RANDOM(3,L)                 AIX Technical Reference                 RANDOM(3,L)



-------------------------------------------------------------------------------
random, srandom, initstate, setstate



PURPOSE

Generates pseudo-random numbers and changes number generators.

LIBRARY

Standard C Library (libc.a)

SYNTAX



               long random ( )             char *initstate (seed, state, bytes)
                                           unsigned int seed;
               srandom (seed);             char *state;
               int seed;                   int bytes;

               char *setstate (state)
               char *state;



DESCRIPTION

The random subroutine generates random numbers using a nonlinear additive
feedback random-number generator.  This generator uses a default table 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 very large,
approximately 16* (2(31)-1).

The srandom subroutine resets the random-number generator to a random starting
point when used after the setstate subroutine.  Like the rand subroutine, the
random generator is initially seeded with a value of 1.

The srandom subroutine, unlike the srand subroutine, does not return the old
seed because the amount of state information used is more than a single word.
Two other subroutines, initstate and setstate, handle restarting and changing
random-number generators.

The initstate subroutine allows a state array, passed in by the state
parameter, to be initialized for future use.  The size of the state array (in
bytes) is contained in the bytes parameter, which is used by the initstate
subroutine to decide how complex the random-number generator should be.  When
more bytes of state information are used, the numbers are more random.  Values
for the amount of state information are:  8, 32, 64, 128, and 256 bytes.
Amounts less than 8 bytes generate an error, while other amounts are rounded
down to the nearest known value.  The seed parameter specifies a starting point



Processed November 7, 1990        RANDOM(3,L)                                 1

RANDOM(3,L)                 AIX Technical Reference                 RANDOM(3,L)



for the random-number sequence and provides for restarting at the same point.
The initstate subroutine returns a pointer to the previous state information
array.

Once a state has been initialized, the setstate subroutine allows rapid
switching between states.  The array defined by state parameter is used for
further random-number generation until the initstate subroutine is called or
the setstate subroutine is called again.  The setstate subroutine returns a
pointer to the previous state array.

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

  o The initstate subroutine can be used, with the desired seed, state array,
    and size of the array, or

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

    With a full 256 bytes of state information, the period of the random-number
    generator is greater than 2(69), which should be sufficient for most
    purposes.

ERROR CONDITIONS

If the initstate subroutine is called with less than 8 bytes of state
information, or if the setstate subroutine detects that the state information
has been damaged, error messages are sent to the standard output.

RELATED INFORMATION

In this book:  "drand48" and "rand, srand."





















Processed November 7, 1990        RANDOM(3,L)                                 2