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Name

Synopsis

long random();

int srandom( unsigned seed);

char *initstate( unsigned seed, char *state, int n);

char *setstate( char *state);

MT-Level

See the NOTES section of this page.

Description

random() uses a non-linear additive feedback random number generator employing a default table of size 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 very large, approximately 16*((2**31)-1).

random() and srandom() have (almost) the same calling sequence and initialization properties as rand() and srand() (see rand(3C) ). The difference is that rand(3C) produces 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. For example,

random()&01

will produce a random binary value.

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. Like rand(3C) , however, random() will, by default, produce a sequence of numbers that can be duplicated by calling srandom() with 1 as the seed.

The initstate() routine allows a state array, passed in as an argument, to be initialized for future use. n specifies the size of state in bytes. initstate() uses n to decide how sophisticated a random number generator it should use--the more state, the better the random numbers will be. Current ‘optimal’ values for the amount of state information are 32, 64, 128, and 256 bytes. If the amount of state information is less than 32 bytes, a simple linear congruential random number generator is used. Using less than 8 bytes causes an error. The seed for the initialization (which specifies a starting point for the random number sequence, and provides for restarting at the same point) is also an argument. initstate() returns a pointer to the previous state information array.

Once a state has been initialized, the setstate() routine provides for rapid switching between states. setstate() returns a pointer to the previous state array; its argument state array is used for further random number generation until the next call to initstate() or setstate().

Once a state array has been initialized, it may be restarted at a different point either by calling initstate() (with the desired seed, the state array, and its size) or by calling both setstate() (with the state array) and srandom() (with the desired seed). The advantage of calling both setstate() and srandom() is that the size of the state array does not have to be remembered after it is initialized.

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.

Return Values

Examples

/* Initialize an array and pass it in to initstate. */

static long 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;
    initstate(seed, state1, n);
    setstate(state1);
    printf("%d0,random());
}

See Also

Notes

Use of these interfaces in multi-thread applications is unsupported.

random() and srandom() function at about two-thirds the speed of rand(3C) .

Table of Contents

• Name
• Synopsis
• MT-Level
• Description
• Return Values
• Examples
• See Also
• Notes