bsd/man/man4/random.4

   1 .Dd September 6, 2001
   2 .Dt RANDOM 4
   3 .Os Darwin
   4 .Sh NAME
   5 .Nm random
   6 ,
   7 .Nm urandom
   8 .Nd random data source devices.
   9 .Sh SYNOPSIS
  10 .Cd "pseudo-device random"
  11 .Sh DESCRIPTION
  12 The
  13 .Nm
  14 device produces uniformly distributed random byte values
  15 of potentially high quality.
  16 .Pp
  17 To obtain random bytes, open
  18 .Nm /dev/random
  19 for reading and read from it.
  20 .Pp
  21 The same random data is also available from
  22 .Xr getentropy 2 .
  23 Using the
  24 .Xr getentropy 2
  25 system call interface will provide resiliency to file descriptor exhaustion, chroot, or sandboxing which can make
  26 .Nm /dev/random
  27 unavailable.  Additionally, the
  28 .Xr arc4random 3
  29 API provides a fast userspace random number generator built on the
  30 .Nm
  31 data source and is preferred over directly accessing the system's random device.
  32 .Pp
  33 .Nm /dev/urandom
  34 is a compatibility nod to Linux. On Linux,
  35 .Nm /dev/urandom
  36 will produce lower quality output if the entropy pool drains, while
  37 .Nm /dev/random
  38 will prefer to block and wait for additional entropy to be collected.
  39 With Yarrow, this choice and distinction is not necessary, and
  40 the two devices behave identically. You may use either.
  41 .Pp
  42 The
  43 .Nm
  44 device implements the
  45 .Nm Yarrow
  46 pseudo random number generator algorithm and maintains its entropy pool.
  47 The kernel automatically seeds the algorithm with additional entropy during normal execution.
  48 .Sh FILES
  49 .Bl -tag -width /dev/urandom -compact
  50 .It Pa /dev/random
  51 .It Pa /dev/urandom
  52 .El
  53 .Sh HISTORY
  54 A
  55 .Nm
  56 device appeared in the Linux operating system.