path: root/doc/Tcl_Main.3

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'\" Copyright (c) 1994 The Regents of the University of California.
'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
'\" Copyright (c) 2000 Ajuba Solutions.
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'\" RCS: @(#) $Id: Tcl_Main.3,v 1.20 2008/12/19 18:23:04 dgp Exp $
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.so man.macros
.TH Tcl_Main 3 8.4 Tcl "Tcl Library Procedures"
.BS
.SH NAME
Tcl_Main, Tcl_SetStartupScript, Tcl_GetStartupScript, Tcl_SetMainLoop \- main program, startup script, and event loop definition for Tcl-based applications
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
\fBTcl_Main\fR(\fIargc, argv, appInitProc\fR)
.sp
\fBTcl_SetStartupScript\fR(\fIpath, encoding\fR)
.sp
Tcl_Obj *
\fBTcl_GetStartupScript\fR(\fIencodingPtr\fR)
.sp
\fBTcl_SetMainLoop\fR(\fImainLoopProc\fR)
.SH ARGUMENTS
.AS Tcl_MainLoopProc *mainLoopProc
.AP int argc in
Number of elements in \fIargv\fR.
.AP char *argv[] in
Array of strings containing command-line arguments.
.AP Tcl_AppInitProc *appInitProc in
Address of an application-specific initialization procedure.
The value for this argument is usually \fBTcl_AppInit\fR.
.AP Tcl_Obj *path in
Name of file to use as startup script, or NULL.
.AP "const char" *encoding in
Encoding of file to use as startup script, or NULL.
.AP "const char" **encodingPtr out
If non-NULL, location to write a copy of the (const char *)
pointing to the encoding name.
.AP Tcl_MainLoopProc *mainLoopProc in
Address of an application-specific event loop procedure.
.BE
.SH DESCRIPTION
.PP
\fBTcl_Main\fR can serve as the main program for Tcl-based shell
applications.  A
.QW "shell application"
is a program
like tclsh or wish that supports both interactive interpretation
of Tcl and evaluation of a script contained in a file given as
a command line argument.  \fBTcl_Main\fR is offered as a convenience
to developers of shell applications, so they do not have to 
reproduce all of the code for proper initialization of the Tcl
library and interactive shell operation.  Other styles of embedding
Tcl in an application are not supported by \fBTcl_Main\fR.  Those
must be achieved by calling lower level functions in the Tcl library
directly.
.PP
The \fBTcl_Main\fR function has been offered by the Tcl library
since release Tcl 7.4.  In older releases of Tcl, the Tcl library
itself defined a function \fBmain\fR, but that lacks flexibility
of embedding style and having a function \fBmain\fR in a library
(particularly a shared library) causes problems on many systems.
Having \fBmain\fR in the Tcl library would also make it hard to use
Tcl in C++ programs, since C++ programs must have special C++
\fBmain\fR functions.
.PP
Normally each shell application contains a small \fBmain\fR function
that does nothing but invoke \fBTcl_Main\fR.
\fBTcl_Main\fR then does all the work of creating and running a
\fBtclsh\fR-like application.
.PP
\fBTcl_Main\fR is not provided by the public interface of Tcl's
stub library.  Programs that call \fBTcl_Main\fR must be linked
against the standard Tcl library.  Extensions (stub-enabled or
not) are not intended to call \fBTcl_Main\fR.
.PP
\fBTcl_Main\fR is not thread-safe.  It should only be called by
a single master thread of a multi-threaded application.  This
restriction is not a problem with normal use described above.
.PP
\fBTcl_Main\fR and therefore all applications based upon it, like
\fBtclsh\fR, use \fBTcl_GetStdChannel\fR to initialize the standard
channels to their default values. See \fBTcl_StandardChannels\fR for
more information.
.PP
\fBTcl_Main\fR supports two modes of operation, depending on
whether the filename and encoding of a startup script has been
established.  The routines \fBTcl_SetStartupScript\fR and
\fBTcl_GetStartupScript\fR are the tools for controlling this
configuration of \fBTcl_Main\fR.
.PP
\fBTcl_SetStartupScript\fR registers the value \fIpath\fR
as the name of the file for \fBTcl_Main\fR to evaluate as
its startup script.  The value \fIencoding\fR is Tcl's name
for the encoding used to store the text in that file.  A
value of \fBNULL\fR for \fIencoding\fR is a signal to use
the system encoding.  A value of \fBNULL\fR for \fIpath\fR
erases any existing registration so that \fBTcl_Main\fR
will not evaluate any startup script.
.PP
\fBTcl_GetStartupScript\fR queries the registered file name
and encoding set by the most recent \fBTcl_SetStartupScript\fR
call in the same thread.  The stored file name is returned,
and the stored encoding name is written to space pointed to
by \fIencodingPtr\fR, when that is not NULL.  
.PP
The file name and encoding values managed by the routines
\fBTcl_SetStartupScript\fR and \fBTcl_GetStartupScript\fR
are stored per-thread.  Although the storage and retrieval
functions of these routines work in any thread, only those
calls in the same master thread as \fBTcl_Main\fR can have
any influence on it.
.PP
The caller of \fBTcl_Main\fR may call \fBTcl_SetStartupScript\fR
first to establish its desired startup script.  If \fBTcl_Main\fR
finds that no such startup script has been established, it consults
the first few arguments in \fIargv\fR.  If they match
?\fB\-encoding \fIname\fR? \fIfileName\fR,
where \fIfileName\fR does not begin with the character \fI\-\fR,
then \fIfileName\fR is taken to be the name of a file containing
a \fIstartup script\fR, and \fIname\fR is taken to be the name
of the encoding of the contents of that file.  \fBTcl_Main\fR
then calls \fBTcl_SetStartupScript\fR with these values.
.PP
\fBTcl_Main\fR then defines in its master interpreter
the Tcl variables \fIargc\fR, \fIargv\fR, \fIargv0\fR, and
\fItcl_interactive\fR, as described in the documentation for \fBtclsh\fR.
.PP
When it has finished its own initialization, but before it processes
commands, \fBTcl_Main\fR calls the procedure given by the
\fIappInitProc\fR argument.  This procedure provides a
.QW hook
for the application to perform its own initialization of the interpreter
created by \fBTcl_Main\fR, such as defining application-specific
commands.  The application initialization routine might also 
call \fBTcl_SetStartupScript\fR to (re-)set the file and encoding
to be used as a startup script.  The procedure must have an interface
that matches the type \fBTcl_AppInitProc\fR:
.PP
.CS
typedef int \fBTcl_AppInitProc\fR(
        Tcl_Interp *\fIinterp\fR);
.CE
.PP
\fIAppInitProc\fR is almost always a pointer to \fBTcl_AppInit\fR; for more
details on this procedure, see the documentation for \fBTcl_AppInit\fR.
.PP
When the \fIappInitProc\fR is finished, \fBTcl_Main\fR calls
\fBTcl_GetStartupScript\fR to determine what startup script has
been requested, if any.  If a startup script has been provided,
\fBTcl_Main\fR attempts to evaluate it.  Otherwise, interactive
mode begins with examination of the variable \fItcl_rcFileName\fR
in the master interpreter.  If that variable exists and holds the
name of a readable file, the contents of that file are evaluated
in the master interpreter.  Then interactive operations begin,
with prompts and command evaluation results written to the standard
output channel, and commands read from the standard input channel
and then evaluated.  The prompts written to the standard output
channel may be customized by defining the Tcl variables \fItcl_prompt1\fR
and \fItcl_prompt2\fR as described in the documentation for \fBtclsh\fR.
The prompts and command evaluation results are written to the standard
output channel only if the Tcl variable \fItcl_interactive\fR in the
master interpreter holds a non-zero integer value.
.PP
\fBTcl_SetMainLoop\fR allows setting an event loop procedure to be run.
This allows, for example, Tk to be dynamically loaded and set its event
loop.  The event loop will run following the startup script.  If you
are in interactive mode, setting the main loop procedure will cause the
prompt to become fileevent based and then the loop procedure is called.
When the loop procedure returns in interactive mode, interactive operation
will continue.
The main loop procedure must have an interface that matches the type
\fBTcl_MainLoopProc\fR:
.PP
.CS
typedef void \fBTcl_MainLoopProc\fR(void);
.CE
.PP
\fBTcl_Main\fR does not return.  Normally a program based on
\fBTcl_Main\fR will terminate when the \fBexit\fR command is
evaluated.  In interactive mode, if an EOF or channel error
is encountered on the standard input channel, then \fBTcl_Main\fR
itself will evaluate the \fBexit\fR command after the main loop
procedure (if any) returns.  In non-interactive mode, after
\fBTcl_Main\fR evaluates the startup script, and the main loop
procedure (if any) returns, \fBTcl_Main\fR will also evaluate
the \fBexit\fR command.
.SH "SEE ALSO"
tclsh(1), Tcl_GetStdChannel(3), Tcl_StandardChannels(3), Tcl_AppInit(3),
exit(n), encoding(n)
.SH KEYWORDS
application-specific initialization, command-line arguments, main program