rand,
rand_r,
srand,
srand_deterministic —
bad pseudo-random number generator
#include
<stdlib.h>
void
srand(
unsigned
int seed);
void
srand_deterministic(
unsigned
int seed);
int
rand(
void);
int
rand_r(
unsigned
int *seed);
Standards insist
that this interface return deterministic results. Unsafe usage is very common,
so OpenBSD changed the subsystem to return
non-deterministic results by default.
To satisfy portable code,
srand() may be called to
initialize the subsystem. In
OpenBSD the
seed variable is ignored, and strong random
number results will be provided from
arc4random(3). In other
systems, the
seed variable primes a
simplistic deterministic algorithm.
If the standardized behavior is required
srand_deterministic() can be substituted for
srand(), then subsequent
rand() calls will return results using the
deterministic algorithm.
The
rand() function returns a result in the range
of 0 to
RAND_MAX. By default, this result
comes from
arc4random(3). If
srand_deterministic() was called, the result will
be computed using the deterministic algorithm.
The
rand_r() is a thread-safe version of
rand(). Storage for the seed must be provided
through the
seed argument, and needs to have
been initialized by the caller. It always operates using the deterministic
algorithm.
arc4random(3),
rand48(3),
random(3)
The
rand() function conforms to
ANSI X3.159-1989
(“ANSI C89”).
The
rand_r() function conforms to
IEEE Std 1003.1-2008
(“POSIX.1”).
The
srand() function does not conform to
ANSI X3.159-1989
(“ANSI C89”), intentionally.
The
srand_deterministic() function is an
OpenBSD extension.
The functions
rand() and
srand() first appeared in
Version 3 AT&T UNIX.