Question

Can I do this in C? Valgrind complains that realloc produces an invalid free?

int main(){
    void* mem;
    // Do stuff
    recurfunc(123, mem);
    // Do stuff
    if(mem)
        free(mem);
    return 0;
}

void recurfunc(int x, void* mem){
     .....
     // Do stuff
     mem = realloc(mem, newsize);
     // Do stuff
     recurfunc(x, mem);
     .....
}

Answer 1

Yes, it does indeed complain, because the void * you’re passing in as mem is a copy of the pointer. Changes to the pointer within the function will not be reflected back to main.

If you want to change mem in the function and have it reflected back, you need to pass a pointer to a pointer.

The following code illustrates this:

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

static void fn (void *p1, void **p2) {
    p1 = realloc(p1, 200);
    *p2 = realloc (*p2, 200);
    printf ("    In call: %p %p\n", p1, *p2);
}

int main (void) {
    void *x1 = malloc (100);
    void *x2 = malloc (100);
    printf ("Before call: %p %p\n", x1, x2);
    fn (x1, &x2);
    printf (" After call: %p %p\n", x1, x2);
    return 0;
}

which outputs:

Before call: 0x896f008 0x896f070
    In call: 0x896f0d8 0x896f1a8
 After call: 0x896f008 0x896f1a8
                    ^
                    +--- Note this bit.

and you can see that the first value is retained in main despite it being changed within the function.