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NAME
initstate, random, setstate, srandom - pseudo-random number functions
SYNOPSIS
#include <stdlib.h> char *initstate(unsigned seed, char *state, size_t size); long random(void); char *setstate(const char *state); void srandom(unsigned seed);
DESCRIPTION
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 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 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.
RETURN VALUE
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.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
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 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.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
drand48() , rand() , the Base Definitions volume of IEEE Std 1003.1-2001, <stdlib.h>
COPYRIGHT
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 .