NAME

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

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <stdlib.h>

       double drand48(void);
       double erand48(unsigned short xsubi[3]);

       long lrand48(void);
       long nrand48(unsigned short xsubi[3]);

       long mrand48(void);
       long jrand48(unsigned short xsubi[3]);

       void srand48(long 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 >= 2.19: */ _DEFAULT_SOURCE
               || /* glibc <= 2.19: */ _SVID_SOURCE

DESCRIPTION

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

       The drand48() and erand48() functions return nonnegative double-precision 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 uniformly 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  generating  the next 48-bit Xi in the sequence.  Then the appropriate
       number 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 initial values for Xi, a, and c.  Array
       argument elements param[0-2] specify 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 attributes(7).

       ┌───────────────────────────────────────────────────────────────┬───────────────┬────────────────────────┐
       │Interface                                                      │ Attribute     │ Value                  │
       ├───────────────────────────────────────────────────────────────┼───────────────┼────────────────────────┤
       │drand48(), erand48(), lrand48(), nrand48(), mrand48(),         │ Thread safety │ MT-Unsafe race:drand48 │
       │jrand48(), srand48(), seed48(), lcong48()                      │               │                        │
       └───────────────────────────────────────────────────────────────┴───────────────┴────────────────────────┘
       The  above  functions  record  global  state information for the random number generator, so they are not
       thread-safe.

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001, SVr4.

SEE ALSO

       rand(3), random(3)