usr/src/lib/libc/port/gen/rand.c

   1 /*
   2  * CDDL HEADER START
   3  *
   4  * The contents of this file are subject to the terms of the
   5  * Common Development and Distribution License (the "License").
   6  * You may not use this file except in compliance with the License.
   7  *
   8  * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
   9  * or http://www.opensolaris.org/os/licensing.
  10  * See the License for the specific language governing permissions
  11  * and limitations under the License.
  12  *
  13  * When distributing Covered Code, include this CDDL HEADER in each
  14  * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
  15  * If applicable, add the following below this CDDL HEADER, with the
  16  * fields enclosed by brackets "[]" replaced with your own identifying
  17  * information: Portions Copyright [yyyy] [name of copyright owner]
  18  *
  19  * CDDL HEADER END
  20  */
  21
  22 /*
  23  * Copyright 2008 Sun Microsystems, Inc.  All rights reserved.
  24  * Use is subject to license terms.
  25  */
  26
  27 /*      Copyright (c) 1988 AT&T */
  28 /*        All Rights Reserved   */
  29
  30 /*
  31  * University Copyright- Copyright (c) 1982, 1986, 1988
  32  * The Regents of the University of California
  33  * All Rights Reserved
  34  *
  35  * University Acknowledgment- Portions of this document are derived from
  36  * software developed by the University of California, Berkeley, and its
  37  * contributors.
  38  */
  39
  40 #pragma ident   "%Z%%M% %I%     %E% SMI"
  41
  42 #include "lint.h"
  43 #include "thr_uberdata.h"
  44 #include <stdlib.h>
  45 #include <atomic.h>
  46
  47 /*
  48  * We need atomic_cas_uint() protection because a multithreaded process
  49  * may be calling rand() from different threads and because multiple
  50  * threads may be calling rand_r() using a pointer to the same seed.
  51  */
  52
  53 static uint_t rand_seed = 1;
  54
  55 #define NEXT_SEED(s)    ((s) * 1103515245 + 12345)
  56 #define NEXT_VALUE(s)   (((s) >> 16) & 0x7fff)
  57
  58 static int
  59 rand_mt(uint_t *seed)
  60 {
  61         uint_t old_seed;
  62         uint_t new_seed;
  63
  64         for (;;) {
  65                 /* force reload on every iteration */
  66                 old_seed = *(volatile uint_t *)seed;
  67                 new_seed = NEXT_SEED(old_seed);
  68                 if (atomic_cas_uint(seed, old_seed, new_seed) == old_seed)
  69                         return (NEXT_VALUE(new_seed));
  70                 SMT_PAUSE();
  71         }
  72 }
  73
  74 int
  75 rand_r(uint_t *seed)
  76 {
  77         if (curthread->ul_uberdata->uberflags.uf_mt)
  78                 return (rand_mt(seed));
  79         return (NEXT_VALUE(*seed = NEXT_SEED(*seed)));
  80 }
  81
  82 void
  83 srand(uint_t seed)
  84 {
  85         rand_seed = seed;
  86 }
  87
  88 int
  89 rand(void)
  90 {
  91         return (rand_r(&rand_seed));
  92 }