How should I pick the value to pass to srand()?

By default, the C random number generator produces the same sequence every time the program is run. In order to generate different sequences, it has to be “seeded” using srand() with a different value each time. The function below to do this is carefully designed. It uses time() to obtain the current time; the alternative clock() is a poor choice because it measures CPU time used, which may vary little from run to run. The actual value of a time_t is not portable, so it computes a “hash” of the bytes in it using a multiply-and-add technique. The factor used for multiplication normally comes out as 257, a prime and therefore a good candidate.

References: Knuth, The Art of Computer Programming, Vol. 2: Seminumerical Algorithms, section 3.2.1; Aho, Sethi, and Ullman, Compilers: Principles, Techniques, and Tools, section 7.6.

#include <limits.h>
#include <stdlib.h>
#include <time.h>

/* Choose and return an initial random seed based on the current time.
   Based on code by Lawrence Kirby <fred@genesis.demon.co.uk>.
   Usage: srand (time_seed ()); */
unsigned
time_seed (void)
{
  time_t timeval;	/* Current time. */
  unsigned char *ptr;	/* Type punned pointed into timeval. */
  unsigned seed;	/* Generated seed. */
  size_t i;

  timeval = time (NULL);
  ptr = (unsigned char *) &timeval;

  seed = 0;
  for (i = 0; i < sizeof timeval; i++)
    seed = seed * (UCHAR_MAX + 2u) + ptr[i];

  return seed;
}