srand48

DRAND48(3)                 Linux Programmer's Manual                DRAND48(3)

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 xsubi[3]);

       long int lrand48(void);

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

       long int mrand48(void);

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

       void srand48(long int seedval);

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

       void lcong48(unsigned short param[7]);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       All functions shown above: _XOPEN_SOURCE
           || /* Glibc since 2.19: */ _DEFAULT_SOURCE
           || /* Glibc versions <= 2.19: */ _SVID_SOURCE

DESCRIPTION
       These functions generate pseudo-random numbers using the linear congru-
       ential algorithm and 48-bit integer arithmetic.

       The  drand48() and erand48() functions return nonnegative double-preci-
       sion floating-point values  uniformly  distributed  over  the  interval
       [0.0, 1.0).

       The  lrand48() and nrand48() functions return nonnegative long integers
       uniformly distributed over the interval [0, 2^31).

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

       The  srand48(),  seed48()  and  lcong48()  functions are initialization
       functions, one of  which  should  be  called  before  using  drand48(),
       lrand48()   or  mrand48().   The  functions  erand48(),  nrand48()  and
       jrand48() do not require an initialization function to be called first.

       All the functions work by generating a sequence of 48-bit integers, Xi,
       according to the linear congruential formula:

           Xn+1 = (aXn + c) mod m, where n >= 0

       The  parameter  m = 2^48, hence 48-bit integer arithmetic is performed.
       Unless lcong48() is called, a and c are given by:

           a = 0x5DEECE66D
           c = 0xB

       The value returned  by  any  of  the  functions  drand48(),  erand48(),
       lrand48(),  nrand48(), mrand48() or jrand48() is computed by first gen-
       erating the next 48-bit Xi in the sequence.  Then the appropriate  num-
       ber  of  bits,  according  to  the type of data item to be returned, is
       copied from the high-order bits of Xi and transformed into the returned
       value.

       The  functions drand48(), lrand48() and mrand48() store the last 48-bit
       Xi generated in an internal buffer.  The functions erand48(), nrand48()
       and  jrand48()  require  the calling program to provide storage for the
       successive Xi values in the array argument xsubi.   The  functions  are
       initialized  by  placing  the initial value of Xi into the array before
       calling the function for the first time.

       The initializer function srand48() sets the high order 32-bits of Xi to
       the  argument  seedval.  The low order 16-bits are set to the arbitrary
       value 0x330E.

       The initializer function seed48() sets the value of Xi  to  the  48-bit
       value  specified  in the array argument seed16v.  The previous value of
       Xi is copied into an internal buffer and a pointer to  this  buffer  is
       returned by seed48().

       The  initialization  function lcong48() allows the user to specify ini-
       tial values for Xi, a and c.  Array argument elements param[0-2]  spec-
       ify  Xi,  param[3-5]  specify  a,  and  param[6]  specifies  c.   After
       lcong48() has been called, a subsequent call  to  either  srand48()  or
       seed48() will restore the standard values of a and c.

ATTRIBUTES
       For  an  explanation  of  the  terms  used  in  this  section,  see at-
       tributes(7).

       +----------------------+---------------+------------------------+
       |Interface             | Attribute     | Value                  |
       +----------------------+---------------+------------------------+
       |drand48(), erand48(), | Thread safety | MT-Unsafe race:drand48 |
       |lrand48(), nrand48(), |               |                        |
       |mrand48(), jrand48(), |               |                        |
       |srand48(), seed48(),  |               |                        |
       |lcong48()             |               |                        |
       +----------------------+---------------+------------------------+
       The above functions record global state information for the random num-
       ber generator, so they are not thread-safe.

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, SVr4.

SEE ALSO
       rand(3), random(3)

COLOPHON
       This  page  is  part of release 5.05 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

                                  2017-09-15                        DRAND48(3)