IEEE Std 1003.1, 2004 Edition

Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.

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

^{[XSI]}#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);

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

The

drand48() anderand48() functions shall return non-negative, double-precision, floating-point values, uniformly distributed over the interval [0.0,1.0).The

lrand48() andnrand48() functions shall return non-negative, long integers, uniformly distributed over the interval [0,2^{31}).The

mrand48() andjrand48() functions shall return signed long integers uniformly distributed over the interval [-2^{31},2^{31}).The

srand48(),seed48(), andlcong48() functions are initialization entry points, one of which should be invoked before eitherdrand48(),lrand48(), ormrand48() is called. (Although it is not recommended practice, constant default initializer values shall be supplied automatically ifdrand48(),lrand48(), ormrand48() is called without a prior call to an initialization entry point.) Theerand48(),nrand48(), andjrand48() 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}, according to the linear congruential formula:X_{n+1}= (aX_{n}+ c)_{mod m}n>= 0The parameter

m= 2^{48}; hence 48-bit integer arithmetic is performed. Unlesslcong48() is invoked, the multiplier valueaand the addend valuecare given by:a= 5DEECE66D_{16}= 273673163155_{8}

c= B_{16}= 13_{8}The value returned by any of the

drand48(),erand48(),jrand48(),lrand48(),mrand48(), ornrand48() 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(), andmrand48() 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. Theerand48(),nrand48(), andjrand48() 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(), andjrand48() allow separate modules of a large program to generate severalindependentstreams of pseudo-random numbers; that is, the sequence of numbers in each stream shallnotdepend 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}.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 ofseed16v[0]. The mid-order 16 bits of X_{i}are set to the low-order 16 bits ofseed16v[1]. The high-order 16 bits of X_{i}are set to the low-order 16 bits ofseed16v[2]. In addition, the previous value of X_{i}is copied into a 48-bit internal buffer, used only byseed48(), and a pointer to this buffer is the value returned byseed48(). 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 viaseed48() when the program is restarted.The initializer function

lcong48() allows the user to specify the initial X_{i}, the multiplier valuea, and the addend valuec. Argument array elementsparam[0-2] specify X_{i},param[3-5] specify the multipliera, andparam[6] specifies the 16-bit addendc. Afterlcong48() is called, a subsequent call to eithersrand48() orseed48() shall restore the standard multiplier and addend values,aandc,specified above.The

drand48(),lrand48(), andmrand48() functions need not be reentrant. A function that is not required to be reentrant is not required to be thread-safe.

As described in the DESCRIPTION above.

No errors are defined.

None.

None.

None.

None.

rand(), the Base Definitions volume of IEEE Std 1003.1-2001,<stdlib.h>

First released in Issue 1. Derived from Issue 1 of the SVID.

A note indicating that these functions need not be reentrant is added to the DESCRIPTION.

The DESCRIPTION is updated to avoid use of the term "must" for application requirements.

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