INFO: Extracting Function ptr from a Variable arg List Function (47961)

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
*/ 
 }