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.

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

       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

RETURN VALUE         top

       As described in the DESCRIPTION above.

ERRORS         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top



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

RATIONALE         top




SEE ALSO         top

       initstate(3p), rand(3p)

       The Base Definitions volume of POSIX.1‐2017, stdlib.h(0p)

COPYRIGHT         top

       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(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)seed48(3p)srand48(3p)