lib/random.c

   1 /* Copyright (C) 1995-2018 Free Software Foundation, Inc.
   2
   3    The GNU C Library is free software; you can redistribute it and/or
   4    modify it under the terms of the GNU Lesser General Public
   5    License as published by the Free Software Foundation; either
   6    version 2.1 of the License, or (at your option) any later version.
   7
   8    The GNU C Library is distributed in the hope that it will be useful,
   9    but WITHOUT ANY WARRANTY; without even the implied warranty of
  10    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  11    Lesser General Public License for more details.
  12
  13    You should have received a copy of the GNU Lesser General Public
  14    License along with the GNU C Library; if not, see
  15    <http://www.gnu.org/licenses/>.  */
  16
  17 /*
  18  * This is derived from the Berkeley source:
  19  *      @(#)random.c    5.5 (Berkeley) 7/6/88
  20  * It was reworked for the GNU C Library by Roland McGrath.
  21  * Rewritten to use reentrant functions by Ulrich Drepper, 1995.
  22  */
  23
  24 /*
  25    Copyright (C) 1983 Regents of the University of California.
  26    All rights reserved.
  27
  28    Redistribution and use in source and binary forms, with or without
  29    modification, are permitted provided that the following conditions
  30    are met:
  31
  32    1. Redistributions of source code must retain the above copyright
  33       notice, this list of conditions and the following disclaimer.
  34    2. Redistributions in binary form must reproduce the above copyright
  35       notice, this list of conditions and the following disclaimer in the
  36       documentation and/or other materials provided with the distribution.
  37    4. Neither the name of the University nor the names of its contributors
  38       may be used to endorse or promote products derived from this software
  39       without specific prior written permission.
  40
  41    THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS "AS IS" AND
  42    ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  43    IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  44    ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  45    FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  46    DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  47    OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  48    HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  49    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  50    OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  51    SUCH DAMAGE.*/
  52
  53 #ifndef _LIBC
  54 # include <libc-config.h>
  55 #endif
  56
  57 /* Specification.  */
  58 #include <stdlib.h>
  59
  60 #ifdef _LIBC
  61 # include <libc-lock.h>
  62 #else
  63 /* Allow memory races; that's random enough.  */
  64 # define __libc_lock_define_initialized(class, name)
  65 # define __libc_lock_lock(name) ((void) 0)
  66 # define __libc_lock_unlock(name) ((void) 0)
  67 #endif
  68
  69 /* An improved random number generation package.  In addition to the standard
  70    rand()/srand() like interface, this package also has a special state info
  71    interface.  The initstate() routine is called with a seed, an array of
  72    bytes, and a count of how many bytes are being passed in; this array is
  73    then initialized to contain information for random number generation with
  74    that much state information.  Good sizes for the amount of state
  75    information are 32, 64, 128, and 256 bytes.  The state can be switched by
  76    calling the setstate() function with the same array as was initialized
  77    with initstate().  By default, the package runs with 128 bytes of state
  78    information and generates far better random numbers than a linear
  79    congruential generator.  If the amount of state information is less than
  80    32 bytes, a simple linear congruential R.N.G. is used.  Internally, the
  81    state information is treated as an array of longs; the zeroth element of
  82    the array is the type of R.N.G. being used (small integer); the remainder
  83    of the array is the state information for the R.N.G.  Thus, 32 bytes of
  84    state information will give 7 longs worth of state information, which will
  85    allow a degree seven polynomial.  (Note: The zeroth word of state
  86    information also has some other information stored in it; see setstate
  87    for details).  The random number generation technique is a linear feedback
  88    shift register approach, employing trinomials (since there are fewer terms
  89    to sum up that way).  In this approach, the least significant bit of all
  90    the numbers in the state table will act as a linear feedback shift register,
  91    and will have period 2^deg - 1 (where deg is the degree of the polynomial
  92    being used, assuming that the polynomial is irreducible and primitive).
  93    The higher order bits will have longer periods, since their values are
  94    also influenced by pseudo-random carries out of the lower bits.  The
  95    total period of the generator is approximately deg*(2**deg - 1); thus
  96    doubling the amount of state information has a vast influence on the
  97    period of the generator.  Note: The deg*(2**deg - 1) is an approximation
  98    only good for large deg, when the period of the shift register is the
  99    dominant factor.  With deg equal to seven, the period is actually much
 100    longer than the 7*(2**7 - 1) predicted by this formula.  */
 101
 102
 103
 104 /* For each of the currently supported random number generators, we have a
 105    break value on the amount of state information (you need at least this many
 106    bytes of state info to support this random number generator), a degree for
 107    the polynomial (actually a trinomial) that the R.N.G. is based on, and
 108    separation between the two lower order coefficients of the trinomial.  */
 109
 110 /* Linear congruential.  */
 111 #define TYPE_0          0
 112 #define BREAK_0         8
 113 #define DEG_0           0
 114 #define SEP_0           0
 115
 116 /* x**7 + x**3 + 1.  */
 117 #define TYPE_1          1
 118 #define BREAK_1         32
 119 #define DEG_1           7
 120 #define SEP_1           3
 121
 122 /* x**15 + x + 1.  */
 123 #define TYPE_2          2
 124 #define BREAK_2         64
 125 #define DEG_2           15
 126 #define SEP_2           1
 127
 128 /* x**31 + x**3 + 1.  */
 129 #define TYPE_3          3
 130 #define BREAK_3         128
 131 #define DEG_3           31
 132 #define SEP_3           3
 133
 134 /* x**63 + x + 1.  */
 135 #define TYPE_4          4
 136 #define BREAK_4         256
 137 #define DEG_4           63
 138 #define SEP_4           1
 139
 140
 141 /* Array versions of the above information to make code run faster.
 142    Relies on fact that TYPE_i == i.  */
 143
 144 #define MAX_TYPES       5       /* Max number of types above.  */
 145
 146
 147 /* Initially, everything is set up as if from:
 148         initstate(1, randtbl, 128);
 149    Note that this initialization takes advantage of the fact that srandom
 150    advances the front and rear pointers 10*rand_deg times, and hence the
 151    rear pointer which starts at 0 will also end up at zero; thus the zeroth
 152    element of the state information, which contains info about the current
 153    position of the rear pointer is just
 154         (MAX_TYPES * (rptr - state)) + TYPE_3 == TYPE_3.  */
 155
 156 static int32_t randtbl[DEG_3 + 1] =
 157   {
 158     TYPE_3,
 159
 160     -1726662223, 379960547, 1735697613, 1040273694, 1313901226,
 161     1627687941, -179304937, -2073333483, 1780058412, -1989503057,
 162     -615974602, 344556628, 939512070, -1249116260, 1507946756,
 163     -812545463, 154635395, 1388815473, -1926676823, 525320961,
 164     -1009028674, 968117788, -123449607, 1284210865, 435012392,
 165     -2017506339, -911064859, -370259173, 1132637927, 1398500161,
 166     -205601318,
 167   };
 168
 169
 170 static struct random_data unsafe_state =
 171   {
 172 /* FPTR and RPTR are two pointers into the state info, a front and a rear
 173    pointer.  These two pointers are always rand_sep places apart, as they
 174    cycle through the state information.  (Yes, this does mean we could get
 175    away with just one pointer, but the code for random is more efficient
 176    this way).  The pointers are left positioned as they would be from the call:
 177         initstate(1, randtbl, 128);
 178    (The position of the rear pointer, rptr, is really 0 (as explained above
 179    in the initialization of randtbl) because the state table pointer is set
 180    to point to randtbl[1] (as explained below).)  */
 181
 182     .fptr = &randtbl[SEP_3 + 1],
 183     .rptr = &randtbl[1],
 184
 185 /* The following things are the pointer to the state information table,
 186    the type of the current generator, the degree of the current polynomial
 187    being used, and the separation between the two pointers.
 188    Note that for efficiency of random, we remember the first location of
 189    the state information, not the zeroth.  Hence it is valid to access
 190    state[-1], which is used to store the type of the R.N.G.
 191    Also, we remember the last location, since this is more efficient than
 192    indexing every time to find the address of the last element to see if
 193    the front and rear pointers have wrapped.  */
 194
 195     .state = &randtbl[1],
 196
 197     .rand_type = TYPE_3,
 198     .rand_deg = DEG_3,
 199     .rand_sep = SEP_3,
 200
 201     .end_ptr = &randtbl[sizeof (randtbl) / sizeof (randtbl[0])]
 202 };
 203 \f
 204 /* POSIX.1c requires that there is mutual exclusion for the 'rand' and
 205    'srand' functions to prevent concurrent calls from modifying common
 206    data.  */
 207 __libc_lock_define_initialized (static, lock)
 208 \f
 209 /* Initialize the random number generator based on the given seed.  If the
 210    type is the trivial no-state-information type, just remember the seed.
 211    Otherwise, initializes state[] based on the given "seed" via a linear
 212    congruential generator.  Then, the pointers are set to known locations
 213    that are exactly rand_sep places apart.  Lastly, it cycles the state
 214    information a given number of times to get rid of any initial dependencies
 215    introduced by the L.C.R.N.G.  Note that the initialization of randtbl[]
 216    for default usage relies on values produced by this routine.  */
 217 void
 218 __srandom (unsigned int x)
 219 {
 220   __libc_lock_lock (lock);
 221   (void) __srandom_r (x, &unsafe_state);
 222   __libc_lock_unlock (lock);
 223 }
 224
 225 weak_alias (__srandom, srandom)
 226 weak_alias (__srandom, srand)
 227 \f
 228 /* Initialize the state information in the given array of N bytes for
 229    future random number generation.  Based on the number of bytes we
 230    are given, and the break values for the different R.N.G.'s, we choose
 231    the best (largest) one we can and set things up for it.  srandom is
 232    then called to initialize the state information.  Note that on return
 233    from srandom, we set state[-1] to be the type multiplexed with the current
 234    value of the rear pointer; this is so successive calls to initstate won't
 235    lose this information and will be able to restart with setstate.
 236    Note: The first thing we do is save the current state, if any, just like
 237    setstate so that it doesn't matter when initstate is called.
 238    Returns a pointer to the old state.  */
 239 char *
 240 __initstate (unsigned int seed, char *arg_state, size_t n)
 241 {
 242   int32_t *ostate;
 243   int ret;
 244
 245   __libc_lock_lock (lock);
 246
 247   ostate = &unsafe_state.state[-1];
 248
 249   ret = __initstate_r (seed, arg_state, n, &unsafe_state);
 250
 251   __libc_lock_unlock (lock);
 252
 253   return ret == -1 ? NULL : (char *) ostate;
 254 }
 255
 256 weak_alias (__initstate, initstate)
 257 \f
 258 /* Restore the state from the given state array.
 259    Note: It is important that we also remember the locations of the pointers
 260    in the current state information, and restore the locations of the pointers
 261    from the old state information.  This is done by multiplexing the pointer
 262    location into the zeroth word of the state information. Note that due
 263    to the order in which things are done, it is OK to call setstate with the
 264    same state as the current state
 265    Returns a pointer to the old state information.  */
 266 char *
 267 __setstate (char *arg_state)
 268 {
 269   int32_t *ostate;
 270
 271   __libc_lock_lock (lock);
 272
 273   ostate = &unsafe_state.state[-1];
 274
 275   if (__setstate_r (arg_state, &unsafe_state) < 0)
 276     ostate = NULL;
 277
 278   __libc_lock_unlock (lock);
 279
 280   return (char *) ostate;
 281 }
 282
 283 weak_alias (__setstate, setstate)
 284 \f
 285 /* If we are using the trivial TYPE_0 R.N.G., just do the old linear
 286    congruential bit.  Otherwise, we do our fancy trinomial stuff, which is the
 287    same in all the other cases due to all the global variables that have been
 288    set up.  The basic operation is to add the number at the rear pointer into
 289    the one at the front pointer.  Then both pointers are advanced to the next
 290    location cyclically in the table.  The value returned is the sum generated,
 291    reduced to 31 bits by throwing away the "least random" low bit.
 292    Note: The code takes advantage of the fact that both the front and
 293    rear pointers can't wrap on the same call by not testing the rear
 294    pointer if the front one has wrapped.  Returns a 31-bit random number.  */
 295
 296 long int
 297 __random (void)
 298 {
 299   int32_t retval;
 300
 301   __libc_lock_lock (lock);
 302
 303   (void) __random_r (&unsafe_state, &retval);
 304
 305   __libc_lock_unlock (lock);
 306
 307   return retval;
 308 }
 309
 310 weak_alias (__random, random)