DRAND48(3)                 POSIX Programmer's Manual                DRAND48(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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

SYNOPSIS
       #include <stdlib.h>

       double drand48(void);
       double erand48(unsigned short xsubi[3]);
       long jrand48(unsigned short xsubi[3]);
       void lcong48(unsigned short param[7]);
       long lrand48(void);
       long mrand48(void);
       long nrand48(unsigned short xsubi[3]);
       unsigned short *seed48(unsigned short seed16v[3]);
       void srand48(long seedval);

DESCRIPTION
       This family of functions shall generate pseudo-random numbers using a
       linear congruential algorithm and 48-bit integer arithmetic.

       The drand48() and erand48() functions shall return non-negative,
       double-precision, floating-point values, uniformly distributed over the
       interval [0.0,1.0).

       The lrand48() and nrand48() functions shall return non-negative, long
       integers, uniformly distributed over the interval [0,231).

       The mrand48() and jrand48() functions shall return signed long integers
       uniformly distributed over the interval [-231,231).

       The srand48(), seed48(), and lcong48() functions are initialization
       entry points, one of which should be invoked before either drand48(),
       lrand48(), or mrand48() is called. (Although it is not recommended
       practice, constant default initializer values shall be supplied
       automatically if drand48(), lrand48(), or mrand48() is called without a
       prior call to an initialization entry point.) The erand48(), nrand48(),
       and jrand48() functions do not require an initialization entry point to
       be called first.

       All the routines work by generating a sequence of 48-bit integer
       values, _X__i , according to the linear congruential formula:

              Xn+1 = (aX__n +_c)mod _m        _n>= 0

       The parameter _m=2_^48; hence 48-bit integer arithmetic is performed.
       Unless lcong48() is invoked, the multiplier value _a and the addend
       value _c are given by:

              _a = 5DEECE66D16 = 2736731631558

              _c = B16 = 138

       The value returned by any of the drand48(), erand48(), jrand48(),
       lrand48(), mrand48(), or nrand48() functions is computed by first
       generating the next 48-bit _X__i in the sequence. Then the appropriate
       number of bits, according to the type of data item to be returned, are
       copied from the high-order (leftmost) bits of _X__i and transformed into
       the returned value.

       The drand48(), lrand48(), and mrand48() functions store the last 48-bit
       _X__i generated in an internal buffer; that is why the application shall
       ensure that these are initialized prior to being invoked. The
       erand48(), nrand48(), and jrand48() functions require the calling
       program to provide storage for the successive _X__i values in the array
       specified as an argument when the functions are invoked. That is why
       these routines do not have to be initialized; the calling program
       merely has to place the desired initial value of _X__i into the array and
       pass it as an argument.  By using different arguments, erand48(),
       nrand48(), and jrand48() allow separate modules of a large program to
       generate several independent streams of pseudo-random numbers; that is,
       the sequence of numbers in each stream shall not depend upon how many
       times the routines are called to generate numbers for the other
       streams.

       The initializer function srand48() sets the high-order 32 bits of _X__i
       to the low-order 32 bits contained in its argument. The low-order 16
       bits of _X__i are set to the arbitrary value 330E_16 .

       The initializer function seed48() sets the value of _X__i to the 48-bit
       value specified in the argument array. The low-order 16 bits of _X__i are
       set to the low-order 16 bits of seed16v[_0].  The mid-order 16 bits of
       _X__i are set to the low-order 16 bits of seed16v[_1].  The high-order 16
       bits of _X__i are set to the low-order 16 bits of seed16v[_2].  In
       addition, the previous value of _X__i is copied into a 48-bit internal
       buffer, used only by seed48(), and a pointer to this buffer is the
       value returned by seed48().  This returned pointer, which can just be
       ignored if not needed, is useful if a program is to be restarted from a
       given point at some future time--use the pointer to get at and store
       the last _X__i value, and then use this value to reinitialize via
       seed48() when the program is restarted.

       The initializer function lcong48() allows the user to specify the
       initial _X__i , the multiplier value _a, and the addend value _c. Argument
       array elements param[0-2] specify _X__i , param[3-5] specify the
       multiplier _a, and param[_6] specifies the 16-bit addend _c. After
       lcong48() is called, a subsequent call to either srand48() or seed48()
       shall restore the standard multiplier and addend values, _a and c,
       specified above.

       The drand48(), lrand48(), and mrand48() functions need not be thread-
       safe.

RETURN VALUE
       As described in the DESCRIPTION above.

ERRORS
       No errors are defined.

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       These functions should be avoided whenever non-trivial requirements
       (including safety) have to be fulfilled.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       initstate(), rand()

       The Base Definitions volume of POSIX.1-2017, <stdlib.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                           DRAND48(3)
