GNU/Linux 
CentOS 2.1AS(Slurm) 

rand(3) 
rand, srand − random number generator.
#include <stdlib.h>
int rand(void);
void srand(unsigned int seed);
The rand() function returns a pseudorandom integer between 0 and RAND_MAX.
The srand() function sets its argument as the seed for a new sequence of pseudorandom 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 automatically seeded with a value of 1.
The rand() function returns a value between 0 and RAND_MAX. The srand() returns no value.
The versions of rand() and srand() in the Linux C Library use the same random number generator as random() and srandom(), so the lowerorder bits should be as random as the higherorder bits. However, on older rand() implementations, the lowerorder bits are much less random than the higherorder 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 highorder bits, as in
j=1+(int) (10.0*rand()/(RAND_MAX+1.0));
and never by anything resembling
j=1+(rand() % 10);
(which uses lowerorder bits)."
Randomnumber generation is a complex topic. The Numerical Recipes in C book (see reference above) provides an excellent discussion of practical randomnumber generation issues in Chapter 7 (Random Numbers).
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 Programming, volume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts: AddisonWesley Publishing Company, 1981.
SVID 3, BSD 4.3, ISO 9899
random(3), srandom(3), initstate(3), setstate(3)
rand(3) 