random, srandom, and initstate
Joern Rennecke
amylaar@spamcop.net
Tue Apr 24 10:20:00 GMT 2012
Quoting Joern Rennecke <amylaar@spamcop.net>:
> Hmm, locking would seriously reduce the class of multithreading scenarious
> where random can work. If an interrupt tried to use it while it is already
> active, deadlock would result - similar to malloc.
> OTOH it should avoid all the issues in having to change our data structures
> to support random (3) reentrancy - allowing us to use the NetBSD version
> with just minor changes.
> And it should be good enough for gcc.c-torture/unsorted/udivmod4.c and
> its ilk.
I've tried an implementation; tested on x86_64-unknown-linux-gnu X sh-elf
with gcc/testsuite/gcc.dg/vshift-{1,3}.c rolled back to revision 181613 .
Below is the diff to the NetBSD original, and the second attachment is the
patch for newlib.
I'm not sure about random_r / srandom_r : should I add a dummy struct _reent *
parameter so that the implementation could later/optionally be changed to
work without locking?
-------------- next part --------------
--- libc/stdlib/random.c-ORIG 2012-03-28 00:48:12.280125217 +0200
+++ libc/stdlib/random.c 2012-03-28 02:11:42.344120901 +0200
@@ -39,32 +39,29 @@ __RCSID("$NetBSD: random.c,v 1.2 2005/12
#endif
#endif /* LIBC_SCCS and not lint */
-#include "namespace.h"
#include <assert.h>
#include <errno.h>
#include <stdlib.h>
-#include "reentrant.h"
-
-#ifdef __weak_alias
-__weak_alias(initstate,_initstate)
-__weak_alias(random,_random)
-__weak_alias(setstate,_setstate)
-__weak_alias(srandom,_srandom)
-#endif
-
+#include <reent.h>
+#include <sys/lock.h>
+#include <assert.h>
+#define _DIAGASSERT assert
-#ifdef _REENTRANT
-static mutex_t random_mutex = MUTEX_INITIALIZER;
-#endif
+#ifndef __SINGLE_THREAD__
+__LOCK_INIT(static, random_mutex);
+#define mutex_lock(a) __lock_acquire(a)
+#define mutex_unlock(a) __lock_release(a)
#else
-#include <lib/libkern/libkern.h>
#define mutex_lock(a) (void)0
#define mutex_unlock(a) (void)0
#endif
+#endif
-static void srandom_unlocked(unsigned int);
-static long random_unlocked(void);
+#ifdef _REENT_ONLY
+static
+#endif
+long random(void);
#define USE_BETTER_RANDOM
@@ -91,7 +88,7 @@ static long random_unlocked(void);
* state information, which will allow a degree seven polynomial. (Note:
* the zeroeth word of state information also has some other information
* stored in it -- see setstate() for details).
- *
+ *
* The random number generation technique is a linear feedback shift register
* approach, employing trinomials (since there are fewer terms to sum up that
* way). In this approach, the least significant bit of all the numbers in
@@ -262,8 +259,11 @@ static int *end_ptr = &randtbl[DEG_3 + 1
* introduced by the L.C.R.N.G. Note that the initialization of randtbl[]
* for default usage relies on values produced by this routine.
*/
-static void
-srandom_unlocked(unsigned int x)
+#ifdef _REENT_ONLY
+static
+#endif
+void
+srandom(unsigned int x)
{
int i;
@@ -296,16 +296,16 @@ srandom_unlocked(unsigned int x)
fptr = &state[rand_sep];
rptr = &state[0];
for (i = 0; i < 10 * rand_deg; i++)
- (void)random_unlocked();
+ (void)random();
}
}
void
-srandom(unsigned long x)
+srandom_r(unsigned long x)
{
mutex_lock(&random_mutex);
- srandom_unlocked((unsigned int) x);
+ srandom((unsigned int) x);
mutex_unlock(&random_mutex);
}
@@ -317,12 +317,12 @@ srandom(unsigned long x)
* the break values for the different R.N.G.'s, we choose the best (largest)
* one we can and set things up for it. srandom() is then called to
* initialize the state information.
- *
+ *
* Note that on return from srandom(), we set state[-1] to be the type
* multiplexed with the current value of the rear pointer; this is so
* successive calls to initstate() won't lose this information and will be
* able to restart with setstate().
- *
+ *
* Note: the first thing we do is save the current state, if any, just like
* setstate() so that it doesn't matter when initstate is called.
*
@@ -376,7 +376,7 @@ initstate(
}
state = (int *) (int_arg_state + 1); /* first location */
end_ptr = &state[rand_deg]; /* must set end_ptr before srandom */
- srandom_unlocked((unsigned int) seed);
+ srandom((unsigned int) seed);
if (rand_type == TYPE_0)
int_arg_state[0] = rand_type;
else
@@ -464,8 +464,11 @@ setstate(char *arg_state) /* pointer to
*
* Returns a 31-bit random number.
*/
-static long
-random_unlocked(void)
+#ifdef _REENT_ONLY
+static
+#endif
+long
+random(void)
{
int i;
int *f, *r;
@@ -495,12 +498,12 @@ random_unlocked(void)
}
long
-random(void)
+random_r(void)
{
long r;
mutex_lock(&random_mutex);
- r = random_unlocked();
+ r = random();
mutex_unlock(&random_mutex);
return (r);
}
-------------- next part --------------
2012-03-27 Joern Rennecke <joern.rennecke@embecosm.com>
* libc/stdlib/random.c: New file, adapted from NetBSD.
* libc/stdlib/Makefile.am (GENERAL_SOURCES): Add random.c
* libc/stdlib/Makefile.in: Regenerate.
Index: libc/stdlib/Makefile.am
===================================================================
RCS file: /cvs/src/src/newlib/libc/stdlib/Makefile.am,v
retrieving revision 1.30
diff -p -u -r1.30 Makefile.am
--- libc/stdlib/Makefile.am 16 Apr 2009 18:24:35 -0000 1.30
+++ libc/stdlib/Makefile.am 28 Mar 2012 00:18:06 -0000
@@ -45,6 +45,7 @@ GENERAL_SOURCES = \
mprec.c \
mstats.c \
rand.c \
+ random.c \
rand_r.c \
realloc.c \
reallocf.c \
--- /dev/null 2012-03-15 10:58:48.842803751 +0100
+++ libc/stdlib/random.c 2012-03-28 02:11:42.344120901 +0200
@@ -0,0 +1,509 @@
+/* $NetBSD: random.c,v 1.2 2005/12/21 14:19:45 christos Exp $ */
+
+/*
+ * Copyright (c) 1983, 1993
+ * The Regents of the University of California. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ * 3. Neither the name of the University nor the names of its contributors
+ * may be used to endorse or promote products derived from this software
+ * without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#if !defined(_KERNEL) && !defined(_STANDALONE)
+#include <sys/cdefs.h>
+#if defined(LIBC_SCCS) && !defined(lint)
+#if 0
+static char sccsid[] = "@(#)random.c 8.2 (Berkeley) 5/19/95";
+#else
+__RCSID("$NetBSD: random.c,v 1.2 2005/12/21 14:19:45 christos Exp $");
+#endif
+#endif /* LIBC_SCCS and not lint */
+
+
+#include <assert.h>
+#include <errno.h>
+#include <stdlib.h>
+#include <reent.h>
+#include <sys/lock.h>
+#include <assert.h>
+#define _DIAGASSERT assert
+
+#ifndef __SINGLE_THREAD__
+__LOCK_INIT(static, random_mutex);
+#define mutex_lock(a) __lock_acquire(a)
+#define mutex_unlock(a) __lock_release(a)
+#else
+#define mutex_lock(a) (void)0
+#define mutex_unlock(a) (void)0
+#endif
+#endif
+
+#ifdef _REENT_ONLY
+static
+#endif
+long random(void);
+
+#define USE_BETTER_RANDOM
+
+/*
+ * random.c:
+ *
+ * An improved random number generation package. In addition to the standard
+ * rand()/srand() like interface, this package also has a special state info
+ * interface. The initstate() routine is called with a seed, an array of
+ * bytes, and a count of how many bytes are being passed in; this array is
+ * then initialized to contain information for random number generation with
+ * that much state information. Good sizes for the amount of state
+ * information are 32, 64, 128, and 256 bytes. The state can be switched by
+ * calling the setstate() routine with the same array as was initiallized
+ * with initstate(). By default, the package runs with 128 bytes of state
+ * information and generates far better random numbers than a linear
+ * congruential generator. If the amount of state information is less than
+ * 32 bytes, a simple linear congruential R.N.G. is used.
+ *
+ * Internally, the state information is treated as an array of ints; the
+ * zeroeth element of the array is the type of R.N.G. being used (small
+ * integer); the remainder of the array is the state information for the
+ * R.N.G. Thus, 32 bytes of state information will give 7 ints worth of
+ * state information, which will allow a degree seven polynomial. (Note:
+ * the zeroeth word of state information also has some other information
+ * stored in it -- see setstate() for details).
+ *
+ * The random number generation technique is a linear feedback shift register
+ * approach, employing trinomials (since there are fewer terms to sum up that
+ * way). In this approach, the least significant bit of all the numbers in
+ * the state table will act as a linear feedback shift register, and will
+ * have period 2^deg - 1 (where deg is the degree of the polynomial being
+ * used, assuming that the polynomial is irreducible and primitive). The
+ * higher order bits will have longer periods, since their values are also
+ * influenced by pseudo-random carries out of the lower bits. The total
+ * period of the generator is approximately deg*(2**deg - 1); thus doubling
+ * the amount of state information has a vast influence on the period of the
+ * generator. Note: the deg*(2**deg - 1) is an approximation only good for
+ * large deg, when the period of the shift register is the dominant factor.
+ * With deg equal to seven, the period is actually much longer than the
+ * 7*(2**7 - 1) predicted by this formula.
+ *
+ * Modified 28 December 1994 by Jacob S. Rosenberg.
+ * The following changes have been made:
+ * All references to the type u_int have been changed to unsigned long.
+ * All references to type int have been changed to type long. Other
+ * cleanups have been made as well. A warning for both initstate and
+ * setstate has been inserted to the effect that on Sparc platforms
+ * the 'arg_state' variable must be forced to begin on word boundaries.
+ * This can be easily done by casting a long integer array to char *.
+ * The overall logic has been left STRICTLY alone. This software was
+ * tested on both a VAX and Sun SpacsStation with exactly the same
+ * results. The new version and the original give IDENTICAL results.
+ * The new version is somewhat faster than the original. As the
+ * documentation says: "By default, the package runs with 128 bytes of
+ * state information and generates far better random numbers than a linear
+ * congruential generator. If the amount of state information is less than
+ * 32 bytes, a simple linear congruential R.N.G. is used." For a buffer of
+ * 128 bytes, this new version runs about 19 percent faster and for a 16
+ * byte buffer it is about 5 percent faster.
+ *
+ * Modified 07 January 2002 by Jason R. Thorpe.
+ * The following changes have been made:
+ * All the references to "long" have been changed back to "int". This
+ * fixes memory corruption problems on LP64 platforms.
+ */
+
+/*
+ * For each of the currently supported random number generators, we have a
+ * break value on the amount of state information (you need at least this
+ * many bytes of state info to support this random number generator), a degree
+ * for the polynomial (actually a trinomial) that the R.N.G. is based on, and
+ * the separation between the two lower order coefficients of the trinomial.
+ */
+#define TYPE_0 0 /* linear congruential */
+#define BREAK_0 8
+#define DEG_0 0
+#define SEP_0 0
+
+#define TYPE_1 1 /* x**7 + x**3 + 1 */
+#define BREAK_1 32
+#define DEG_1 7
+#define SEP_1 3
+
+#define TYPE_2 2 /* x**15 + x + 1 */
+#define BREAK_2 64
+#define DEG_2 15
+#define SEP_2 1
+
+#define TYPE_3 3 /* x**31 + x**3 + 1 */
+#define BREAK_3 128
+#define DEG_3 31
+#define SEP_3 3
+
+#define TYPE_4 4 /* x**63 + x + 1 */
+#define BREAK_4 256
+#define DEG_4 63
+#define SEP_4 1
+
+/*
+ * Array versions of the above information to make code run faster --
+ * relies on fact that TYPE_i == i.
+ */
+#define MAX_TYPES 5 /* max number of types above */
+
+static const int degrees[MAX_TYPES] = { DEG_0, DEG_1, DEG_2, DEG_3, DEG_4 };
+static const int seps[MAX_TYPES] = { SEP_0, SEP_1, SEP_2, SEP_3, SEP_4 };
+
+/*
+ * Initially, everything is set up as if from:
+ *
+ * initstate(1, &randtbl, 128);
+ *
+ * Note that this initialization takes advantage of the fact that srandom()
+ * advances the front and rear pointers 10*rand_deg times, and hence the
+ * rear pointer which starts at 0 will also end up at zero; thus the zeroeth
+ * element of the state information, which contains info about the current
+ * position of the rear pointer is just
+ *
+ * MAX_TYPES * (rptr - state) + TYPE_3 == TYPE_3.
+ */
+
+/* LINTED */
+static int randtbl[DEG_3 + 1] = {
+ TYPE_3,
+#ifdef USE_BETTER_RANDOM
+ 0x991539b1, 0x16a5bce3, 0x6774a4cd,
+ 0x3e01511e, 0x4e508aaa, 0x61048c05,
+ 0xf5500617, 0x846b7115, 0x6a19892c,
+ 0x896a97af, 0xdb48f936, 0x14898454,
+ 0x37ffd106, 0xb58bff9c, 0x59e17104,
+ 0xcf918a49, 0x09378c83, 0x52c7a471,
+ 0x8d293ea9, 0x1f4fc301, 0xc3db71be,
+ 0x39b44e1c, 0xf8a44ef9, 0x4c8b80b1,
+ 0x19edc328, 0x87bf4bdd, 0xc9b240e5,
+ 0xe9ee4b1b, 0x4382aee7, 0x535b6b41,
+ 0xf3bec5da,
+#else
+ 0x9a319039, 0x32d9c024, 0x9b663182,
+ 0x5da1f342, 0xde3b81e0, 0xdf0a6fb5,
+ 0xf103bc02, 0x48f340fb, 0x7449e56b,
+ 0xbeb1dbb0, 0xab5c5918, 0x946554fd,
+ 0x8c2e680f, 0xeb3d799f, 0xb11ee0b7,
+ 0x2d436b86, 0xda672e2a, 0x1588ca88,
+ 0xe369735d, 0x904f35f7, 0xd7158fd6,
+ 0x6fa6f051, 0x616e6b96, 0xac94efdc,
+ 0x36413f93, 0xc622c298, 0xf5a42ab8,
+ 0x8a88d77b, 0xf5ad9d0e, 0x8999220b,
+ 0x27fb47b9,
+#endif /* USE_BETTER_RANDOM */
+};
+
+/*
+ * fptr and rptr are two pointers into the state info, a front and a rear
+ * pointer. These two pointers are always rand_sep places aparts, as they
+ * cycle cyclically through the state information. (Yes, this does mean we
+ * could get away with just one pointer, but the code for random() is more
+ * efficient this way). The pointers are left positioned as they would be
+ * from the call
+ *
+ * initstate(1, randtbl, 128);
+ *
+ * (The position of the rear pointer, rptr, is really 0 (as explained above
+ * in the initialization of randtbl) because the state table pointer is set
+ * to point to randtbl[1] (as explained below).
+ */
+static int *fptr = &randtbl[SEP_3 + 1];
+static int *rptr = &randtbl[1];
+
+/*
+ * The following things are the pointer to the state information table, the
+ * type of the current generator, the degree of the current polynomial being
+ * used, and the separation between the two pointers. Note that for efficiency
+ * of random(), we remember the first location of the state information, not
+ * the zeroeth. Hence it is valid to access state[-1], which is used to
+ * store the type of the R.N.G. Also, we remember the last location, since
+ * this is more efficient than indexing every time to find the address of
+ * the last element to see if the front and rear pointers have wrapped.
+ */
+static int *state = &randtbl[1];
+static int rand_type = TYPE_3;
+static int rand_deg = DEG_3;
+static int rand_sep = SEP_3;
+static int *end_ptr = &randtbl[DEG_3 + 1];
+
+/*
+ * srandom:
+ *
+ * Initialize the random number generator based on the given seed. If the
+ * type is the trivial no-state-information type, just remember the seed.
+ * Otherwise, initializes state[] based on the given "seed" via a linear
+ * congruential generator. Then, the pointers are set to known locations
+ * that are exactly rand_sep places apart. Lastly, it cycles the state
+ * information a given number of times to get rid of any initial dependencies
+ * introduced by the L.C.R.N.G. Note that the initialization of randtbl[]
+ * for default usage relies on values produced by this routine.
+ */
+#ifdef _REENT_ONLY
+static
+#endif
+void
+srandom(unsigned int x)
+{
+ int i;
+
+ if (rand_type == TYPE_0)
+ state[0] = x;
+ else {
+ state[0] = x;
+ for (i = 1; i < rand_deg; i++) {
+#ifdef USE_BETTER_RANDOM
+ int x1, hi, lo, t;
+
+ /*
+ * Compute x[n + 1] = (7^5 * x[n]) mod (2^31 - 1).
+ * From "Random number generators: good ones are hard
+ * to find", Park and Miller, Communications of the ACM,
+ * vol. 31, no. 10,
+ * October 1988, p. 1195.
+ */
+ x1 = state[i - 1];
+ hi = x1 / 127773;
+ lo = x1 % 127773;
+ t = 16807 * lo - 2836 * hi;
+ if (t <= 0)
+ t += 0x7fffffff;
+ state[i] = t;
+#else
+ state[i] = 1103515245 * state[i - 1] + 12345;
+#endif /* USE_BETTER_RANDOM */
+ }
+ fptr = &state[rand_sep];
+ rptr = &state[0];
+ for (i = 0; i < 10 * rand_deg; i++)
+ (void)random();
+ }
+}
+
+void
+srandom_r(unsigned long x)
+{
+
+ mutex_lock(&random_mutex);
+ srandom((unsigned int) x);
+ mutex_unlock(&random_mutex);
+}
+
+/*
+ * initstate:
+ *
+ * Initialize the state information in the given array of n bytes for future
+ * random number generation. Based on the number of bytes we are given, and
+ * the break values for the different R.N.G.'s, we choose the best (largest)
+ * one we can and set things up for it. srandom() is then called to
+ * initialize the state information.
+ *
+ * Note that on return from srandom(), we set state[-1] to be the type
+ * multiplexed with the current value of the rear pointer; this is so
+ * successive calls to initstate() won't lose this information and will be
+ * able to restart with setstate().
+ *
+ * Note: the first thing we do is save the current state, if any, just like
+ * setstate() so that it doesn't matter when initstate is called.
+ *
+ * Returns a pointer to the old state.
+ *
+ * Note: The Sparc platform requires that arg_state begin on an int
+ * word boundary; otherwise a bus error will occur. Even so, lint will
+ * complain about mis-alignment, but you should disregard these messages.
+ */
+char *
+initstate(
+ unsigned long seed, /* seed for R.N.G. */
+ char *arg_state, /* pointer to state array */
+ size_t n) /* # bytes of state info */
+{
+ void *ostate = (void *)(&state[-1]);
+ int *int_arg_state;
+
+ _DIAGASSERT(arg_state != NULL);
+
+ int_arg_state = (int *)(void *)arg_state;
+
+ mutex_lock(&random_mutex);
+ if (rand_type == TYPE_0)
+ state[-1] = rand_type;
+ else
+ state[-1] = MAX_TYPES * (int)(rptr - state) + rand_type;
+ if (n < BREAK_0) {
+ mutex_unlock(&random_mutex);
+ return (NULL);
+ } else if (n < BREAK_1) {
+ rand_type = TYPE_0;
+ rand_deg = DEG_0;
+ rand_sep = SEP_0;
+ } else if (n < BREAK_2) {
+ rand_type = TYPE_1;
+ rand_deg = DEG_1;
+ rand_sep = SEP_1;
+ } else if (n < BREAK_3) {
+ rand_type = TYPE_2;
+ rand_deg = DEG_2;
+ rand_sep = SEP_2;
+ } else if (n < BREAK_4) {
+ rand_type = TYPE_3;
+ rand_deg = DEG_3;
+ rand_sep = SEP_3;
+ } else {
+ rand_type = TYPE_4;
+ rand_deg = DEG_4;
+ rand_sep = SEP_4;
+ }
+ state = (int *) (int_arg_state + 1); /* first location */
+ end_ptr = &state[rand_deg]; /* must set end_ptr before srandom */
+ srandom((unsigned int) seed);
+ if (rand_type == TYPE_0)
+ int_arg_state[0] = rand_type;
+ else
+ int_arg_state[0] = MAX_TYPES * (int)(rptr - state) + rand_type;
+ mutex_unlock(&random_mutex);
+ return((char *)ostate);
+}
+
+/*
+ * setstate:
+ *
+ * Restore the state from the given state array.
+ *
+ * Note: it is important that we also remember the locations of the pointers
+ * in the current state information, and restore the locations of the pointers
+ * from the old state information. This is done by multiplexing the pointer
+ * location into the zeroeth word of the state information.
+ *
+ * Note that due to the order in which things are done, it is OK to call
+ * setstate() with the same state as the current state.
+ *
+ * Returns a pointer to the old state information.
+ *
+ * Note: The Sparc platform requires that arg_state begin on a long
+ * word boundary; otherwise a bus error will occur. Even so, lint will
+ * complain about mis-alignment, but you should disregard these messages.
+ */
+char *
+setstate(char *arg_state) /* pointer to state array */
+{
+ int *new_state;
+ int type;
+ int rear;
+ void *ostate = (void *)(&state[-1]);
+
+ _DIAGASSERT(arg_state != NULL);
+
+ new_state = (int *)(void *)arg_state;
+ type = (int)(new_state[0] % MAX_TYPES);
+ rear = (int)(new_state[0] / MAX_TYPES);
+
+ mutex_lock(&random_mutex);
+ if (rand_type == TYPE_0)
+ state[-1] = rand_type;
+ else
+ state[-1] = MAX_TYPES * (int)(rptr - state) + rand_type;
+ switch(type) {
+ case TYPE_0:
+ case TYPE_1:
+ case TYPE_2:
+ case TYPE_3:
+ case TYPE_4:
+ rand_type = type;
+ rand_deg = degrees[type];
+ rand_sep = seps[type];
+ break;
+ default:
+ mutex_unlock(&random_mutex);
+ return (NULL);
+ }
+ state = (int *) (new_state + 1);
+ if (rand_type != TYPE_0) {
+ rptr = &state[rear];
+ fptr = &state[(rear + rand_sep) % rand_deg];
+ }
+ end_ptr = &state[rand_deg]; /* set end_ptr too */
+ mutex_unlock(&random_mutex);
+ return((char *)ostate);
+}
+
+/*
+ * random:
+ *
+ * If we are using the trivial TYPE_0 R.N.G., just do the old linear
+ * congruential bit. Otherwise, we do our fancy trinomial stuff, which is
+ * the same in all the other cases due to all the global variables that have
+ * been set up. The basic operation is to add the number at the rear pointer
+ * into the one at the front pointer. Then both pointers are advanced to
+ * the next location cyclically in the table. The value returned is the sum
+ * generated, reduced to 31 bits by throwing away the "least random" low bit.
+ *
+ * Note: the code takes advantage of the fact that both the front and
+ * rear pointers can't wrap on the same call by not testing the rear
+ * pointer if the front one has wrapped.
+ *
+ * Returns a 31-bit random number.
+ */
+#ifdef _REENT_ONLY
+static
+#endif
+long
+random(void)
+{
+ int i;
+ int *f, *r;
+
+ if (rand_type == TYPE_0) {
+ i = state[0];
+ state[0] = i = (i * 1103515245 + 12345) & 0x7fffffff;
+ } else {
+ /*
+ * Use local variables rather than static variables for speed.
+ */
+ f = fptr; r = rptr;
+ *f += *r;
+ /* chucking least random bit */
+ i = ((unsigned int)*f >> 1) & 0x7fffffff;
+ if (++f >= end_ptr) {
+ f = state;
+ ++r;
+ }
+ else if (++r >= end_ptr) {
+ r = state;
+ }
+
+ fptr = f; rptr = r;
+ }
+ return(i);
+}
+
+long
+random_r(void)
+{
+ long r;
+
+ mutex_lock(&random_mutex);
+ r = random();
+ mutex_unlock(&random_mutex);
+ return (r);
+}
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