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RAND(3) Library Functions Manual RAND(3)

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.
December 9, 2014 OpenBSD-current