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RANDOM(3) Library Functions Manual RANDOM(3)

random, srandom, srandom_deterministic, srandomdev, initstate, setstatepseudo-random number generator; routines for changing generators

#include <stdlib.h>


srandom(unsigned int seed);

srandom_deterministic(unsigned int seed);


char *
initstate(unsigned int seed, char *state, size_t n);

char *
setstate(char *state);

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, () or () 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 () can be substituted for srandom(), then subsequent random() calls will return results using the deterministic algorithm.

In non-deterministic (default) mode, the () function returns results from arc4random(3) in the range from 0 to (2**31)-1.

In deterministic mode, the () function uses a non-linear additive feedback random number generator employing a default table of size 31 long integers to return successive pseudo-random numbers in the range from 0 to (2**31)-1. The period of this random number generator is very large, approximately 16*((2**31)-1), but the results are a deterministic sequence from the seed. The deterministic sequence algorithm changed a number of times since original development, is underspecified, and should not be relied upon to remain consistent between platforms and over time.

The () routine allows a state array, passed in as an argument, to be initialized for future use. The size of the state array (in bytes) is used by initstate() to decide how sophisticated a random number generator it should use — the more state, the better the random numbers will be. (Current "optimal" values for the amount of state information are 8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to the nearest known amount. Using less than 8 bytes will cause an error.) The seed for the initialization (which specifies a starting point for the random number sequence, and provides for restarting at the same point) is also an argument. The initstate() function returns a pointer to the previous state information array.

Once a state has been initialized, the () routine provides for rapid switching between states. The setstate() function returns a pointer to the previous state array; its argument state array is used for further random number generation until the next call to initstate() or setstate().

Once a state array has been initialized, it may be restarted at a different point either by calling () (with the desired seed, the state array, and its size) or by calling both setstate() (with the state array) and srandom() (with the desired seed). The advantage of calling both setstate() and srandom() is that the size of the state array does not have to be remembered after it is initialized.

Use of (), initstate(), or setstate() forces the subsystem into deterministic mode.

If initstate() is called with less than 8 bytes of state information, or if setstate() detects that the state information has been garbled, error messages are printed on the standard error output.

arc4random(3), drand48(3), rand(3), random(4)

The random(), initstate(), and setstate() functions conform to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”).

The srandom() function does not conform to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”), intentionally.

The srandomdev() function is an extension.

The srandom_deterministic() function is an OpenBSD extension.

These functions appeared in 4.2BSD.

Earl T. Cohen

February 12, 2021 OpenBSD-current