### 19.8 Pseudo-Random Numbers

This section describes the GNU facilities for generating a series of
pseudo-random numbers. The numbers generated are not truly random;
typically, they form a sequence that repeats periodically, with a period
so large that you can ignore it for ordinary purposes. The random
number generator works by remembering a seed value which it uses
to compute the next random number and also to compute a new seed.

Although the generated numbers look unpredictable within one run of a
program, the sequence of numbers is *exactly the same* from one run
to the next. This is because the initial seed is always the same. This
is convenient when you are debugging a program, but it is unhelpful if
you want the program to behave unpredictably. If you want a different
pseudo-random series each time your program runs, you must specify a
different seed each time. For ordinary purposes, basing the seed on the
current time works well.

You can obtain repeatable sequences of numbers on a particular machine type
by specifying the same initial seed value for the random number
generator. There is no standard meaning for a particular seed value;
the same seed, used in different C libraries or on different CPU types,
will give you different random numbers.

The GNU C Library supports the standard ISO C random number functions
plus two other sets derived from BSD and SVID. The BSD and ISO C
functions provide identical, somewhat limited functionality. If only a
small number of random bits are required, we recommend you use the
ISO C interface, `rand`

and `srand`

. The SVID functions
provide a more flexible interface, which allows better random number
generator algorithms, provides more random bits (up to 48) per call, and
can provide random floating-point numbers. These functions are required
by the XPG standard and therefore will be present in all modern Unix
systems.