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Linux 2.6 - man page for erand48 (linux section 3posix)

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

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, float-
       ing-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 [0,2**31).

       The mrand48() and jrand48() functions shall return signed long integers uniformly distrib-
       uted 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()	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_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  multiplier  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 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 multi-
       plier  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, Standard for Information Technology  --  Portable	Operating  System
       Interface  (POSIX), The Open Group Base Specifications 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
       Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003				       DRAND48(P)


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