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
2**31-1. The period of this random-number generator is approximately 16 x
(2**31-1). The size of the state array determines the period of the
random-number generator. Increasing the state array size shall increase the
With 256 bytes of state information, the period of the random-number generator
shall be greater than 2**69.
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
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
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
After initialization, a state array can be restarted at a different point in one
of two ways:
The initstate() function can be used, with the
desired seed, state array, and size of the array.
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
IEEE Std 1003.1-2001.
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.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html