10-15-2001
From the linux rand(3) man page:
Quote:
The versions of rand() and srand() in the Linux C Library use the same random number gen
erator as random() and srandom(), so the lower-order bits should be as random as the
higher-order bits. However, on older rand() implementations, the lower-order bits are
much less random than the higher-order bits.
In Numerical Recipes in C: The Art of Scientific Computing (William H. Press, Brian P.
Flannery, Saul A. Teukolsky, William T. Vetterling; New York: Cambridge University Press,
1992 (2nd ed., p. 277)), the following comments are made:
"If you want to generate a random integer between 1 and 10, you should always do
it by using high-order bits, as in
j=1+(int)(10.0*rand()/(RAND_MAX+1.0));
and never by anything resembling
j=1+(rand() % 10);
(which uses lower-order bits)."
In other words, under Linux your algorithm is probably fine, on other architectures it may not be very random. For portability, use the noted syntax or check the man page for your particular rand implementation.
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rand(3C) rand(3C)
NAME
rand(), rand_r(), srand() - simple random-number generator
SYNOPSIS
DESCRIPTION
uses a multiplicative, congruential, random-number generator with period 2^32 that returns successive pseudo-random numbers in the range
from 0 to 2^15-1.
can be called at any time to reset the random-number generator to a random starting point. The generator is initially seeded with a value
of 1.
returns a random number at the address pointed to by the randval parameter. The seed parameter can be set at any time to start the random-
number generator at an arbitrary point.
Note
The spectral properties of leave a great deal to be desired. provides a much better, though more elaborate, random-number generator (see
drand48(3C)).
RETURN VALUE
If seed or randval is NULL, returns 0. Otherwise, returns a psuedo-random integer.
EXAMPLES
The following:
int x, y;
srand(10);
x = rand();
y = rand();
would produce the same results as:
int x, y, s = 10;
x=rand_r(&s);
y=rand_r(&s);
WARNINGS
Users of should note that rand_r() now conforms with POSIX.1c. The old prototype of is supported for compatibility with existing DCE
applications only.
SEE ALSO
drand48(3C), random(3M), thread_safety(5), random(7).
STANDARDS CONFORMANCE
rand(3C)