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.