RANDOM(4) Device Drivers Manual RANDOM(4)

random , urandomrandom data source devices.

pseudo-device random

The random device produces uniformly distributed random byte values of potentially high quality.

To obtain random bytes, open /dev/random for reading and read from it.

The same random data is also available from getentropy(2). Using the getentropy(2) system call interface will provide resiliency to file descriptor exhaustion, chroot, or sandboxing which can make /dev/random unavailable. Additionally, the arc4random(3) API provides a fast userspace random number generator built on the random data source and is preferred over directly accessing the system's random device.

/dev/urandom is a compatibility nod to Linux. On Linux, /dev/urandom will produce lower quality output if the entropy pool drains, while /dev/random will prefer to block and wait for additional entropy to be collected. With Fortuna, this choice and distinction is not necessary, and the two devices behave identically. You may use either.

The random device implements the Fortuna pseudo random number generator algorithm and maintains its entropy pool. The kernel automatically seeds the algorithm with additional entropy during normal execution.


A random device appeared in the Linux operating system.

September 6, 2001 Darwin