ISO C Random Number Functions ----------------------------- This section describes the random number functions that are part of the ISO C standard. To use these facilities, you should include the header file `stdlib.h' in your program. - Macro: int RAND_MAX The value of this macro is an integer constant representing the largest value the `rand' function can return. In the GNU library, it is `2147483647', which is the largest signed integer representable in 32 bits. In other libraries, it may be as low as `32767'. - Function: int rand (void) The `rand' function returns the next pseudo-random number in the series. The value ranges from `0' to `RAND_MAX'. - Function: void srand (unsigned int SEED) This function establishes SEED as the seed for a new series of pseudo-random numbers. If you call `rand' before a seed has been established with `srand', it uses the value `1' as a default seed. To produce a different pseudo-random series each time your program is run, do `srand (time (0))'. POSIX.1 extended the C standard functions to support reproducible random numbers in multi-threaded programs. However, the extension is badly designed and unsuitable for serious work. - Function: int rand_r (unsigned int *SEED) This function returns a random number in the range 0 to `RAND_MAX' just as `rand' does. However, all its state is stored in the SEED argument. This means the RNG's state can only have as many bits as the type `unsigned int' has. This is far too few to provide a good RNG. If your program requires a reentrant RNG, we recommend you use the reentrant GNU extensions to the SVID random number generator. The POSIX.1 interface should only be used when the GNU extensions are not available.
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