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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