INFO: Extracting Function ptr from a Variable arg List Function

• Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax
• Microsoft C/C++ for MS-DOS, version 7.0
• Microsoft Visual C++ for Windows, versions 1.0, 1.5
• Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

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)) );

   C2059: Syntax Error : ')'

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

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

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
   */
   }

Keywords          : CLngIss kbcode kbfasttip
Version           : MS-DOS:6.0,6.00a,6.00ax,7.0; WINDOWS:1.0,1.5,1.51,1.52;  WINDOWS NT:1.0,2.0,2.1,4.0,5.0
Platform          : MS-DOS NT WINDOWS
Issue type        : kbinfo