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(),     │ Thread safety │ MT-Unsafe race:drand48 │
       │mrand48(), 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)