drand48

DRAND48(3)		  Linux Programmer’s Manual		   DRAND48(3)



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

SYNOPSIS
       #include <stdlib.h>

       double drand48(void);

       double erand48(unsigned short xsubi[3]);

       long int lrand48(void);

       long int nrand48(unsigned short xsubi[3]);

       long int mrand48(void);

       long int jrand48(unsigned short xsubi[3]);

       void srand48(long int seedval);

       unsigned short *seed48(unsigned short seed16v[3]);

       void lcong48(unsigned short param[7]);

DESCRIPTION
       These functions generate pseudo-random numbers using the	 linear	 con-
       gruential algorithm and 48-bit integer arithmetic.

       The  drand48() and erand48() functions return non-negative double-pre-
       cision floating-point values uniformly distributed between [0.0, 1.0).

       The  lrand48()  and nrand48() functions return non-negative long inte-
       gers uniformly distributed between 0 and 2^31.

       The mrand48() and jrand48() functions return signed long integers uni-
       formly distributed between -2^31 and 2^31.

       The  srand48(),	seed48()  and  lcong48() functions are initialization
       functions, one of which	should	be  called  before  using  drand48(),
       lrand48()  or  mrand48().   The	functions  erand48(),  nrand48()  and
       jrand48() do not require	 an  initialization  function  to  be  called
       first.

       All  the	 functions  work by generating a sequence of 48-bit integers,
       Xi, according to the linear congruential formula:

	      Xn+1 = (aXn + c) mod m, where n >= 0

       The parameter m = 2^48, hence 48-bit integer arithmetic is  performed.
       Unless lcong48() is called, a and c are given by:

	      a = 0x5DEECE66D
	      c = 0xB

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

       The functions drand48(), lrand48() and mrand48() store the last 48-bit
       Xi   generated  in  an  internal	 buffer.   The	functions  erand48(),
       nrand48() and jrand48() require the calling program to provide storage
       for  the	 successive Xi values in the array argument xsubi.  The func-
       tions are initialized by placing the initial  value  of	Xi  into  the
       array before calling the function for the first time.

       The  initializer	 function srand48() sets the high order 32-bits of Xi
       to the argument seedval.	 The low order 16-bits are set to  the	arbi-
       trary value 0x330E.

       The  initializer	 function seed48() sets the value of Xi to the 48-bit
       value specified in the array argument seed16v.  The previous value  of
       Xi  is  copied into an internal buffer and a pointer to this buffer is
       returned by seed48().

       The initialization function lcong48() allows the user to specify	 ini-
       tial values for Xi, a and c.  Array argument elements param[0-2] spec-
       ify Xi,	param[3-5]  specify  a,	 and  param[6]	specifies  c.	After
       lcong48()  has  been  called, a subsequent call to either srand48() or
       seed48() will restore the standard values of a and c.

CONFORMING TO
       SVID 3

NOTES
       These functions are declared obsolete by SVID  3,  which	 states	 that
       rand(3) should be used instead.

SEE ALSO
       rand(3), random(3)



				  1993-07-02			   DRAND48(3)