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RAND(3) FreeBSD Library Functions Manual RAND(3)
NAME
rand, srand, rand_r - bad random number generator
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <stdlib.h>
void
srand(unsigned seed);
int
rand(void);
int
rand_r(unsigned *ctx);
DESCRIPTION
The functions described in this manual page are not cryptographically
secure. Applications which require unpredictable random numbers should
use arc4random(3) instead.
The rand() function computes a sequence of pseudo-random integers in the
range of 0 to RAND_MAX, inclusive.
The srand() function seeds the algorithm with the seed parameter.
Repeatable sequences of rand() output may be obtained by calling srand()
with the same seed. rand() is implicitly initialized as if srand(1) had
been invoked explicitly.
In FreeBSD 13, rand() is implemented using the same 128-byte state LFSR
generator algorithm as random(3). However, the legacy rand_r() function
is not (and can not be, because of its limited *ctx size). rand_r()
implements the historical, poor-quality Park-Miller 32-bit LCG and should
not be used in new designs.
IMPLEMENTATION NOTES
Since FreeBSD 13, rand() is implemented with the same generator as
random(3), so the low-order bits should no longer be significantly worse
than the high-order bits.
SEE ALSO
arc4random(3), random(3), random(4)
STANDARDS
The rand() and srand() functions conform to ISO/IEC 9899:1990
("ISO C90").
The rand_r() function is not part of ISO/IEC 9899:1990 ("ISO C90") and is
marked obsolescent in IEEE Std 1003.1-2008 ("POSIX.1"). It may be
removed in a future revision of POSIX.
CAVEATS
Prior to FreeBSD 13, rand() used the historical Park-Miller generator
with 32 bits of state and produced poor quality output, especially in the
lower bits. rand() in earlier versions of FreeBSD, as well as other
If broader portability or better performance is desired, any of the
widely available and permissively licensed SFC64/32, JSF64/32, PCG64/32,
or SplitMix64 algorithm implementations may be embedded in your
application. These algorithms have the benefit of requiring less space
than random(3) and being quite fast (in header inline implementations).
FreeBSD 14.0-RELEASE-p6 February 1, 2020 FreeBSD 14.0-RELEASE-p6