drand48(3C)                                                     drand48(3C)

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

SYNOPSIS
     #include <stdlib.h>

     double drand48(void);

     double erand48(unsigned short int xsubi[3]);

     long int lrand48(void);

     long int nrand48(unsigned short int xsubi[3]);

     long int mrand48(void);

     long int jrand48(unsigned short int xsubi[3]);

     void srand48(long int seedval);

     unsigned short int *seed48(unsigned short int seed16v[3]);

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

DESCRIPTION
     This family of functions generates pseudo-random numbers using the
     well-known linear congruent algorithm and 48-bit integer arithmetic.

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

     lrand48() and nrand48() return non-negative long integers uniformly
     distributed over the interval [0, 2^31).

     Functions mrand48() and jrand48() return signed long integers uni-
     formly distributed over the interval [-2^31, 2^31).

     Functions srand48(), seed48(), and lcong48() 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 will be supplied
     automatically if drand48(), lrand48(), or mrand48() is called without
     a prior call to an initialization entry point. Functions erand48(),
     nrand48(), and jrand48() 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 sub i according to the linear congruent formula:

     (see the Programmer's Reference Manual)



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drand48(3C)                                                     drand48(3C)

              X sub{n+1} = (aX sub n + c) sub{ mod m}        n>=0

     The parameter m is 2^48; hence 48-bit integer arithmetic is performed.
     Unless lcong48() has been invoked, the multiplier value a and the
     addend value c are given by

           a = 5DEECE66D sub 16 = 273673163155 sub 8

           c = B sub 16 = 13 sub 8

     The value returned by any of the functions drand48(), erand48(),
     lrand48(), nrand48(), mrand48(), or jrand48() is computed by first
     generating the next 48-bit X sub i in the sequence. Then the appropri-
     ate number of bits, according to the type of data item to be returned,
     are copied from the high-order, leftmost, bits of X sub i and
     transformed into the returned value. The functions drand48(),
     lrand48(), and mrand48() store the last X sub i generated in an inter-
     nal buffer. X sub i must be initialized prior to being invoked. For
     the functions erand48(), nrand48(), and jrand48(), the calling program
     must provide storage for the successive X sub i values in the array
     specified as an argument when the functions are invoked. These rou-
     tines do not have to be initialized; the calling program must place
     the desired initial value of X sub i into the array and pass it as an
     argument. By using different arguments, functions erand48(),
     nrand48(), and jrand48() allow separate modules of a large program to
     generate several independent streams of pseudo-random numbers, i.e.,
     the sequence of numbers in each stream will not depend upon how many
     times the routines have been called to generate numbers for the other
     streams.

     The initializer function srand48() sets the high-order 32 bits of
     X sub i to the 32 bits contained in its argument. The low-order 16
     bits of Xi are set to the arbitrary value 330E16.

     The initializer function seed48() sets the value of X sub i to the
     48-bit value specified in the argument array. In addition, the previ-
     ous value of X sub 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 sub i value, and then use this value to reinitialize via seed48()
     when the program is restarted.

     The initialization function lcong48() allows the user to specify the
     initial X sub i, the multiplier value a and the addend value c. Argu-
     ment array elements param[0-2] specify X sub i, param[3-5] the multi-
     plier a, and param[6] specifies the 16-bit addend c. After lcong48()
     has been called, a subsequent call to either srand48() or seed48()
     will restore the "standard" multiplier and addend values, a and c,
     specified on the previous page.



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drand48(3C)                                                     drand48(3C)

SEE ALSO
     rand(3C).




















































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