random, srandom, initstate, setstate - pseudorandom number functions

#include <stdlib.h> long random(void); void srandom(unsigned intseed); char *initstate(unsigned intseed, char *state, size_tsize); char *setstate(const char *state);

The random() function uses a nonlinear additive feedback random-number generator employing a default state array size of 31 long integers to return successive pseudo-random 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 increases the period. The srandom() function initializes the current state array using the value ofseed. The random() and srandom() functions have (almost) the same calling sequence and initialization properties as rand() and srand() (seerand(3C)). The difference is thatrand(3C)pro- duces a much less random sequence-in fact, the low dozen bits generated by rand go through a cyclic pattern. All the bits generated by random() are usable. The algorithm from rand() is used by srandom() to generate the 31 state integers. Because of this, different srandom() seeds often produce, within an offset, the same sequence of low order bits from random(). If low order bits are used directly, random() should be initialized with setstate() using high quality random values. Unlike srand(), srandom() does not return the old seed because the amount of state information used is much more than a single word. Two other routines are provided to deal with restarting/changing random number generators. With 256 bytes of state information, the period of the random-number generator is greater than 2**69, which should be sufficient for most purposes. Likerand(3C), random() produces by default a sequence of numbers that can be duplicated by calling srandom() with 1 as the seed. The initstate() and setstate() functions handle restarting and changing random-number generators. The initstate() function allows a state array, pointed to by thestateargu- ment, to be initialized for future use. The size argument, which specifies the size in bytes of the state array, is used by initstate() to decide what type of random-number generator to use; the larger the state array, the more ran- dom 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. For values smaller than 8, random() uses a simple linear congruential random number generator. Theseedargu- ment specifies a starting point for the random-number sequence and provides for restarting at the same point. The initstate() function returns a pointer to the previous state information array. If initstate() has not been called, then random() behaves as though initstate() had been called withseed=1 andsize=128. If initstate() is called withsize<8, then random() uses a simple linear congruential random number generator. Once a state has been initialized, setstate() allows switch- ing between state arrays. The array defined by thestateargument is used for further random-number generation until initstate() is called or setstate() is called again. The setstate() function returns a pointer to the previous state array.

The random() function returns the generated pseudo-random number. The srandom() function returns no value. Upon successful completion, initstate() and setstate() return a pointer to the previous state array. Otherwise, a null pointer is returned.

No errors are defined.

After initialization, a state array can be restarted at a different point in one of two ways:oThe initstate() function can be used, with the desired seed, state array, and size of the array.oThe 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.

Example 1: Initialize an array. The following example demonstrates the use of initstate() to intialize an array. It also demonstrates how to initialize an array and pass it to setstate(). # include <stdlib.h> static unsigned int state0[32]; static unsigned int state1[32] = { 3, 0x9a319039, 0x32d9c024, 0x9b663182, 0x5da1f342, 0x7449e56b, 0xbeb1dbb0, 0xab5c5918, 0x946554fd, 0x8c2e680f, 0xeb3d799f, 0xb11ee0b7, 0x2d436b86, 0xda672e2a, 0x1588ca88, 0xe369735d, 0x904f35f7, 0xd7158fd6, 0x6fa6f051, 0x616e6b96, 0xac94efdc, 0xde3b81e0, 0xdf0a6fb5, 0xf103bc02, 0x48f340fb, 0x36413f93, 0xc622c298, 0xf5a42ab8, 0x8a88d77b, 0xf5ad9d0e, 0x8999220b, 0x27fb47b9 }; main() { unsigned seed; int n; seed = 1; n = 128; (void)initstate(seed, (char *)state0, n); printf("random() = %d00, random()); (void)setstate((char *)state1); printf("random() = %d00, random()); }

Seeattributes(5)for descriptions of the following attri- butes: ____________________________________________________________ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | |_____________________________|_____________________________|| MT-Level | See NOTES below. | |_____________________________|_____________________________|

drand48(3C),rand(3C),attributes(5)

The random() and srandom() functions are unsafe in mul- tithreaded applications. Use of these functions in multithreaded applications is unsupported. For initstate() and setstate(), thestateargument must be aligned on an int boundary. Newer and better performing random number generators such as addrans() and lcrans() are available with the SUNWspro pack- age.

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