### SYNOPSIS

**#include <stdlib.h>**

**double drand48(void);**

**"double erand48(unsigned short "***xsubi***[3]);**

**long int lrand48(void);**

**"long int nrand48(unsigned short "***xsubi***[3]);**

**long int mrand48(void);**

**"long int jrand48(unsigned short "***xsubi***[3]);**

**"void srand48(long int "***seedval***);**

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

These functions generate pseudo-random numbers using the linear congruential algorithm and 48-bit integer arithmetic. The **drand48()** and **erand48()** functions return non-negative double-precision floating-point values uniformly distributed between [0.0, 1.0).

The **lrand48()** and **nrand48()** functions return non-negative long integers uniformly distributed between 0 and 2^31.

The **mrand48()** and **jrand48()** functions return signed long integers uniformly distributed between -2^31 and 2^31.

The **srand48()**, **seed48()** and **lcong48()** functions are initialization functions, one of which should be called before using **drand48()**, **lrand48()** or **mrand48()**. The functions **erand48()**, **nrand48()** and **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 **lcong48()** is called, *a* and *c* are given by:

**a = 0x5DEECE66D**

**c = 0xB**

The value returned by any of the functions **drand48()**, **erand48()**, **lrand48()**, **nrand48()**, **mrand48()** or **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 **drand48()**, **lrand48()** and **mrand48()** store the last 48-bit *Xi* generated in an internal buffer. The functions **erand48()**, **nrand48()** and **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 **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 **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 **seed48()**.