pointer from integer without a cast warning

Question

Can someone please explain to me why the following code will compile fine when it is all in the one C file, but when i put the make_queue_data() function into another C file and compile it, it gives me an "assignment makes pointer from integer without a cast" warning?

#include <stdlib.h>
#include <stdio.h>

typedef struct pqueue_data_t
{
    int priority;
    void *queue_data;
} pqueue_data_t;

void*
safe_malloc (size_t size)
{
    void *mem_block = NULL;
    if ((mem_block = calloc (1, size)) == NULL) {
        fprintf (stderr, "ERROR: safe_malloc() cannot allocate memory.");
        exit (EXIT_FAILURE);
    }
    return (mem_block);
}

pqueue_data_t * 
make_queue_data(void *data, int priority)
{
    pqueue_data_t *pdata;
    pdata = (pqueue_data_t *) safe_malloc(sizeof(pqueue_data_t));
    pdata->priority = priority;
    pdata->queue_data = data;
    return (pdata);
}

int *
alloc_data (int val)
{
    int *rv = (int *)safe_malloc(sizeof(int));
    *rv = val;
    return (rv);
}

int
main (int argc, char **argv)
{
    pqueue_data_t *temp;
    temp = make_queue_data(alloc_data(34), 0); /* problem line */
    printf("%d\n", *((int *)temp->queue_data));
    return EXIT_SUCCESS;
}

this isnt the whole of my code, i just cut and pasted the relevant parts into the one.

any help would be greatly appreciated, as i have been bashing my head against this wall for a couple of hours trying to find where the problem is..

[Please don't cast the return value of `malloc()` (or your workalike clone) in C](http://stackoverflow.com/a/605858/28169). — unwind, Oct 07 '13 at 14:20
You're showing us code that compiles, and asking about other code that doesn't compile. We can only guess what that other code looks like. — Keith Thompson, Oct 07 '13 at 15:22

score 3 · Answer 1 · answered Oct 07 '13 at 14:19

3

It might be (and here I'm guessing) because you don't have a prototype of safe_malloc in the other source file.

Or it might be because you don't have the pqueue_data_t defined in the other source file (it should probably be in a header file instead).

answered Oct 07 '13 at 14:19

Some programmer dude

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score 1 · Answer 2 · answered Oct 07 '13 at 14:22

1

Make sure the function prototype is available in all source files that use the function. If you define a function in one file and use it in another the other file still needs to know the function prototype.

When the compiler sees a function-call that it knows no prototype or definition of it assumes the function to have a return type int, when you assign this to a pointer the compiler warns you about it. This is also the reason you should never cast the result of malloc in case you forget to include stdlib.h.

answered Oct 07 '13 at 14:22

Kninnug

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thanks, i have changed the cast from malloc, im not sure why i did that in the first place actually.. with the other one, the function prototypes for safe_malloc() and make_queue_data() are all actually in header files that are #included in the test source file, but it still is giving me the warning, should i cut-and-paste them into the actual test file itself? – guskenny83 Oct 07 '13 at 14:34

pointer from integer without a cast warning

2 Answers2