Have sodium_memcmp(a,b,s) return -1 if a != b, document return value.

README.markdown

@@ -173,6 +173,11 @@ In order to compare memory zones in constant time, Sodium provides:
     int      sodium_memcmp(const void * const b1_, const void * const b2_,
                            size_t size);
 
+`sodium_memcmp()` returns `0` if `size` bytes at `b1_` and `b2_` are
+equal, another value if they are not. Unlike `memcmp()`,
+`sodium_memcmp()` cannot be used to put `b1_` and `b2_` into a defined
+order.
+
 And a convenience function for converting a binary buffer to a
 hexadecimal string:
 

src/libsodium/sodium/utils.c

@@ -38,7 +38,7 @@ sodium_memcmp(const void * const b1_, const void * const b2_, size_t size)
     for (i = 0U; i < size; i++) {
         d |= b1[i] ^ b2[i];
     }
-    return (int) d;
+    return (int) ((1 & ((d - 1) >> 8)) - 1);
 }
 
 unsigned char *

test/default/sodium_utils.exp

@@ -1,6 +1,6 @@
 0
 0
-255
+-1
 0
 0
 30313233343536373839414243444546