Q47961: INFO: Extracting Function ptr from a Variable arg List Function

Article: Q47961
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,2.0,2.1,4.0,5.0
Operating System(s): 
Keyword(s): kbcode kbLangC kbVC100 kbVC150 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Last Modified: 13-NOV-2001

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The information in this article applies to:

- Microsoft C for MS-DOS 
- Microsoft C/C++ for MS-DOS 
- Microsoft Visual C++, versions 1.0, 1.5, 2.0, 2.1, 4.0 
- Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
- Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
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SUMMARY
=======

In Microsoft C the va_arg() macro may be used to remove an item from a variable
argument list. When the element to be removed is complex, however, you may have
to typedef the item type. This is necessary because the va_arg() macro requires
you to provide the type of the item to be removed from the list. For a pointer
to a function, if you were to put the item type (the declaration of the function
ptr) as follows:

     FuncPtr = va_arg (arg_list, (int(*)(void)) );

The compiler would yield the following error:

     C2059: Syntax Error : ')'

You could try to get around this by grabbing the function pointer as a data
pointer, and then typecasting it to a function pointer, as in the following
line:

     FuncPtr = (int (*)(void)) va_arg (arg_list, char *);

This will work; however, you will get the following warning:

  C4074: Non-standard extension used, cast of data pointer to function pointer

The best solution is to typedef the function pointer. With a typedef, you avoid
both the error and the warning.

MORE INFORMATION
================

The following sample code passes a function pointer to a variable argument list
function, changes that pointer, and returns it to the calling procedure. To
implement this, however, you need to pass the function pointer by reference. For
this reason, a pointer is typedef'd to a function pointer. Once in the variable
argument list function, the pointer is dereferenced to the function pointer to
get at the actual function pointer.

Sample Code
-----------

  #include <stdio.h>
  #include <stdarg.h>

  typedef void (*FuncPtr)(void);
  typedef FuncPtr *PFuncPtr;

  void hello(void);
  void varfunc(int, ...);

   FuncPtr func1, func2;
   int i;

  void main(void)  {
      func1 = hello;
      printf ("Function pointer: func1 now points to Hello()\n");
      printf ("About to use func1 to make a call to Hello()\n\n");
      func1();
      printf ("Now lets pass these function pointers to our variable");
      printf (" argument list\n");
      varfunc(2, &func1, &func2);   /* To alter ptr, pass by reference */ 
      printf ("\nAfter the call to the variable argument procedure,\n");
      printf ("function pointer: func2 now points to Hello()\n");
      printf ("About to use func2 to make a call to Hello()\n\n");
      func2();
   }

  void hello(void)  {
      printf ("Hello, I'm in the procedure HELLO()\n");
   }

  void varfunc(int i, ...)  {
      va_list arg_ptr;
      PFuncPtr tmpfptr;                /* Temporary ptr to function ptr
  */ 

      printf ("\nIn function VARFUNC\n");
      va_start (arg_ptr, i);
      printf ("%d arguments were passed\n", i);
      tmpfptr = va_arg (arg_ptr, PFuncPtr);  /* Grab pointer to fnptr */ 
      *tmpfptr = hello;                      /* Assign fnptr addr of fn
  */ 
      tmpfptr = va_arg (arg_ptr, PFuncPtr);  /* Grab pointer to fnptr */ 
      *tmpfptr = hello;                      /* Assign fnptr addr of fn
  */ 
   }

Additional query words:

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Keywords          : kbcode kbLangC kbVC100 kbVC150 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbZNotKeyword8 kbvc150 kbvc100 kbCCompSearch kbZNotKeyword3 kbVC500 kbVC200 kbVC210 kbVC32bitSearch kbVC500Search
Version           : :1.0,1.5,2.0,2.1,4.0,5.0
Issue type        : kbinfo

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