# drand48 man page

## Prolog

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

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

## Synopsis

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

## Description

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

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

The *lrand48*() and *nrand48*() functions shall return non-negative, long integers, uniformly distributed over the interval [0,2^{31}).

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

The *srand48*(), *seed48*(), and *lcong48*() functions are initialization entry points, one of which should be invoked before either *drand48*(), *lrand48*(), or *mrand48*() is called. (Although it is not recommended practice, constant default initializer values shall be supplied automatically if *drand48*(), *lrand48*(), or *mrand48*() is called without a prior call to an initialization entry point.) The *erand48*(), *nrand48*(), and *jrand48*() 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, $X\_i\text{},$ according to the linear congruential formula:

${X}_{n+1}\text{}=\text{}(aX\_n\text{}\text{}+\text{}c{)}_{mod\text{}m}\text{}\text{}\text{}\text{}\text{}\text{}\text{}\text{}n\ge \text{}0$

The parameter $m\text{}=\text{}2\text{^}48$; hence 48-bit integer arithmetic is performed. Unless *lcong48*() is invoked, the multiplier value $a$ and the addend value $c$ are given by:

$a\text{}=\text{}\text{5DEECE66D}{\text{}}_{16}\text{}=\text{}273673163155{\text{}}_{8}$

$c\text{}=\text{}B{\text{}}_{16}\text{}=\text{}13{\text{}}_{8}$

The value returned by any of the *drand48*(), *erand48*(), *jrand48*(), *lrand48*(), *mrand48*(), or *nrand48*() functions is computed by first generating the next 48-bit $X\_i$ in the sequence. Then the appropriate number of bits, according to the type of data item to be returned, are copied from the high-order (leftmost) bits of $X\_i$ and transformed into the returned value.

The *drand48*(), *lrand48*(), and *mrand48*() functions store the last 48-bit $X\_i$ generated in an internal buffer; that is why the application shall ensure that these are initialized prior to being invoked. The *erand48*(), *nrand48*(), and *jrand48*() functions require the calling program to provide storage for the successive $X\_i$ values in the array specified as an argument when the functions are invoked. That is why these routines do not have to be initialized; the calling program merely has to place the desired initial value of $X\_i$ into the array and pass it as an argument. By using different arguments, *erand48*(), *nrand48*(), and *jrand48*() 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 shall *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 $X\_i$ to the low-order 32 bits contained in its argument. The low-order 16 bits of $X\_i$ are set to the arbitrary value $330E\_16\text{}.$

The initializer function *seed48*() sets the value of $X\_i$ to the 48-bit value specified in the argument array. The low-order 16 bits of $X\_i$ are set to the low-order 16 bits of *seed16v*[*0*]. The mid-order 16 bits of $X\_i$ are set to the low-order 16 bits of *seed16v*[*1*]. The high-order 16 bits of $X\_i$ are set to the low-order 16 bits of *seed16v*[*2*]. In addition, the previous value of $X\_i$ is copied into a 48-bit internal buffer, used only by *seed48*(), and a pointer to this buffer is the value returned by *seed48*(). This returned pointer, which can just 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 $X\_i$ value, and then use this value to reinitialize via *seed48*() when the program is restarted.

The initializer function *lcong48*() allows the user to specify the initial $X\_i\text{},$ the multiplier value $a,$ and the addend value $c.$ Argument array elements *param*[*0-2*] specify $X\_i\text{},$*param*[*3-5*] specify the multiplier $a,$ and *param*[*6*] specifies the 16-bit addend $c.$ After *lcong48*() is called, a subsequent call to either *srand48*() or *seed48*() shall restore the standard multiplier and addend values, *a* and *c,* specified above.

The *drand48*(), *lrand48*(), and *mrand48*() functions need not be thread-safe.

## Return Value

As described in the Description above.

## Errors

No errors are defined.

*The following sections are informative.*

## Examples

None.

## Application Usage

None.

## Rationale

None.

## Future Directions

None.

## See Also

*rand()*

The Base Definitions volume of POSIX.1‐2008, **<stdlib.h>**

## Copyright

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html .

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