drand48(3)                                            Library Functions Manual                                           drand48(3)

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

       Standard C library (libc, -lc)

       #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:
               || /* glibc >= 2.19: */ _DEFAULT_SOURCE
               || /* glibc <= 2.19: */ _SVID_SOURCE

       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.

       For an explanation of the terms used in this section, see attributes(7).

       │Interface                                                                         │ Attribute     │ Value                  │
       │drand48(), erand48(), lrand48(), nrand48(), mrand48(), jrand48(), srand48(),      │ Thread safety │ MT-Unsafe race:drand48 │
       │seed48(), lcong48()                                                               │               │                        │

       The above functions record global state information for the random number generator, so they are not thread-safe.

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

       rand(3), random(3)

Linux man-pages 6.03                                         2023-02-10                                                  drand48(3)