memcmp —
compare byte string
#include <sys/systm.h>
int
  
  memcmp(const
    void *b1, const void
    *b2, size_t
  len);
The memcmp() function compares byte string
  b1 against byte string b2. Both
  strings are assumed to be len bytes long.
The memcmp() function returns zero if the two strings
  are identical, otherwise returns the difference between the first two
  differing bytes (treated as unsigned char values, so that
  ‘\200’ is greater than
  ‘\0’, for example). Zero-length strings
  are always identical.
The memcmp() function conforms to ANSI
  X3.159-1989 (“ANSI C89”).