Do not rely on compiler warnings.

buggy-gcc-Wundefined.c

#include <stdio.h>

int foo(void) {
    return 1;
}

int buggy(void)
{
    int len;
    /* GCC with -Wall -Wextra will not warn.
     * If you replace `while` by `if` it does. */
    while (foo() > 0) ++len;
    return len;
}

int main()
{
   printf("%d\n", buggy());
   return 0;
}