**Introduction**

The following questions tend to be asked regularly on the comp.lang.tcl newsgroup:
   * ''I have a large [C]/[C++] program- how do I make it scriptable with [Tcl]?''
   * ''I have a large C/C++ program- how do I add a [Tk] [GUI] to it?''

At a high level, there are three different solutions to consider.

   1.  Embed Tcl calls in my C/C++ code.

   2.  Wrap my C/C++ code to make it callable from Tcl.
       Replace the C/C++ main() with a tclsh/wish main program

   3.  Something else

----

**Embed Tcl Calls in C/C++ code**

While the text
"[Why adding Tcl calls to a C/C++ application is a bad idea]"
is an interesting dialog concerning the appropriate
approach to performing this task,
where can one find specific coding examples and documentation
regarding what one needs to do, in a C program, if one needs to create
an interpreter and then execute tcl commands making use of that interpreter.

Well, one example comes ''out of the box'' with both Tcl and Tk.
Take a look at the tcl/unix/tclAppInit.c (or tcl/win/tclAppInit.c) files
in the tcl source distribution (which is the mainline C module for the
tclsh command), and in the Tk source distribution, tk/unix/tkAppInit.c
or tk/win/winMain.c (I don't understand why the name difference here...).
For MacOS, see tk/mac/tkMacAppInit.c and tk/macosx/tkMacOSXAppInit.c
(again, I don't understand the reasoning for file name changes).
These provide at least a basic skeleton for initializing an interpreter.
Unfortunately, they leave you in an interpretive mode which most people
don't want to happen.  So another example is needed.

I do know there are two kinds of Tcl actions one can invoke from C/C++
Since Tcl is ''just'' a C library, some Tcl actions can be invoked by calling
the appropriate Tcl function call.

However, some things in Tcl must be done by invoking [Tcl_Eval], after
appropriately setting up a Tcl interpreter.

Can someone provide some sample C or C++ code that shows setting up the
interpreter, then perhaps invoking things each way?

A dream scenario would do this for Tcl/Tk since that is a superset of
the same kind of request for Tcl...

----

**First Example of invoking Tcl**

[AM] Here is some (massaged) code that I use in one application. The idea is:

   * Set up a basic interpreter
   * Register the specific commands
   * Make sure that the interpreter is available when a scripted task is called

It ''does not'' use the Tcl script library, only the basic built-in commands are available, but this is a very simple set-up after all.

The function InitScript() is used to set up a script library private to the application and to get a functioning Tcl interpreter. The function EvalScriptCommand() is used elsewhere and simply wraps the details so that I do not need to use Tcl routines everywhere.

 #include <stdlib.h>

 /*
  * Static global data
  */

 static Tcl_Interp * tcl_interp ; /* Script interpreter */

 int
 InitScript(
    void                         /* Nothing */
   )                               /* Return okay or not */
 {
    char       * pstr   ;
    FILE       * infile   ;
    int          retval   ;
    int          rc       ;
    size_t       filesize ;


    tcl_interp = Tcl_CreateInterp() ;

    if ( tcl_interp == NULL )
    {
       fprintf( stderr, "Could not create interpreter!\n" ) ;
       return 1 ;
    }
    /* Register the commands specific to my application


    Tcl_CreateObjCommand( tcl_interp, "session", GppSessionCmd,
       (ClientData) NULL, GppDummyDestroy ) ;

    /* Read the configuration file with specific script code
       (in reality, I use a function that searches for the file and opens it!
       Hence I need to do more work ... If you have the file name, then use
       Tcl_EvalFile()!)
    */
    infile = fopen( "scripts.conf", "r" ) ;

    if ( infile == NULL )
    {
      return 1 ;
    }

    /* Now, read the whole file ...
    */
    fseek( infile, 0L, SEEK_END ) ;
    filesize = ftell( infile ) ;
    pstr     = (char * ) malloc( (filesize+1) * sizeof(char) ) ;
    if ( pstr == NULL )
    {
       return 1;
    }
    fseek( infile, 0L, SEEK_SET ) ;

    fread( pstr, filesize, 1, infile ) ;
    pstr[filesize] = '\0' ;

    rc = Tcl_Eval( tcl_interp, pstr ) ;

    if ( rc != TCL_OK )
    {
       fprintf( stderr, "Error loading script library\n" ) ;
       return 1 ;
    }

    free( pstr ) ;

    return 0;
 }

 /* Function for encapsulating the details
 */
 EvalScriptCommand(
   char *  command )
 {
    if ( Tcl_Eval( tcl_interp, command ) == TCL_OK )
    {
       return 0 ;
    }
    else
    {
       return 1 ;
    }
 }
//compile with g++ and run using ./exec

----

**Second Example of Invoking Tcl from a C Application**

David Gravereaux writes, on comp.lang.tcl, this code fragment for invoking Tcl from a C program:
=====
Tcl_Interp *interp; 


int Init (char *argv0) 
{ 
    Tcl_FindExecutable(argv0); 
    interp = Tcl_CreateInterp(); 
    if (Tcl_Init(interp) != TCL_OK) { 
        return TCL_ERROR; 
    } 
    return TCL_OK; 



} 


void TearDown () {Tcl_Finalize();} 

int EvalFile (char *fileName) 
{ 
    return Tcl_EvalFile(interp, fileName); 



} 

=====

If EvalFile() returns TCL_ERROR, get the error with `Tcl_GetStringResult()` . 


----
**Wrap C/C++ code to make it callable from Tcl**

[[Replace this line with a pointer or details]]

----
**See also**
   * [Writing Tcl-Based Applications in C]

----
!!!!!!
%| [Category Tutorial] |%
!!!!!!