# drand48 - Man Page

generate uniformly distributed pseudo-random numbers

## Synopsis

#include <stdlib.h> double drand48(void); double erand48(unsigned shortxsubi[3]); long lrand48(void); long nrand48(unsigned shortxsubi[3]); long mrand48(void); long jrand48(unsigned shortxsubi[3]); void srand48(longseedval); unsigned short *seed48(unsigned shortseed16v[3]); void lcong48(unsigned shortparam[7]);

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

All functions shown above:

_XOPEN_SOURCE || /* Glibc since 2.19: */ _DEFAULT_SOURCE || /* Glibc <= 2.19: */ _SVID_SOURCE

## Description

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*.

## Attributes

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

Interface | Attribute | Value |
---|---|---|

drand48(), erand48(), lrand48(), nrand48(), mrand48(), jrand48(), srand48(), seed48(), lcong48() | Thread safety | MT-Unsafe race:drand48 |

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

## Conforming to

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

## See Also

## Colophon

This page is part of release 5.13 of the Linux *man-pages* project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

## Referenced By

drand48_r(3), rand(3), random(3), random_r(3), stress-ng(1), zshmodules(1).

The man pages erand48(3), jrand48(3), lcong48(3), lrand48(3), mrand48(3), nrand48(3), seed48(3) and srand48(3) are aliases of drand48(3).