drand48(3) - NetBSD Manual Pages

RAND48(3)               NetBSD Library Functions Manual              RAND48(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 = 0x5deece66d = 25214903917 and
     the addend c = 0xb = 11.  The modulus 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.

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

     lrand48() and nrand48() 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.

     mrand48() and jrand48() return values of type long in the range [-2**31,
     2**31-1].  The high-order (32) bits of r(n+1) are loaded into the
     returned value.

     drand48(), lrand48(), and mrand48() use an internal buffer to store r(n).
     For these functions the initial value of r(0) = 0x1234abcd330e =
     20017429951246.

     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
     unsigned shorts, where the zeroth member holds the least significant
     bits.

     All functions share the same multiplicand and addend.

     srand48() 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 multiplicand
     and addend of the algorithm are reset to the default values given above.

     seed48() also initializes the internal buffer r(n) of drand48(),
     lrand48(), and mrand48(), but here all 48 bits of the seed can be speci-
     fied in an array of 3 unsigned 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.  seed48() returns
     a pointer to an array of 3 unsigned 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 unsigned shorts is passed as parameter; the first three shorts
     are used to initialize the seed; the second three are used to initialize
     the multiplicand; 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).


SEE ALSO

     rand(3), random(3)


AUTHORS

     Martin Birgmeier

NetBSD 10.99                   February 22, 2020                  NetBSD 10.99