Copyright © 1997 The Open Group

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

`#include <stdlib.h> double drand48(void); double erand48(unsigned short int`

xsubi[3]); long int jrand48(unsigned short intxsubi[3]); void lcong48(unsigned short intparam[7]); long int lrand48(void); long int mrand48(void); long int nrand48(unsigned short intxsubi[3]); unsigned short int *seed48(unsigned short intseed16v[3]); void srand48(long intseedval);

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

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

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

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

srand48(),seed48()andlcong48()are initialisation entry points, one of which should be invoked before eitherdrand48(),lrand48()ormrand48()is called. (Although it is not recommended practice, constant default initialiser values will be supplied automatically ifdrand48(),lrand48()ormrand48()is called without a prior call to an initialisation entry point.) Theerand48(),nrand48()andjrand48()functions do not require an initialisation 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 they must be initialised 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 initialised; 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 willnotdepend upon how many times the routines are called to generate numbers for the other streams.The initialiser 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 initialiser 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 re-initialise viaseed48()when the program is restarted.The initialiser 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()will restore the standard multiplier and addend values,aandc, specified above.The

drand48(),lrand48()andmrand48()interfaces need not be reentrant.

As described in the DESCRIPTION above.

No errors are defined.

None.

None.

None.

rand(),<stdlib.h>.

Derived from Issue 1 of the SVID.

Copyright © 1997 The Open Group

[ Main Index | XSH | XCU | XBD | XCURSES | XNS ]