Apparently according to the c-faqs link elsethread

    execl takes a variable-length, null-pointer-terminated list of character pointer arguments, and is correctly called like this:
    execl("/bin/sh", "sh", "-c", "date", (char *)0);

Due to ececl being a variadic function it can not take advantage of a prototype to instruct the compiler that one of its arguments needs to be treated as a pointer context.