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

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

       initstate, random, setstate, srandom - pseudo-random number functions

       #include <stdlib.h>

       char *initstate(unsigned seed, char *state, size_t size);
       long random(void);
       char *setstate(const char *state);
       void srandom(unsigned seed);

       The  random()  function	shall  use a non-linear additive feedback random-number generator
       employing a default state array size of 31 long integers to return successive  pseudo-ran-
       dom numbers in the range from 0 to 2**31-1.  The period of this random-number generator is
       approximately 16 x (2**31-1).  The size of the state array determines the  period  of  the
       random-number generator. Increasing the state array size shall increase the period.

       With  256  bytes  of state information, the period of the random-number generator shall be
       greater than 2**69.

       Like rand(), random() shall produce by default a sequence of numbers that  can  be  dupli-
       cated by calling srandom() with 1 as the seed.

       The srandom() function shall initialize the current state array using the value of seed.

       The initstate() and setstate() functions handle restarting and changing random-number gen-
       erators. The initstate() function allows a state array, pointed to by the state	argument,
       to  be initialized for future use. The size argument, which specifies the size in bytes of
       the state array, shall be used by initstate() to decide what type of random-number genera-
       tor to use; the larger the state array, the more random the numbers. Values for the amount
       of state information are 8, 32, 64, 128, and 256 bytes. Other values greater than 8  bytes
       are  rounded  down  to  the  nearest  one  of  these values. If initstate() is called with
       8<=size<32, then random() shall use a simple linear congruential random number  generator.
       The  seed  argument specifies a starting point for the random-number sequence and provides
       for restarting at the same point. The initstate() function shall return a pointer  to  the
       previous state information array.

       If  initstate()	has not been called, then random() shall behave as though initstate() had
       been called with seed=1 and size=128.

       Once a state has been initialized, setstate() allows switching between state  arrays.  The
       array  defined  by  the	state argument shall be used for further random-number generation
       until initstate() is called or setstate() is called again. The setstate()  function  shall
       return a pointer to the previous state array.

       If initstate() is called with size less than 8, it shall return NULL.

       The random() function shall return the generated pseudo-random number.

       The srandom() function shall not return a value.

       Upon  successful completion, initstate() and setstate() shall return a pointer to the pre-
       vious state array; otherwise, a null pointer shall be returned.

       No errors are defined.

       The following sections are informative.


       After initialization, a state array can be restarted at a different point in  one  of  two

	1. The	initstate() function can be used, with the desired seed, state array, and size of
	   the array.

	2. The setstate() function, with the desired state, can be used,  followed  by	srandom()
	   with the desired seed. The advantage of using both of these functions is that the size
	   of the state array does not have to be saved once it is initialized.

       Although some implementations of random() have written messages to  standard  error,  such
       implementations do not conform to IEEE Std 1003.1-2001.

       Issue 5 restored the historical behavior of this function.

       Threaded  applications  should  use erand48(), nrand48(), or jrand48() instead of random()
       when an independent random number sequence in multiple threads is required.



       drand48() , rand() , the Base Definitions volume of IEEE Std 1003.1-2001, <stdlib.h>

       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				     INITSTATE(P)

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