rand() uses a multiplicative congruential random number generator with period 2³² to return successive pseudo-random numbers in the range from 0 to "2³¹ -1."

srand() can be called at any time to reset the random-number generator to a random starting point. The generator is initially seeded with a value of 1.

drand48(3C), rand(3C), random(3C)

Use of these interfaces should be restricted to only applications written on BSD platforms. Use of these interfaces with any of the system libraries or in multi-thread applications is unsupported.

The spectral properties of rand() leave a great deal to be desired. drand48(3C) and random(3C) provide much better, though more elaborate, random-number generators.

The low bits of the numbers generated are not very random; use the middle bits. In particular the lowest bit alternates between 0 and 1.