what should strlen() really return in this code?

Question

#include 
#include 
#include 
int main(void)
{
    char qq[] = {\'a\' , \'b\' , \'c\' , \'d\'};
    char qqq[] = \"abcd\";         


        

Answer 1

strlen will work its way forward until it finds a \0 character, counting how many characters there are along the way.

Because qq does not have a \0 in it, strlen will keep going into other, unrelated memory. Eventually, by chance, it encounters a \0 byte.

Apparently for you that is after 15 bytes (or 11 bytes beyond the end of qq).
But it is not deterministic or guaranteed, and is very unsafe.
It is very possible that strlen will end up trying to read invalid memory before it happens to find a \0 character, in which case your program will seg-fault.