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Tcl_Main(3) Tcl Library Procedures Tcl_Main(3)
Tcl_Main, Tcl_SetStartupScript, Tcl_GetStartupScript, Tcl_SetMainLoop -
main program, startup script, and event loop definition for Tcl-based
Tcl_Main(argc, argv, appInitProc)
int argc (in) Number of elements in
char *argv (in) Array of strings
arguments. On Windows,
when using -DUNICODE, the
parameter type changes to
Tcl_AppInitProc *appInitProc (in) Address of an application-
procedure. The value for
this argument is usually
Tcl_Obj *path (in) Name of file to use as
startup script, or NULL.
const char *encoding (in) Encoding of file to use as
startup script, or NULL.
const char **encodingPtr (out) If non-NULL, location to
write a copy of the (const
char *) pointing to the
Tcl_MainLoopProc *mainLoopProc (in) Address of an application-
specific event loop
Tcl_Main can serve as the main program for Tcl-based shell
applications. A "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.
Tcl_Main 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 Tcl_Main. Those must be achieved by calling lower level
functions in the Tcl library directly.
The Tcl_Main 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 main, but that lacks flexibility of embedding style and having
a function main in a library (particularly a shared library) causes
problems on many systems. Having main in the Tcl library would also
make it hard to use Tcl in C++ programs, since C++ programs must have
special C++ main functions.
Normally each shell application contains a small main function that
does nothing but invoke Tcl_Main. Tcl_Main then does all the work of
creating and running a tclsh-like application.
Tcl_Main is not provided by the public interface of Tcl's stub library.
Programs that call Tcl_Main must be linked against the standard Tcl
library. Extensions (stub-enabled or not) are not intended to call
Tcl_Main is not thread-safe. It should only be called by a single main
thread of a multi-threaded application. This restriction is not a
problem with normal use described above.
Tcl_Main and therefore all applications based upon it, like tclsh, use
Tcl_GetStdChannel to initialize the standard channels to their default
values. See Tcl_StandardChannels for more information.
Tcl_Main supports two modes of operation, depending on whether the
filename and encoding of a startup script has been established. The
routines Tcl_SetStartupScript and Tcl_GetStartupScript are the tools
for controlling this configuration of Tcl_Main.
Tcl_SetStartupScript registers the value path as the name of the file
for Tcl_Main to evaluate as its startup script. The value encoding is
Tcl's name for the encoding used to store the text in that file. A
value of NULL for encoding is a signal to use the system encoding. A
value of NULL for path erases any existing registration so that
Tcl_Main will not evaluate any startup script.
Tcl_GetStartupScript queries the registered file name and encoding set
by the most recent Tcl_SetStartupScript call in the same thread. The
stored file name is returned, and the stored encoding name is written
to space pointed to by encodingPtr, when that is not NULL.
The file name and encoding values managed by the routines
Tcl_SetStartupScript and Tcl_GetStartupScript are stored per-thread.
Although the storage and retrieval functions of these routines work in
any thread, only those calls in the same main thread as Tcl_Main can
have any influence on it.
The caller of Tcl_Main may call Tcl_SetStartupScript first to establish
its desired startup script. If Tcl_Main finds that no such startup
script has been established, it consults the first few arguments in
argv. If they match ?-encoding name? fileName, where fileName does not
begin with the character -, then fileName is taken to be the name of a
file containing a startup script, and name is taken to be the name of
the encoding of the contents of that file. Tcl_Main then calls
Tcl_SetStartupScript with these values.
Tcl_Main then defines in its main interpreter the Tcl variables argc,
argv, argv0, and tcl_interactive, as described in the documentation for
When it has finished its own initialization, but before it processes
commands, Tcl_Main calls the procedure given by the appInitProc
argument. This procedure provides a "hook" for the application to
perform its own initialization of the interpreter created by Tcl_Main,
such as defining application-specific commands. The application
initialization routine might also call Tcl_SetStartupScript to (re-)set
the file and encoding to be used as a startup script. The procedure
must have an interface that matches the type Tcl_AppInitProc:
typedef int Tcl_AppInitProc(
AppInitProc is almost always a pointer to Tcl_AppInit; for more details
on this procedure, see the documentation for Tcl_AppInit.
When the appInitProc is finished, Tcl_Main calls Tcl_GetStartupScript
to determine what startup script has been requested, if any. If a
startup script has been provided, Tcl_Main attempts to evaluate it.
Otherwise, interactive mode begins with examination of the variable
tcl_rcFileName in the main interpreter. If that variable exists and
holds the name of a readable file, the contents of that file are
evaluated in the main 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 tcl_prompt1 and tcl_prompt2
as described in the documentation for tclsh. The prompts and command
evaluation results are written to the standard output channel only if
the Tcl variable tcl_interactive in the main interpreter holds a non-
zero integer value.
Tcl_SetMainLoop 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 Tcl_MainLoopProc:
typedef void Tcl_MainLoopProc(void);
Tcl_Main does not return. Normally a program based on Tcl_Main will
terminate when the exit command is evaluated. In interactive mode, if
an EOF or channel error is encountered on the standard input channel,
then Tcl_Main itself will evaluate the exit command after the main loop
procedure (if any) returns. In non-interactive mode, after Tcl_Main
evaluates the startup script, and the main loop procedure (if any)
returns, Tcl_Main will also evaluate the exit command.
tclsh(1), Tcl_GetStdChannel(3), Tcl_StandardChannels(3),
Tcl_AppInit(3), exit(n), encoding(n)
application-specific initialization, command-line arguments, main
Tcl 8.4 Tcl_Main(3)