Linux / Unix Command: srand48 |

Command Library |

## 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 shortxsubi[3]);long int lrand48(void);long int nrand48(unsigned shortxsubi[3]);long int mrand48(void);long int jrand48(unsigned shortxsubi[3]);void srand48(long intseedval);unsigned short *seed48(unsigned shortseed16v[3]);void lcong48(unsigned shortparam[7]);

## DESCRIPTION

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()**.

The initialization function **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 **lcong48()**
has been called, a subsequent call to either **srand48()** or
**seed48()** will restore the standard values of *a* and *c*.

## CONFORMING TO

SVID 3## SEE ALSO

**rand**(3),

**random**(3)

**Important:**
Use the *man* command (*% man*) to see how a command is used on your
particular computer.