The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


initstate, random, setstate, srandom - pseudorandom number functions


#include <stdlib.h>

char *initstate(unsigned int seed, char *state, size_t size);
long random(void);
char *setstate(const char *state);
void srandom(unsigned int seed);


The random() function uses a non-linear additive feedback random-number generator employing a default state array size of 31 long integers to return successive pseudo-random numbers in the range from 0 to 231-1. The period of this random-number generator is approximately 16 x (231-1). The size of the state array determines the period of the random-number generator. Increasing the state array size increases the period.

With 256 bytes of state information, the period of the random-number generator is greater than 269.

Like rand(), random() produces by default a sequence of numbers that can be duplicated by calling srandom() with 1 as the seed.

The srandom() function initialises the current state array using the value of seed.

The initstate() and setstate() functions handle restarting and changing random-number generators. The initstate() function allows a state array, pointed to by the state argument, to be initialised for future use. The size argument, which specifies the size in bytes of the state array, is used by initstate() to decide what type of random-number generator to use; the larger the state array, the more random the numbers. Values for the amount of state information are 8, 32, 64, 128, and 256 bytes. Other values greater than 8 bytes are rounded down to the nearest one of these values. For values greater than or equal to 8, or less than 32 random() uses a simple linear congruential random number generator. The seed argument specifies a starting point for the random-number sequence and provides for restarting at the same point. The initstate() function returns a pointer to the previous state information array.

If initstate() has not been called, then random() behaves as though initstate() had been called with seed=1 and size=128.

If initstate() is called with 8 <= size <32, then random() uses a simple linear congruential random number generator.

Once a state has been initialised, setstate() allows switching between state arrays. The array defined by the state argument is used for further random-number generation until initstate() is called or setstate() is called again. The setstate() function returns a pointer to the previous state array.


If initstate() is called with size less than 8, it returns NULL.

The random() function returns the generated pseudo-random number.

The srandom() function returns no value.

Upon successful completion, initstate() and setstate() return a pointer to the previous state array. Otherwise, a null pointer is returned.


No errors are defined.




After initialisation, a state array can be restarted at a different point in one of two ways:

Although some implementations of random() have written messages to standard error, such implementations do not conform to this specification.

Issue 5 restores the historical behaviour of this function.

Threaded applications should use rand_r(), erand48(), nrand48() or jrand48() instead of random() when an independent random number sequence in multiple threads is required.




drand48(), rand(), <stdlib.h>.

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
[ Main Index | XSH | XCU | XBD | XCURSES | XNS ]