Is it possible to cast printf as a parameter to another function?

Question

I am working on a linked list library and here is a function I wrote:

/**
 * go through a linked list and perform function func for every node of the
 * linked list
 *
 * func is a function pointer to the function you would to apply on the node.
 * it should return 0 if it is successful and non-zero value otherwise.
 */
void traverse_list(linkedlist * ll, int (* func)(void * args)){
    node * temp;

    temp = ll->head;
    while( temp != NULL ){
        if((* func)(temp->val))
            fprintf(stderr,"Error processing value!\n");
        temp = temp->next;
    }
}

My question is simple, I tried something like travers_list(testlinkedlist,printf) but it just cannot work(printf not printing anything out), what I am doing wrong? Can I do it at all, if I can, how?

You can definitely do it. Not sure the syntax off the top of my head. — Jonathan Wood, Jan 07 '13 at 20:31
You can cast between function pointers, but what is the actual error message? — , Jan 07 '13 at 20:31
possible duplicate of [How do function pointers in C work?](http://stackoverflow.com/questions/840501/how-do-function-pointers-in-c-work) — Falmarri, Jan 07 '13 at 20:31
@Falmarri: I think this question is about the syntax for *variadic* function pointers, so not a duplicate. — Mechanical snail, Jan 07 '13 at 20:33
Closely related: http://stackoverflow.com/questions/2118889/what-happens-if-i-cast-a-function-pointer-changing-the-number-of-parameters — Mechanical snail, Jan 07 '13 at 20:33
Sorry I made a mistake, it can compile but printf just does not work. — Bob Fang, Jan 07 '13 at 20:33
What if you change `func` declaration to one that matches `printf` : `int (* func)(const char *format, ...)` ? — qrdl, Jan 07 '13 at 20:37

Jason · Accepted Answer · 2013-01-07T20:56:06.597

2

Here's a code snippet to help you out:

#include <stdio.h>

typedef int (*func)(const char* format, ...);

int main()
{
    func a = printf;
    a("Hello World\n");
    return 0;
}

Now, if you want to create your own function that takes a variable number of arguments in C, this page from the GNU manual is a good resource to use that explains how variadic functions work.

edited Jan 07 '13 at 20:56

answered Jan 07 '13 at 20:35

Jason

31,834
7
59
78

what does ... mean in typedef? – Bob Fang Jan 07 '13 at 20:35
@dorafmon: It means the function takes a variable number of arguments -- as `printf` does. – Keith Thompson Jan 07 '13 at 20:36
It signifies a variadic function ... i.e., there can be an unlimited number of following arguments, although those arguments will need to match the actual format of `printf` in this case since that is what you want to create a pointer to. – Jason Jan 07 '13 at 20:36
I just realized a problem here, how can you access variable that is represented by ... inside a function? – Bob Fang Jan 07 '13 at 20:51
http://www.gnu.org/software/libc/manual/html_node/Variadic-Functions.html#Variadic-Functions – Jason Jan 07 '13 at 20:54
@Jason Cheers dude. Very helpful indeed. – Bob Fang Jan 07 '13 at 20:59

score 1 · Answer 2 · answered Jan 07 '13 at 20:42

1

Create your own function type that takes your list element as a parameter. There is no point in creating traversal procedure taking function as argument if the only function that matches is printf. (printf has quite unique signature)

answered Jan 07 '13 at 20:42

Agent_L

4,960
28
30

2

static int printfAnInt(void * val){ printf("%d\n",*((int *)val)); return 0; } Thats what i did before. – Bob Fang Jan 07 '13 at 20:46
@dorafmon Exactly. So you can use more functions than just printf, or predefine parameters. Consider: `printfAnIntToFile(void * val){ sprintf(globalDumplogHandle, "%d\n",*((int *)val)); return 0; }` – Agent_L Jan 07 '13 at 20:51
What does globalDumplogHandle mean here? – Bob Fang Jan 07 '13 at 20:54
@dorafmon globally defined handle to a file, `FILE*`. opened before calling `traverse_list` and closed after. – Agent_L Jan 07 '13 at 20:57
Thanks dude! It's something new to me. – Bob Fang Jan 07 '13 at 20:58

cmc · Answer 3 · 2013-01-07T20:41:39.330

You should cast printf to your function's argument type:

traverse_list(my_list, (int (*) (void*))&printf);

Remember to cast it back before using it, or this is going to end up in undefined behavior.

(I'm assuming you do not want to change the parameters of your function here.)

EDIT:

If what you're really asking is what parameters should your function take, then it should be a pointer to function that corresponds to the synopsis of printf, which you can find in man 3 printf:

int printf(const char *format, ...);

Is it possible to cast printf as a parameter to another function?

3 Answers3