Tribblix: manual page: drand48.3c

DRAND48(3C) Standard C Library Functions DRAND48(3C)

NAME

drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48,
seed48, lcong48 - generate uniformly distributed pseudo-random
numbers

SYNOPSIS

#include <stdlib.h>

double drand48(void)

double erand48(unsigned short x(i)[3]);

long lrand48(void)

long nrand48(unsigned short x(i)[3]);

long mrand48(void)

long jrand48(unsigned short x(i)[3]);

void srand48(long seedval);

unsigned short *seed48(unsigned short seed16v[3]);

void lcong48(unsigned short param[7]);

DESCRIPTION

This family of functions generates pseudo-random numbers using the
well-known linear congruential algorithm and 48-bit integer
arithmetic.

Functions drand48() and erand48() return non-negative double-
precision floating-point values uniformly distributed over the
interval [0.0, 1.0).

Functions lrand48() and nrand48() return non-negative long integers
uniformly distributed over the interval [0, 2^31].

Functions mrand48() and jrand48() return signed long integers
uniformly distributed over the interval [-2^31, 2^31].

Functions srand48(), seed48(), and lcong48() are initialization entry
points, one of which should be invoked before either drand48(),
lrand48(), or mrand48() is called. (Although it is not recommended
practice, constant default initializer values will be supplied
automatically if drand48(), lrand48(), or mrand48() is called without
a prior call to an initialization entry point.) Functions erand48(),
nrand48(), and jrand48() do not require an initialization entry point
to be called first.

All the routines work by generating a sequence of 48-bit integer
values, X(i ), according to the linear congruential formula

X(n+1)= (aX (n)+c)(mod m) n>=0.

The parameter m = 2^48; hence 48-bit integer arithmetic is performed.
Unless lcong48() has been invoked, the multiplier value a and the
addend value c are given by
a = 5DEECE66D(16) = 273673163155(8)
c = B(16) = 13(8)

The value returned by any of the functions drand48(), erand48(),
lrand48(), nrand48(), mrand48(), or jrand48() is computed by first
generating the next 48-bit X(i) in the sequence. Then the appropriate
number of bits, according to the type of data item to be returned,
are copied from the high-order (leftmost) bits of X(i) and
transformed into the returned value.

The functions drand48(), lrand48(), and mrand48() store the last
48-bit X(i) generated in an internal buffer. X(i) must be initialized
prior to being invoked. The functions erand48(), nrand48(), and
jrand48() require the calling program to provide storage for the
successive X(i) values in the array specified as an argument when the
functions are invoked. These routines do not have to be initialized;
the calling program must place the desired initial value of X(i) into
the array and pass it as an argument. By using different arguments,
functions erand48(), nrand48(), and jrand48() allow separate modules
of a large program to generate several independent streams of pseudo-
random numbers, that is, the sequence of numbers in each stream will
not depend upon how many times the routines have been called to
generate numbers for the other streams.

The initializer function srand48() sets the high-order 32 bits of
X(i) to the 32 bits contained in its argument. The low-order 16 bits
of X(i) are set to the arbitrary value 330E(16) .

The initializer function seed48() sets the value of X(i) to the
48-bit value specified in the argument array. In addition, the
previous value of X(i) is copied into a 48-bit internal buffer, used
only by seed48(), and a pointer to this buffer is the value returned
by seed48(). This returned pointer, which can just be ignored if not
needed, is useful if a program is to be restarted from a given point
at some future time -- use the pointer to get at and store the last
X(i) value, and then use this value to reinitialize using seed48()
when the program is restarted.

The initialization function lcong48() allows the user to specify the
initial X(i) the multiplier value a, and the addend value c.
Argument array elements param[0-2] specify X(i), param[3-5] specify
the multiplier a, and param[6] specifies the 16-bit addend c. After
lcong48() has been called, a subsequent call to either srand48() or
seed48() will restore the ``standard'' multiplier and addend values,
a and c, specified above.

For a more powerful random number generator, see arc4random(3C).

ATTRIBUTES

NAME

SYNOPSIS

DESCRIPTION

ATTRIBUTES

SEE ALSO