This section describes a set of random number generation functions that
are derived from BSD. There is no advantage to using these functions
with the GNU C library; we support them for BSD compatibility only.
The prototypes for these functions are in stdlib.h.
— Function: long int random (void)
This function returns the next pseudo-random number in the sequence.
The value returned ranges from 0 to RAND_MAX.
Note: Temporarily this function was defined to return a
int32_t value to indicate that the return value always contains
32 bits even if long int is wider. The standard demands it
differently. Users must always be aware of the 32-bit limitation,
though.
— Function: void srandom (unsigned int seed)
The srandom function sets the state of the random number
generator based on the integer seed. If you supply a seed value
of 1, this will cause random to reproduce the default set
of random numbers.
To produce a different set of pseudo-random numbers each time your
program runs, do srandom (time (0)).
The initstate function is used to initialize the random number
generator state. The argument state is an array of size
bytes, used to hold the state information. It is initialized based on
seed. The size must be between 8 and 256 bytes, and should be a
power of two. The bigger the state array, the better.
The return value is the previous value of the state information array.
You can use this value later as an argument to setstate to
restore that state.
— Function: void * setstate (void *state)
The setstate function restores the random number state
information state. The argument must have been the result of
a previous call to initstate or setstate.
The return value is the previous value of the state information array.
You can use this value later as an argument to setstate to
restore that state.
If the function fails the return value is NULL.
The four functions described so far in this section all work on a state
which is shared by all threads. The state is not directly accessible to
the user and can only be modified by these functions. This makes it
hard to deal with situations where each thread should have its own
pseudo-random number generator.
The GNU C library contains four additional functions which contain the
state as an explicit parameter and therefore make it possible to handle
thread-local PRNGs. Beside this there are no difference. In fact, the
four functions already discussed are implemented internally using the
following interfaces.
The stdlib.h header contains a definition of the following type:
— Data Type: struct random_data
Objects of type struct random_data contain the information
necessary to represent the state of the PRNG. Although a complete
definition of the type is present the type should be treated as opaque.
The functions modifying the state follow exactly the already described
functions.
The random_r function behaves exactly like the random
function except that it uses and modifies the state in the object
pointed to by the first parameter instead of the global state.
— Function: int srandom_r (unsigned int seed, struct random_data *buf)
The srandom_r function behaves exactly like the srandom
function except that it uses and modifies the state in the object
pointed to by the second parameter instead of the global state.
— Function: int initstate_r (unsigned int seed, char *restrict statebuf, size_t statelen, struct random_data *restrict buf)
The initstate_r function behaves exactly like the initstate
function except that it uses and modifies the state in the object
pointed to by the fourth parameter instead of the global state.
The setstate_r function behaves exactly like the setstate
function except that it uses and modifies the state in the object
pointed to by the first parameter instead of the global state.
Published under the terms of the GNU General Public License
