drand48(3p) — Linux manual page


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

PROLOG         top

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

       delim $$

NAME         top

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

SYNOPSIS         top

       #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         top

       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:

              $X sub{n+1} " " = " " (aX_ n" "^+^c) sub{roman mod " " m} " "
              " "  " "  " "  " "  " "  " "  " " n>= " " 0$

       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:

              $a " " mark = " " roman "5DEECE66D"^sub 16 " " = " " roman
              273673163155^sub 8$

              $c " " lineup = " " roman B^sub 16 " " = " " roman 13^sub 8$

       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 $roman 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

       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-

RETURN VALUE         top

       As described in the DESCRIPTION above.

ERRORS         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top




RATIONALE         top




SEE ALSO         top


       The Base Definitions volume of POSIX.1‐2008, stdlib.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/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                 2013                         DRAND48(3P)

Pages that refer to this page: stdlib.h(0p)erand48(3p)initstate(3p)jrand48(3p)lcong48(3p)lrand48(3p)mrand48(3p)nrand48(3p)rand(3p)rand_r(3p)seed48(3p)srand48(3p)