Can't free a Malloc'd string

Question

This one should be quick, I think.

EDIT: This is for my CS113 class. I just needed to free all the memory. If Valgrind found any memory leaks, I'd lose points. :P

Regardless, I figured out that it apparently just required me to free stuff in Main that related to the return value of zero_pad. Once I did so, it worked fine. I'd mark this post as "complete" if I knew how.

char *zero_pad(struct cpu_t *cpu, char *string)
{
    char *zero_string = malloc(cpu->word_size + 1); 
    int num_zeros = ((cpu->word_size) - strlen(string));
    int i;

    for(i = 0; i < num_zeros; i++)
    {
        zero_string[i] = '0';
    }

    return strncat(zero_string, string, strlen(string));    
}

I need to free zero_string, since I allocated it. However, I have no idea how. If I free it before I return, then I've lost that data and can't return it. If I try to free it after, the function has already returned and thus can't go on to freeing it.

I tried to use strcpy to copy the string in zero_string into a new string, but I must have been doing it wrong, because I just ended up with a massive mess.

So, what do you all think?

Answer 1

What is cpu->word_size ? Are you sure that cpu->word_size >= strlen(string) in any case ?

Anyway, the returned buffer is mallocated so it is the caller's responsibility to free it. Another possibility is to use a static variable if you know the maximum size needed. This is the code.

#define MAX_SIZE 128 /* adjust this */
char *zero_pad(struct cpu_t *cpu, char *string)
{
    static char zero_string[MAX_SIZE]; 
    int num_zeros = ((cpu->word_size) - strlen(string));
    int i;

    for(i = 0; i < num_zeros; i++)
    {
        zero_string[i] = '0';
    }

    strcpy(zero_string + i, string); 
    return zero_string;   
}

Obviously, the caller should be informed that the returned buffer is always the same (the caller has to duplicate the returned buffer if needed, e.g. using strdup() and later free()).