Weird gcc warning and sanitizer crash

Question

I've encountered some weird gcc warning in my project. Let's look at this simple example in 3 files:

struct.h

typedef struct {
    int a;
    long b;
    char *c;
} myStruct;

func.c

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

myStruct* func() {
    myStruct* new = (myStruct*) malloc(sizeof(myStruct));
    new->a = 42;
    new->b = 84;
    new->c = "lol_ok\n";
    return new;
}

void prn(myStruct* x) {
    printf("%d\n", x->a);
}

main.c

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

#include "struct.h"

int main() {
    myStruct* ms = func();
    prn(ms);
    return 0;
}

So I get the following warning:

main.c: In function ‘main’:
main.c:8:24: warning: initialization makes pointer from integer without a cast
         myStruct* ms = func();

Moreover when I build it with -Wall -Wextra I get more:

main.c: In function ‘main’:
main.c:8:9: warning: implicit declaration of function ‘func’ [-Wimplicit-function-declaration]
         myStruct* ms = func();
         ^
main.c:8:24: warning: initialization makes pointer from integer without a cast
         myStruct* ms = func();
                        ^
main.c:9:2: warning: implicit declaration of function ‘prn’ [-Wimplicit-function-declaration]
  prn(ms);

What does it all mean? It also crashes if built with -fsanitize=undefined -fsanitize=address and that's weird. Why?

Answer 1

Lack of prototype.

Include the prototype for func() in struct.h.

myStruct* func(void);

When there's no prototype visible for func(), compiler assumes (pre-C99) that it returns an int. But func() actually returns an myStruct*.

Note that this implicit int rule has been removed from C99. So technically, your code is ill-formed in C99 and C11.

Increasing your warning level would help. gcc provides an option to catch this:

-Wimplicit-function-declaration