.Dd September 6, 2001
.Dt RANDOM 4
.Os Darwin
.Sh NAME
.Nm random
,
.Nm urandom
.Nd random data source devices.
.Sh SYNOPSIS
.Cd "pseudo-device random"
.Sh DESCRIPTION
The
.Nm
device produces uniformly distributed random byte values
of potentially high quality.
.Pp
To obtain random bytes, open
.Nm /dev/random
for reading and read from it.
.Pp
The same random data is also available from
.Xr getentropy 2 .
Using the
.Xr getentropy 2
system call interface will provide resiliency to file descriptor exhaustion, chroot, or sandboxing which can make
.Nm /dev/random
unavailable.  Additionally, the
.Xr arc4random 3
API provides a fast userspace random number generator built on the
.Nm
data source and is preferred over directly accessing the system's random device.
.Pp
.Nm /dev/urandom
is a compatibility nod to Linux. On Linux,
.Nm /dev/urandom
will produce lower quality output if the entropy pool drains, while
.Nm /dev/random
will prefer to block and wait for additional entropy to be collected.
With Yarrow, this choice and distinction is not necessary, and
the two devices behave identically. You may use either.
.Pp
The
.Nm
device implements the
.Nm Yarrow
pseudo random number generator algorithm and maintains its entropy pool.
The kernel automatically seeds the algorithm with additional entropy during normal execution.
.Sh FILES
.Bl -tag -width /dev/urandom -compact
.It Pa /dev/random
.It Pa /dev/urandom
.El
.Sh HISTORY
A
.Nm
device appeared in the Linux operating system.