Question

It’s a textbook C code

void strcpy_new(char *s, char *t) {
    while ((*s = *t) != '\0') {
        s++;
        t++;
    }
}

int main(int argc, const char * argv[])
{

    char *s = "this is line a";
    char *t = "this is line b";
    printf("%s", s);
    strcpy_new(s, t);
    printf("%s", s);
    return 0;
}

when I run it with Xcode, I got EXEC_BAD_ACCESS.

Answer 1

The reason you get EXEC_BAD_ACCESS is because those string literals "this is line a" and "this is line b" are stored in read-only memory. Attempting to write to it (*s = *t) is undefined behavior and you are receiving a crash because of it.

To remedy this code you should allocate some memory for s so that it is large enough to hold the second string (t):

    char s[] = "this is line a"; // contrived example, s and t are the same length
    char *t = "this is line b";
    strcpy_new(s, t);