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1) POSIX requires rand(3) return values to be in the [0, RAND_MAX] range,

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but ACM formula we use have internal state (and return value) in the
[1, 0x7ffffffe] range, so our RAND_MAX (0x7fffffff) is never reached
because it is off by one, zero is not reached too.

Correct both RAND_MAX and rand(3) return value, shifting last one
to the 0 by 1 subtracted, resulting POSIXed [0, 0x7ffffffd(=new RAND_MAX)]
range.

2) Add a checks for not overflowing on too big seeds. It may happens on
the machines, where sizeof(unsigned int) > 32 bits.

Reviewed by:    bde [1]
MFC after:      2 weeks
This commit is contained in:
Andrey A. Chernov 2013-07-03 21:21:54 +00:00
parent 6f691f7ee1
commit 476d9314d6
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=252608
2 changed files with 17 additions and 5 deletions
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2
include/stdlib.h
View File

@ -69,7 +69,7 @@ typedef struct {
#define EXIT_FAILURE 1
#define EXIT_SUCCESS 0
#define RAND_MAX 0x7fffffff
#define RAND_MAX 0x7ffffffd
__BEGIN_DECLS
#ifdef _XLOCALE_H_

20
lib/libc/stdlib/rand.c
View File

@ -67,15 +67,15 @@ do_rand(unsigned long *ctx)
*/
long hi, lo, x;
/* Can't be initialized with 0, so use another value. */
if (*ctx == 0)
*ctx = 123459876;
/* Must be in [1, 0x7ffffffe] range at this point. */
hi = *ctx / 127773;
lo = *ctx % 127773;
x = 16807 * lo - 2836 * hi;
if (x < 0)
x += 0x7fffffff;
return ((*ctx = x) % ((u_long)RAND_MAX + 1));
*ctx = x;
/* Transform to [0, 0x7ffffffd] range. */
return (x - 1);
#endif /* !USE_WEAK_SEEDING */
}
@ -84,6 +84,10 @@ int
rand_r(unsigned int *ctx)
{
u_long val = (u_long) *ctx;
#ifndef USE_WEAK_SEEDING
/* Transform to [1, 0x7ffffffe] range. */
val = (val % 0x7ffffffe) + 1;
#endif
int r = do_rand(&val);
*ctx = (unsigned int) val;
@ -104,6 +108,10 @@ srand(seed)
u_int seed;
{
next = seed;
#ifndef USE_WEAK_SEEDING
/* Transform to [1, 0x7ffffffe] range. */
next = (next % 0x7ffffffe) + 1;
#endif
}
@ -125,6 +133,10 @@ sranddev()
mib[0] = CTL_KERN;
mib[1] = KERN_ARND;
sysctl(mib, 2, (void *)&next, &len, NULL, 0);
#ifndef USE_WEAK_SEEDING
/* Transform to [1, 0x7ffffffe] range. */
next = (next % 0x7ffffffe) + 1;
#endif
}