NAME

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

 SYNOPSIS

#include <stdlib.h>

double drand48(void);
double erand48(unsigned short int xsubi[3]);
long int jrand48(unsigned short int xsubi[3]);
void lcong48(unsigned short int param[7]);
long int lrand48(void);
long int mrand48(void);
long int nrand48(unsigned short int xsubi[3]);
unsigned short int *seed48(unsigned short int seed16v[3]);
void srand48(long int seedval);

 DESCRIPTION

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

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

The lrand48() and nrand48() functions return non-negative, long integers, uniformly distributed over the interval [0,2³¹].

The mrand48() and jrand48() functions return signed long integers uniformly distributed over the interval [-2³¹,2³¹].

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

 

Xn+1 = (aXn + c)mod m   n>= 0

The parameter m = 2⁴⁸; hence 48-bit integer arithmetic is performed. Unless lcong48() is invoked, the multiplier value a and the addend value c are given by:

 

a = 5DEECE66D16 = 2736731631558

c = B16 = 138

The value returned by any of the drand48(), erand48(), jrand48(), lrand48(), mrand48() or nrand48() functions 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 drand48(), lrand48() and mrand48() functions store the last 48-bit X_i generated in an internal buffer; that is why they must be initialised prior to being invoked. The erand48(), nrand48() and jrand48() functions 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. That is why these routines do not have to be initialised; the calling program merely has to place the desired initial value of X_i into the array and pass it as an argument. By using different arguments, 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 are called to generate numbers for the other streams.

The initialiser function srand48() sets the high-order 32 bits of X_i to the low-order 32 bits contained in its argument. The low-order 16 bits of X_i are set to the arbitrary value 330E₁₆ .

The initialiser function seed48() sets the value of X_i to the 48-bit value specified in the argument array. The low-order 16 bits of X_i are set to the low-order 16 bits of seed16v[0]. The mid-order 16 bits of X_i are set to the low-order 16 bits of seed16v[1]. The high-order 16 bits of X_i are set to the low-order 16 bits of seed16v[2]. 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 re-initialise via seed48() when the program is restarted.

The initialiser 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() is called, a subsequent call to either srand48() or seed48() will restore the standard multiplier and addend values, a and c, specified above.

The drand48(), lrand48() and mrand48() interfaces need not be reentrant.

 RETURN VALUE

As described in the DESCRIPTION above.

 ERRORS

No errors are defined.

 EXAMPLES

None.

 APPLICATION USAGE

None.

 FUTURE DIRECTIONS

None.

 SEE ALSO

rand(), <stdlib.h>.

DERIVATION

Derived from Issue 1 of the SVID.