INITSTATE(P) POSIX Programmer's Manual INITSTATE(P)
initstate, random, setstate, srandom - pseudo-random number functions
char *initstate(unsigned seed, char *state, size_t size);
char *setstate(const char *state);
void srandom(unsigned seed);
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-ran-
dom 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 period.
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 dupli-
cated 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 gen-
erators. 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 genera-
tor 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 pre-
vious 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
1. The initstate() function can be used, with the desired seed, state array, and size of
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 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.
drand48() , rand() , the Base Definitions volume of IEEE Std 1003.1-2001, <stdlib.h>
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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 INITSTATE(P)