Страницы справочника OS X

Эта страница руководства является частью версии 5.0 Инструментов XCode

Читать страницы руководства

RAND48(3)                BSD Library Functions Manual                RAND48(3)

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

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdlib.h>

     double
     drand48(void);

     double
     erand48(unsigned short xsubi[3]);

     long
     jrand48(unsigned short xsubi[3]);

     void
     lcong48(unsigned short param[7]);

     long
     lrand48(void);

     long
     mrand48(void);

     long
     nrand48(unsigned short xsubi[3]);

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

     void
     srand48(long seedval);

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.
     The default value for the multiplicand `a' is 0x5deece66d (25214903917).  The default value for the the
     addend `c' is 0xb (11).  The modulo is always fixed at m = 2 ** 48.  r(n) is called the seed of the
     random number generator.

     For the six generator routines described next, the first computational 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 highorder
     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.

     The mrand48() and jrand48() functions 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.

     The drand48(), lrand48(), and mrand48() functions 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-supplied 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 multiplicand 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 multiplicand; and the last short is used to ini-tialize initialize
     tialize 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

BSD                             October 8, 1993                            BSD