Question

I am reading a C book. In the description of the rand() function, they say:

rand returns a pseudo-random integer in the range 0 to RAND_MAX.  RAND_MAX is implementation dependent but at least 32767.

I don’t understand; what is a “pseudo-random integer”?

Thanks.

Answer 1

Informally, a pseudorandom number is a number that isn’t truly random, but is “random enough” for most purposes.

Computers are inherently deterministic devices. The processor executes specific commands in a specific order, and programs control how the processor does so. Consequently, it’s hard for programs to generate random numbers because no deterministic process can create a random number. Thus what many programs do is use a pseudorandom number generator, which is a function that produces numbers according to some deterministic formula that appear to be random but actually are not. Most programming languages provide some sort of pseudorandom number generator for general programming use, and when true randomness isn’t needed they work just fine.

However, they have their limitations. In cryptographic settings, in many cases true randomness is required in order to prevent attackers from guessing the workings of a system and compromising it, for example. In this case, it is possible to get truly random numbers by using specialized hardware that can amplify background noise or use quantum effects. This sort of randomness is extremely hard to generate, though, and so it’s not commonly used unless absolute unpredictability is required.