drand48(3) — Linux manual page


drand48(3)              Library Functions Manual              drand48(3)

NAME         top

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

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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

       All functions shown above:
               || /* glibc >= 2.19: */ _DEFAULT_SOURCE
               || /* glibc <= 2.19: */ _SVID_SOURCE

DESCRIPTION         top

       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         top

       For an explanation of the terms used in this section, see
       │ Interface                      Attribute     Value        │
       │ drand48(), erand48(),          │ Thread safety │ MT-Unsafe    │
       │ lrand48(), nrand48(),          │               │ 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         top


HISTORY         top

       POSIX.1-2001, SVr4.

SEE ALSO         top

       rand(3), random(3)

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