rand_r(3)  Linux man page
NAME
rand, rand_r, srand  pseudorandom number generatorSYNOPSIS
#include <stdlib.h> int rand(void); int rand_r(unsigned int *seedp); void srand(unsigned int seed);
DESCRIPTION
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 function rand() is not reentrant or threadsafe, since it uses hidden state that is modified on each call. This might just be the seed value to be used by the next call, or it might be something more elaborate. In order to get reproducible behaviour in a threaded application, this state must be made explicit. The function rand_r() is supplied with a pointer to an unsigned int, to be used as state. This is a very small amount of state, so this function will be a weak pseudorandom generator. Try drand48_r(3) instead.
RETURN VALUE
The rand() and rand_r() functions return a value between 0 and RAND_MAX. The srand() function returns no value.EXAMPLE
POSIX 1003.12003 gives the following example of an implementation of rand() and srand(), possibly useful when one needs the same sequence on two different machines.
static unsigned long next = 1; /* RAND_MAX assumed to be 32767 */ int myrand(void) { next = next * 1103515245 + 12345; return((unsigned)(next/65536) % 32768); } void mysrand(unsigned seed) { next = seed; }
NOTES
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, and on current implementations on different systems, the lowerorder bits are much less random than the higherorder bits. Do not use this function in applications intended to be portable when good randomness is needed.FreeBSD adds a function
void sranddev(void);
that initializes the seed for their bad random generator rand() with a value obtained from their good random generator random(). Strange.
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));

j=1+(rand() % 10);

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.
CONFORMING TO
The functions rand() and srand() conform to SVID 3, BSD 4.3, ISO 9899, POSIX 1003.12003. The function rand_r() is from POSIX 1003.12003.SEE ALSO
drand48(3), random(3)