Ubuntu Manpage: Tcl_StandardChannels - How the Tcl library deals with the standard channels

Provided by: tcl8.6-doc_8.6.14+dfsg-1build1_all

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.

KEYWORDS