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
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
Published under the terms of the GNU General Public License