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Root/sqlite/random.c

1/*
2** 2001 September 15
3**
4** The author disclaims copyright to this source code. In place of
5** a legal notice, here is a blessing:
6**
7** May you do good and not evil.
8** May you find forgiveness for yourself and forgive others.
9** May you share freely, never taking more than you give.
10**
11*************************************************************************
12** This file contains code to implement a pseudo-random number
13** generator (PRNG) for SQLite.
14**
15** Random numbers are used by some of the database backends in order
16** to generate random integer keys for tables or random filenames.
17**
18** $Id: random.c,v 1.1 2003/08/05 23:03:07 graydon Exp $
19*/
20#include "sqliteInt.h"
21#include "os.h"
22
23
24/*
25** Get a single 8-bit random value from the RC4 PRNG. The Mutex
26** must be held while executing this routine.
27**
28** Why not just use a library random generator like lrand48() for this?
29** Because the OP_NewRecno opcode in the VDBE depends on having a very
30** good source of random numbers. The lrand48() library function may
31** well be good enough. But maybe not. Or maybe lrand48() has some
32** subtle problems on some systems that could cause problems. It is hard
33** to know. To minimize the risk of problems due to bad lrand48()
34** implementations, SQLite uses this random number generator based
35** on RC4, which we know works very well.
36*/
37static int randomByte(){
38 int t;
39
40 /* All threads share a single random number generator.
41 ** This structure is the current state of the generator.
42 */
43 static struct {
44 int isInit; /* True if initialized */
45 int i, j; /* State variables */
46 int s[256]; /* State variables */
47 } prng;
48
49 /* Initialize the state of the random number generator once,
50 ** the first time this routine is called. The seed value does
51 ** not need to contain a lot of randomness since we are not
52 ** trying to do secure encryption or anything like that...
53 **
54 ** Nothing in this file or anywhere else in SQLite does any kind of
55 ** encryption. The RC4 algorithm is being used as a PRNG (pseudo-random
56 ** number generator) not as an encryption device.
57 */
58 if( !prng.isInit ){
59 int i;
60 char k[256];
61 prng.j = 0;
62 prng.i = 0;
63 sqliteOsRandomSeed(k);
64 for(i=0; i<256; i++){
65 prng.s[i] = i;
66 }
67 for(i=0; i<256; i++){
68 int t;
69 prng.j = (prng.j + prng.s[i] + k[i]) & 0xff;
70 t = prng.s[prng.j];
71 prng.s[prng.j] = prng.s[i];
72 prng.s[i] = t;
73 }
74 prng.isInit = 1;
75 }
76
77 /* Generate and return single random byte
78 */
79 prng.i = (prng.i + 1) & 0xff;
80 prng.j = (prng.j + prng.s[prng.i]) & 0xff;
81 t = prng.s[prng.i];
82 prng.s[prng.i] = prng.s[prng.j];
83 prng.s[prng.j] = t;
84 t = prng.s[prng.i] + prng.s[prng.j];
85 return prng.s[t & 0xff];
86}
87
88/*
89** Return an random 8-bit integer.
90*/
91int sqliteRandomByte(){
92 int r;
93 sqliteOsEnterMutex();
94 r = randomByte();
95 sqliteOsLeaveMutex();
96 return r;
97}
98
99/*
100** Return a random 32-bit integer. The integer is generated by making
101** 4 calls to sqliteRandomByte().
102*/
103int sqliteRandomInteger(){
104 int r;
105 int i;
106 sqliteOsEnterMutex();
107 r = randomByte();
108 for(i=1; i<4; i++){
109 r = (r<<8) + randomByte();
110 }
111 sqliteOsLeaveMutex();
112 return r;
113}

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