random numbers and multiple srand calls
c, random, srand
Solution
let's break this question into two separate ones.
if you worry about race conditions when accessing random number generator, create a mutex or other synchronization primitive to avoid this.
if you think about calling `srand()` several times, please, don't. the reason behind this is the random generator initialization routine which sets up the variables based on seed has worse random characteristics, gives worse performance than the random generator itself and should be not used as a substitute.
Problem
I'm writing a program that will generate numerous random numbers in loops. I'm trying to make the numbers somewhat less predictable (not only for security, but to avoid collisions on multiple threads). I've noticed that many documents recommended calling `srand` only once in the program. For example: Random numbers in C, the selected answer is "As a general rule, only call srand() once in your program". But why? Why would it be so bad to do something like this: ``` int THIS_THREAD_SEED; int randomness() { ++THIS_THREAD_SEED; int i; for(i=0 i<1000; i++) { unsigned n = rand_r(&THIS_THREAD_SEED) / RAND_MAX; /* do something with n */ } return 0; } int do_something() { int i; for(i=0; i<1000; i++) { randomness(); } } ``` As such, the seed changes once per function call, not once per program. This way, no matter how many threads are running, no two threads will every have the same random number list... Right? UPDATE Assume I either have a unique seed for each thread, or mutex on a global SEED to prevent race conditions.