If-statement segfault, not sure how to fix.

I've been trying to run some code, but I keep running into issues with if-statements of all things. The code basically looks like this:

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define MAX 40

typedef struct stuff{
    int *digit;
}stuff;

int main()
{
    func("0123456789abcdef0123456789abcdef01234567");
}

stuff *func(char *str)
{
    stuff *buff = malloc(40 * sizeof(stuff));
    if(str == NULL || buff == NULL)
        return NULL;
    else
    {
        if(strlen(str) == 40)    // Crashes here
        {
            // Execute code 1
            puts("The string has exactly 40 chars");
            return buff;
        }
        else if(strlen(str) < 40)  // Crashes here too
        {
            // Execute code 2
            puts("The string has less than 40 chars");
            return buff;
        }
        else   // And also here
        {
            // Execute code 3
            puts("The string has more than 40 chars");
            return buff;
        }
    }
}

I've tested each code (1, 2, and 3) individually, so I know they work. I've also tried replacing the strlen(str) in every if-statement with normal integers, and the code worked also.

When I prompted the program to print put strlen(str) I received integer values, so that;s fine too.

When I tried to debug my program, it stated that the program does indeed break when I enter an if-statement, but I'm not sure why. Any ideas?

EDIT: Some one requested my full code to better understand my issue, but since this is a homework assignment I don't want to be accused of cheating, or laziness. Instead, here's a similar (but more complete) code where the exact same crash/segfault, in the exact same place occurs.