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## NAME

## SYNOPSIS

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## SEE ALSO

## COLOPHON

SYNOPSIS

DESCRIPTION

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SEE ALSO

COLOPHON

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

**#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]);**

**void
lcong48(unsigned short** *param***[7]);**

Feature Test
Macro Requirements for glibc (see
**feature_test_macros**(7)):

All functions shown above: _SVID_SOURCE || _XOPEN_SOURCE

These functions generate pseudo-random numbers using the linear congruential algorithm and 48-bit integer arithmetic.

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

The
**lrand48**() and **nrand48**() functions return
nonnegative long integers uniformly distributed over the
interval [0, 2^31).

The
**mrand48**() and **jrand48**() functions return
signed long integers uniformly distributed over the interval
[−2^31, 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*.

For an
explanation of the terms used in this section, see
**attributes**(7).

The above functions record global state information for the random number generator, so they are not thread-safe.

POSIX.1-2001, POSIX.1-2008, SVr4.

**rand**(3),
**random**(3)

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