Q47961

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

Article: Q47961
Version(s): 5.10   | 5.10
Operating System: MS-DOS | OS/2
Flags: ENDUSER | S_quickC S_QuickASM
Last Modified: 16-JAN-1990

To remove an item from a variable argument list, you use the va_arg()
macro. However, when the element to be removed is complex, 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 attempt 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 would work; however, you will get the following warning:

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

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

This problem occurs in C Version 5.10 and in QuickC Version 2.00 and
2.01.

The following example 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.

#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 */
}