How to generate a random int in C?

Question

Is there a function to generate a random int number in C? Or will I have to use a third party library?

Answer 1

Despite all the people suggestion rand() here, you don't want to use rand() unless you have to! The random numbers that rand() produces are often very bad. To quote from the Linux man page:

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.)

Regarding portability, random() is also defined by the POSIX standard for quite some time now. rand() is older, it appeared already in the first POSIX.1 spec (IEEE Std 1003.1-1988), whereas random() first appeared in POSIX.1-2001 (IEEE Std 1003.1-2001), yet the current POSIX standard is already POSIX.1-2008 (IEEE Std 1003.1-2008), which received an update just a year ago (IEEE Std 1003.1-2008, 2016 Edition). So I would consider random() to be very portable.

POSIX.1-2001 also introduced the lrand48() and mrand48() functions, see here:

This family of functions shall generate pseudo-random numbers using a linear congruential algorithm and 48-bit integer arithmetic.

And pretty good pseudo random source is the arc4random() function that is available on many systems. Not part of any official standard, appeared in BSD around 1997 but you can find it on systems like Linux and macOS/iOS.