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)

Linux man-pages 6.7                                2023-10-31                                         drand48(3)