Question

Why does the following code return with a segmentation fault? When I comment out line 7, the seg fault disappears.

int main(void){
      char *s;
      int ln;
      puts("Enter String");
      // scanf("%s", s);
      gets(s);
      ln = strlen(s); // remove this line to end seg fault
      char *dyn_s = (char*) malloc (strlen(s)+1); //strlen(s) is used here as well but doesn't change outcome
      dyn_s = s;
      dyn_s[strlen(s)] = '\0';
      puts(dyn_s);
      return 0;
    }

Cheers!

Answer 1

s is an uninitialized pointer; you are writing to a random location in memory. This will invoke undefined behaviour.

You need to allocate some memory for s. Also, never use gets; there is no way to prevent it overflowing the memory you allocate. Use fgets instead.