# drand48(), erand48(), jrand48(), lcong48(), lrand48(), mrand48(), nrand48(), seed48(), srand48()generate pseudo-random numbers using 48-bit integer arithmetic

## SYNOPSIS

#include <stdlib.h>

double drand48(void);

double erand48(unsigned short xsubi[3]);

long jrand48(unsigned short xsubi[3]);

void lcong48(unsigned short param[7]);

long lrand48(void);

long mrand48(void);

long nrand48(unsigned short xsubi[3]);

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

void srand48(long seedval);

## DESCRIPTION

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

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

All the routines work by generating a sequence of 48-bit integer values, Xi, according to the linear congruential formula:

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 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 (leftmost) bits of Xi and transformed into the return value.

The drand48(), lrand48() and mrand48() functions store the last 48-bit Xi in generated in an internal buffer; that is why they must be initialized prior to being invoked. The erand48(), jrand48() and nrand48() functions require the calling program to provide storage for the successive Xi values in the array specified as a parameter when the functions are invoked. This is why these routines do not have to be initialized; the calling program merely has to place the desired initial value of Xi into the array and pass it as an argument. By using different arguments, erand48(), jrand48() and nrand48() 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 does not depend upon how many times the routines are called to generate numbers for the other streams.

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

The initializer function seed48() sets the value Xi to the 48-bit value specified in the argument array. In addition, the previous value of Xi is copied into a 48-bit internal buffer, used only by seed48(), and a pointer to this buffer is returned by seed48(). This returned pointer, which can 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 Xi value, and then use this value to re-initialize via seed48() when the program is restarted.

The initializer function lcong48() allows the user to specify the initial Xi, the multiplier value a, and the addend value c. After lcong48() is called, a subsequent call to either srand48() or seed48() restores the standard multiplier and addend values, a and c, as specified earlier.

## PARAMETERS

xsubi

Specifies the next value for Xi in the linear congruential formula described earlier. The functions erand48(), jrand48() and nrand48() fill in this array on each iteration; prior to calling any of these functions for the first time, this array should be initialized with the first Xi value.

param

Specifies the initial Xi, the multiplier value a, and the addend value c, for the linear congruential formula described above. Array elements param[0-2] specify Xi, param[3-5] specify the multiplier a, and param[6] specifies the 16-bit addend c.

seed16v

Specifies the value of Xi in the linear congruential formula described above. The low-order 16 bits of Xi are set to the low-order 16 bits of seed16v[0]. The mid-order 16 bits of Xi are set to the low-order 16 bits of seed16v[1]. The high-order 16 bits of Xi are set to the low-order 16 bits of seed16v[2].

seedval

Specifies the high-order 32 bits of Xi. The low-order 16 bits of Xi are set to the arbitrary value 330E16.

## RETURN VALUES

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 seed48() function returns a pointer to a buffer containing the previous value of Xi.

The lcong48() and srand48() functions return nothing.

## CONFORMANCE

UNIX 98

drand48(), lrand48(), mrand48(): Unsafe

erand48(), jrand48(), lcong48(), nrand48(), seed48(), srand48(): MT-Safe.

None.

## AVAILABILITY

PTC MKS Toolkit for Professional Developers
PTC MKS Toolkit for Professional Developers 64-Bit Edition
PTC MKS Toolkit for Enterprise Developers
PTC MKS Toolkit for Enterprise Developers 64-Bit Edition