drand48

NAME

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

SYNOPSIS

#include <stdlib.h>
 double drand48(void);
 I double erand48(unsigned short  xsubi [3]);
 long int lrand48(void);
 I long int nrand48(unsigned short  xsubi [3]);
 long int mrand48(void);
 I long int jrand48(unsigned short  xsubi [3]);
 I void srand48(long int  seedval );
 I unsigned short *seed48(unsigned short  seed16v [3]);
 I void lcong48(unsigned short  param [7]);

DESCRIPTION

These functions generate pseudo-random numbers using the linear congruential algorithm and 48-bit integer arithmetic.
The R drand48 () and R erand48 () functions return non-negative double-precision floating-point values uniformly distributed between [0.0, 1.0).
The R lrand48 () and R nrand48 () functions return non-negative long integers uniformly distributed between 0 and 2^31.
The R mrand48 () and R jrand48 () functions return signed long integers uniformly distributed between -2^31 and 2^31.
The R srand48 (), R seed48 () and R lcong48 () functions are initialization functions, one of which should be called before using R drand48 (), R lrand48 () or R mrand48 (). The functions R erand48 (), R nrand48 () and R jrand48 () do not require an initialization function to be called first.
All the functions work by generating a sequence of 48-bit integers, Xi, according to the linear congruential formula:
Xn+1 = (aXn + c) mod m, where n >= 0
The parameter m = 2^48, hence 48-bit integer arithmetic is performed. Unless R lcong48 () is called, a and c are given by:
a = 0x5DEECE66D c = 0xB
The value returned by any of the functions R drand48 (), R erand48 (), R lrand48 (), R nrand48 (), R mrand48 () or R jrand48 () is computed by first generating the next 48-bit Xi in the sequence. Then the appropriate number of bits, according to the type of data item to be returned, is copied from the high-order bits of Xi and transformed into the returned value.
The functions R drand48 (), R lrand48 () and R mrand48 () store the last 48-bit Xi generated in an internal buffer. The functions R erand48 (), R nrand48 () and R jrand48 () require the calling program to provide storage for the successive Xi values in the array argument xsubi. The functions are initialized by placing the initial value of Xi into the array before calling the function for the first time.
The initializer function R srand48 () sets the high order 32-bits of Xi to the argument seedval. The low order 16-bits are set to the arbitrary value 0x330E.
The initializer function R seed48 () sets the value of Xi to the 48-bit value specified in the array argument seed16v. The previous value of Xi is copied into an internal buffer and a pointer to this buffer is returned by R seed48 ().
The initialization function R lcong48 () allows the user to specify initial values for Xi, a and c. Array argument elements param[0-2] specify Xi, param[3-5] specify a, and param[6] specifies c. After R lcong48 () has been called, a subsequent call to either R srand48 () or R seed48 () will restore the standard values of a and c.

CONFORMING TO

SVr4, POSIX.1-2001.

NOTES

These functions are declared obsolete by SVID 3, which states that rand(3) should be used instead.

SEE ALSO

rand(3), random(3)