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.
The value of this macro is an integer constant representing the largest value the
randfunction can return. In the GNU C Library, it is
2147483647, which is the largest signed integer representable in 32 bits. In other libraries, it may be as low as
randfunction returns the next pseudo-random number in the series. The value ranges from
This function establishes seed as the seed for a new series of pseudo-random numbers. If you call
randbefore a seed has been established with
srand, it uses the value
1as 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.
This function returns a random number in the range 0 to
randdoes. 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 inthas. 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.