Uniform random numbers in an integer interval in C

March 6, 2025

Just using modulo is a poor choice #

• https://c-faq.com/lib/randrange.html
• https://codereview.stackexchange.com/questions/159604/uniform-random-numbers-in-an-integer-interval-in-c
• https://www.man7.org/linux/man-pages/man3/random.3.html

Using random() instead of rand() #

A quote from the rand() manual page on Linux:

The versions of rand() and srand() in the Linux C Library use the same random number generator as random(3) and srandom(3), so the lower-order bits should be as random as the higher-order bits. However, on older rand() implementations, and on current implementations on different systems, the lower-order bits are much less random than the higher-order bits. Do not use this function in applications intended to be portable when good randomness is needed. (Use random(3) instead.)

random_range() #

All of this results in the following implementation (srandom() initialization not shown):

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int random_range(int min, int max) {

  int range;
  long int chunkSize;
  long int endOfLastChunk;
  long int r;

  range = max - min + 1; 

  chunkSize = (RAND_MAX) / (long) range;
  endOfLastChunk = chunkSize * (long) range;

  r = random();

  while(r >= endOfLastChunk) {
    r = random();
  }

  return(min + r / chunkSize);
}