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Although not specified by POSIX, arc4random() is another possibility for systems that support it. The arc4random(3) manual page [OpenBSD] states:

arc4random() fits into a middle ground not covered by other subsystems such as the strong, slow, and resource expensive random devices described in random(4) versus the fast but poor quality interfaces ... provides higher quality of data than those described in rand(3), random(3), and drand48(3).

To achieve the best random numbers possible, an implementation-specific function must be used. When unpredictability is crucial and speed is not an issue, as in the creation of strong cryptographic keys, use a true entropy source, such as /dev/random, or a hardware device capable of generating random numbers. Note that the /dev/random device can block for a long time if there are not enough events going on to generate sufficient entropy.

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