  -  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 applica-
       tion.  They  are  ssttddiinn,  ssttddoouutt  and ssttddeerrrr. 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.


AAPPIIss
       The public API procedures dealing directly with standard  channels  are
       TTccllGGeettSSttddCChhaannnneell and TTccllSSeettSSttddCChhaannnneell. Additional public APIs to con-
       sider are TTccllRReeggiisstteerrCChhaannnneell, TTccllCCrreeaatteeCChhaannnneell and TTccllGGeettCChhaannnneell.

IINNIITTIIAALLIIZZAATTIIOONN OOFF TTCCLL SSTTAANNDDAARRDD CCHHAANNNNEELLSS
       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  TTccllSSeettSSttddCChhaannnneell.  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  TTccllGGeettCChhaannnneellNNaammeess
                     (or the ffiillee cchhaannnneellss script command), or

              (b)    when  information about any standard channel is requested
                     with a call to  TTccllGGeettSSttddCChhaannnneell,  or  with  a  call  to
                     TTccllGGeettCChhaannnneell  which specifies one of the standard names
                     (ssttddiinn, ssttddoouutt and ssttddeerrrr).

              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 TTccllRReeggiisstteerrCChhaannnneell.

              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.


RREE-IINNIITTIIAALLIIZZAATTIIOONN OOFF TTCCLL SSTTAANNDDAARRDD CCHHAANNNNEELLSS
       Once a Tcl standard channel is initialized through one of  the  methods
       above,  closing  this  Tcl standard channel will cause the next call to
       TTccllCCrreeaatteeCChhaannnneell to make the new channel  the  new  standard  channel,
       too. If more than one Tcl standard channel was closed TTccllCCrreeaatteeCChhaannnneell
       will fill the empty slots in the order ssttddiinn, ssttddoouutt and ssttddeerrrr.

       TTccllCCrreeaatteeCChhaannnneell 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.


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


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


SEE ALSO
       TclCreateChannel(3),     TclRegisterChannel(3),    TclGetChannel(3),
       TclGetStdChannel(3), TclSetStdChannel(3),  TkInitConsoleChannels(3),
       tclsh(1), wish(1), TclMain(3), TkMainEx(3)


KKEEYYWWOORRDDSS
       standard channels



Tcl                                   7.5                 Standard Channels(3)