The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


initstate, random, setstate, srandom - pseudo-random number functions


[XSI] [Option Start] #include <stdlib.h>

char *initstate(unsigned
seed, char *state, size_t size);
long random(void);
char *setstate(char *
void srandom(unsigned
seed); [Option End]


The random() function shall use 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 shall increase the period.

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

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

The srandom() function shall initialize 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 initialized for future use. The size argument, which specifies the size in bytes of the state array, shall be 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. If initstate() is called with 8<=size<32, then random() shall use 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 shall return a pointer to the previous state information array.

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

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


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

The random() function shall return the generated pseudo-random number.

The srandom() function shall not return a value.

Upon successful completion, initstate() and setstate() shall return a pointer to the previous state array; otherwise, a null pointer shall be returned.


No errors are defined.

The following sections are informative.




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

  1. The initstate() function can be used, with the desired seed, state array, and size of the array.

  2. The setstate() function, with the desired state, can be used, followed by srandom() with the desired seed. The advantage of using both of these functions is that the size of the state array does not have to be saved once it is initialized.

Although some implementations of random() have written messages to standard error, such implementations do not conform to POSIX.1-2017.

Issue 5 restored the historical behavior of this function.

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

These functions should be avoided whenever non-trivial requirements (including safety) have to be fulfilled.






drand48, rand

XBD <stdlib.h>


First released in Issue 4, Version 2.

Issue 5

Moved from X/OPEN UNIX extension to BASE.

In the DESCRIPTION, the phrase "values smaller than 8" is replaced with "values greater than or equal to 8, or less than 32", "size<8" is replaced with "8<=size <32", and a new first paragraph is added to the RETURN VALUE section. A note is added to the APPLICATION USAGE indicating that these changes restore the historical behavior of the function.

Issue 6

In the DESCRIPTION, duplicate text "For values greater than or equal to 8 ..." is removed.

IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/30 is applied, removing rand_r() from the list of suggested functions in the APPLICATION USAGE section.

Issue 7

The type of the first argument to setstate() is changed from const char * to char *.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0179 [743] is applied.

End of informative text.


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