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



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

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, uni-
       formly distributed over the interval [0.0,1.0).

       The lrand48() and nrand48() functions shall return non-negative, long integers, uniformly distributed over  the
       interval [0,2**31).

       The mrand48() and jrand48() functions shall return signed long integers uniformly distributed over the interval
       [-2**31,2**31).

       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() func-
       tions 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 congruen-
       tial formula: X_n+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 multi-
       plier value a and the addend value c are given by: a = 5DEECE66D_16 = 273673163155_8

       c = B_16 = 13_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  mod-
       ules 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 reentrant. A function that is not required to  be
       reentrant is not required to 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
       None.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       rand(), the Base Definitions volume of IEEE Std 1003.1-2001, <stdlib.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  Issue  6,  Copyright (C) 2001-2003 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 Stan-
       dard,  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 .



IEEE/The Open Group                  2003                          DRAND48(3P)