(libc.info.gz) Pseudo-Random Numbers

(libc.info.gz) Errors in Math Functions
(libc.info.gz) FP Function Optimizations
 
 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 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.
(libc.info.gz) Pseudo-Random Numbers

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