[wahern]

The portable API is getentropy, which glibc provides as a simple wrapper around getrandom. getentropy was added to POSIX, and is also available on most modern unix systems, including FreeBSD, Illumos, NetBSD, macOS, OpenBSD, and Solaris.

arc4random has been provided by glibc 2.36 (2022), and is available on all the above-mentioned systems as well. If you don't want to make a syscall per request (outside Linux), just use arc4random; it'll be the fastest method available. musl libc lacks arc4random, unfortunately, but you can always ship a small wrapper.

Systems that support arc4random also support arc4random_uniform, which is a way to get an unbiased unsigned integer between 0 and N (up to 2^32-1). That's probably the most important reason to use the arc4random family.