random: Fix behavior of kernel option "-rs".

[pintos-anon] / src / lib / random.c

diff --git a/src/lib/random.c b/src/lib/random.c
index c9973d07b3102bca4f4f0f1a1e8a8c7d2086e717..6a963e21b42ccce71551e66e8dad806edd542d9a 100644 (file)
--- a/src/lib/random.c
+++ b/src/lib/random.c
@@ -1,12 +1,26 @@
 #include "random.h"
 #include <stdbool.h>
 #include <stdint.h>
+#include "debug.h"
 
-/* RC4-based pseudo-random state. */
-static uint8_t s[256];
-static uint8_t s_i, s_j;
-static bool inited;
+/* RC4-based pseudo-random number generator (PRNG).
 
+   RC4 is a stream cipher.  We're not using it here for its
+   cryptographic properties, but because it is easy to implement
+   and its output is plenty random for non-cryptographic
+   purposes.
+
+   See http://en.wikipedia.org/wiki/RC4_(cipher) for information
+   on RC4.*/
+
+/* RC4 state. */
+static uint8_t s[256];          /* S[]. */
+static uint8_t s_i, s_j;        /* i, j. */
+
+/* Already initialized? */
+static bool inited;     
+
+/* Swaps the bytes pointed to by A and B. */
 static inline void
 swap_byte (uint8_t *a, uint8_t *b) 
 {
@@ -15,6 +29,7 @@ swap_byte (uint8_t *a, uint8_t *b)
   *b = t;
 }
 
+/* Initializes or reinitializes the PRNG with the given SEED. */
 void
 random_init (unsigned seed)
 {
@@ -24,7 +39,7 @@ random_init (unsigned seed)
 
   if (inited)
     return;
-  
+
   for (i = 0; i < 256; i++) 
     s[i] = i;
   for (i = j = 0; i < 256; i++) 
@@ -37,12 +52,15 @@ random_init (unsigned seed)
   inited = true;
 }
 
+/* Writes SIZE random bytes into BUF. */
 void
 random_bytes (void *buf_, size_t size) 
 {
   uint8_t *buf;
 
-  ASSERT (inited);
+  if (!inited)
+    random_init (0);
+
   for (buf = buf_; size-- > 0; buf++)
     {
       uint8_t s_k;
@@ -56,7 +74,9 @@ random_bytes (void *buf_, size_t size)
     }
 }
 
-/* Returns a pseudo-random unsigned long. */
+/* Returns a pseudo-random unsigned long.
+   Use random_ulong() % n to obtain a random number in the range
+   0...n (exclusive). */
 unsigned long
 random_ulong (void) 
 {