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### 2.33 `rand`, `srand`—pseudo-random numbers

Synopsis

    #include <stdlib.h>
    int rand(void);
    void srand(unsigned int seed);
    int rand_r(unsigned int *seed);

Description
`rand` returns a different integer each time it is called; each integer is chosen by an algorithm designed to be unpredictable, so that you can use `rand` when you require a random number. The algorithm depends on a static variable called the “random seed”; starting with a given value of the random seed always produces the same sequence of numbers in successive calls to `rand`.

You can set the random seed using `srand`; it does nothing beyond storing its argument in the static variable used by `rand`. You can exploit this to make the pseudo-random sequence less predictable, if you wish, by using some other unpredictable value (often the least significant parts of a time-varying value) as the random seed before beginning a sequence of calls to `rand`; or, if you wish to ensure (for example, while debugging) that successive runs of your program use the same “random” numbers, you can use `srand` to set the same random seed at the outset.

Returns
`rand` returns the next pseudo-random integer in sequence; it is a number between `0` and `RAND_MAX` (inclusive).

`srand` does not return a result.

Notes
`rand` and `srand` are unsafe for multi-threaded applications. `rand_r` is thread-safe and should be used instead.

Portability
`rand` is required by ANSI, but the algorithm for pseudo-random number generation is not specified; therefore, even if you use the same random seed, you cannot expect the same sequence of results on two different systems.

`rand` requires no supporting OS subroutines.

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