s_free
#define s_free(x) (((x) != NULL) ? (free(x), (x) = NULL) : (void *)0)
#define s_free(x) (((x) != NULL) ? (free(x), (x) = NULL) : (void *)0)
#define s_free(x) (((x) != NULL) ? (free(x), (x) = NULL) : (void *)0)
s_free(string *sp)