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drand48(3)

NAME

     drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48,
     lcong48 -- pseudo random number generators and initialization routines

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <stdlib.h>

     double
     drand48(void);

     double
     erand48(unsigned short xseed[3]);

     long
     lrand48(void);

     long
     nrand48(unsigned short xseed[3]);

     long
     mrand48(void);

     long
     jrand48(unsigned short xseed[3]);

     void
     srand48(long seed);

     unsigned short *
     seed48(unsigned short xseed[3]);

     void
     lcong48(unsigned short p[7]);

DESCRIPTION

     The rand48() family of functions generates pseudo-random numbers using a
     linear congruential algorithm working on integers 48 bits in size.  The
     particular formula employed is r(n+1) = (a * r(n) + c) mod m where the
     default values are for the multiplicand a = 0xfdeece66d = 25214903917 and
     the addend c = 0xb = 11.  The modulo is always fixed at m = 2 ** 48.
     r(n) is called the seed of the random number generator.

     For all the six generator routines described next, the first computa-
     tional step is to perform a single iteration of the algorithm.

     The drand48() and erand48() functions return values of type double.  The
     full 48 bits of r(n+1) are loaded into the mantissa of the returned
     value, with the exponent set such that the values produced lie in the
     interval [0.0, 1.0).

     The lrand48() and nrand48() functions return values of type long in the
     range [0, 2**31-1].  The high-order (31) bits of r(n+1) are loaded into
     the lower bits of the returned value, with the topmost (sign) bit set to
     zero.

     On the other hand, erand48(), nrand48(), and jrand48() use a user-sup-
     plied buffer to store the seed r(n), which consists of an array of 3
     shorts, where the zeroth member holds the least significant bits.

     All functions share the same multiplicand and addend.

     The srand48() function is used to initialize the internal buffer r(n) of
     drand48(), lrand48(), and mrand48() such that the 32 bits of the seed
     value are copied into the upper 32 bits of r(n), with the lower 16 bits
     of r(n) arbitrarily being set to 0x330e.  Additionally, the constant mul-
     tiplicand and addend of the algorithm are reset to the default values
     given above.

     The seed48() function also initializes the internal buffer r(n) of
     drand48(), lrand48(), and mrand48(), but here all 48 bits of the seed can
     be specified in an array of 3 shorts, where the zeroth member specifies
     the lowest bits.  Again, the constant multiplicand and addend of the
     algorithm are reset to the default values given above.  The seed48()
     function returns a pointer to an array of 3 shorts which contains the old
     seed.  This array is statically allocated, thus its contents are lost
     after each new call to seed48().

     Finally, lcong48() allows full control over the multiplicand and addend
     used in drand48(), erand48(), lrand48(), nrand48(), mrand48(), and
     jrand48(), and the seed used in drand48(), lrand48(), and mrand48().  An
     array of 7 shorts is passed as argument; the first three shorts are used
     to initialize the seed; the second three are used to initialize the mul-
     tiplicand; and the last short is used to initialize the addend.  It is
     thus not possible to use values greater than 0xffff as the addend.

     Note that all three methods of seeding the random number generator always
     also set the multiplicand and addend for any of the six generator calls.

     For a more powerful random number generator, see random(3).

AUTHORS

     Martin Birgmeier

SEE ALSO

     rand(3), random(3)

FreeBSD 5.4			October 8, 1993 		   FreeBSD 5.4

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