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<TITLE>rand</TITLE>
<body bgcolor="#ffffcc">
<hr>
<pre>
<h3>RAND(3) Linux Programmer's Manual RAND(3)
</h3>
<h3>NAME
</h3> rand, srand - random number generator.
<h3>SYNOPSIS
</h3> #include <stdlib.h>
int rand(void);
void srand(unsigned int seed);
<h3>DESCRIPTION
</h3> The rand() function returns a pseudo-random integer
between 0 and RAND_MAX.
The srand() function sets its argument as the seed for a
new sequence of pseudo-random integers to be returned by
rand(). These sequences are repeatable by calling srand()
with the same seed value.
If no seed value is provided, the rand() function is auto-
matically seeded with a value of 1.
<h3>RETURN VALUE
</h3> The rand() function returns a value between 0 and
RAND_MAX. The srand() returns no value.
<h3>NOTES
</h3> The versions of rand() and srand() in the Linux C Library
use the same random number generator as random() and sran-
dom(), so the lower-order bits should be as random as the
higher-order bits. However, on older rand() implementa-
tions, 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, 1990 (1st ed, p. 207)), the following comments are
made:
"If you want to generate a random integer between 1
and 10, you should always do it by
j=1+(int) (10.0*rand()/(RAND_MAX+1.0));
and never by anything resembling
j=1+((int) (1000000.0*rand()) % 10);
(which uses lower-order bits)."
Random-number generation is a complex topic. The Numeri-
cal Recipes in C book (see reference above) provides an
excellent discussion of practical random-number generation
issues in Chapter 7 (Random Numbers).
<h3>GNU 18 May 1995 1
</h3>
<h3>RAND(3) Linux Programmer's Manual RAND(3)
</h3>
For a more theoretical discussion which also covers many
practical issues in depth, please see Chapter 3 (Random
Numbers) in Donald E. Knuth's The Art of Computer Program-
ming, volume 2 (Seminumerical Algorithms), 2nd ed.; Read-
ing, Massachusetts: Addison-Wesley Publishing Company,
1981.
<h3>CONFORMING TO
</h3> SVID 3, BSD 4.3, ISO 9899
</pre>
<hr>
<h3>SEE ALSO
</h3><p>
<a href=random.htm>random</a>,
<a href=srandom.htm>srandom</a>,
<a href=initstate.htm>initstate</a>,
<a href=setstate.htm>setstate</a>,
<pre>
<h3>GNU 18 May 1995 2
</h3>
</pre>
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