Oct 28, 2015 · Like rand(), rand_r() returns a pseudo-random integer in the range [0, RAND_MAX]. The seedp argument is a pointer to an unsigned int that is used to store state between calls. If rand_r() is called with the same initial value for the integer pointed to by seedp, and that value is not modified between calls, then the same pseudo-random sequence ...

Sep 27, 2011 · The c++ rand() function gives you a number from 0 to RAND_MAX (a constant defined in <cstdlib>), which is at least 32767. (from the c++ documentation) The modulus (%) operator gives the remainder after dividing. When you use it with rand() you are using it to set an upper limit (n) on what the random number can be.

Oct 24, 2013 · The equivalent half-open range is [0, n), and rand() % n == rand() % (n-0) + 0. So the lesson is: don't confuse half-open ranges for closed ranges. A second lesson is that this shows another way in which <random> is easier to use than rand() and manually computing your own distributions.

If we use more than 53 bits, we get rounding bias. Some programmers write code like rand() / (double)RAND_MAX, but rand() might return only 31 bits, or only 15 bits in Windows. OpenSSL's RAND_bytes() seeds itself, perhaps by reading /dev/urandom in Linux.

Jul 30, 2009 · Related: How to generate a random int in C?. Here is my answer there, which contains the definition for my int utils_rand(int min, int max) func, which returns a random number using rand() which is in the specific range from min to …

Jul 24, 2013 · You need to seed the random number generator, from man 3 rand. If no seed value is provided, the rand() function is automatically seeded with a value of 1. and. The srand() function sets its argument as the seed for a new sequence of …

Aug 21, 2010 · Now, if you are interested on the reasons why the above is true, here are the gory details on how rand() works: rand() is what's called a "linear congruential generator." This means that it employs an equation of the form: x n+1 = (*a****x n + ***b*) mod m. where x n is the n th random number, and a and b are some predetermined integers.

On older rand() implementations, and on current implementations on different systems, the lower-order bits are much less random than the higher-order bits. When available, random() does not suffer of this issue. In add, modern version of rand() use the same random number generator as random(). So rand() may be correct, but it is not garanteed.

Oct 18, 2018 · None of the answers here explains the real reason of being rand() bad. rand() is a pseudo-random number generator (PRNG), but this doesn't mean it must be bad. Actually, there are very good PRNGs, which are statistically hard or …

Mar 14, 2013 · From the rand man page: The function rand() is not reentrant or thread-safe, since it uses hidden state that is modified on each call. So don't use it with threaded code. Use rand_r (or drand48_r if you're on linux/glibc). Seed each RNG with a different value (you could seed a first RNG in the main thread to produce random seeds for the ones in ...