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random.4: Improve discussion of /dev/urandom, blocking reads, and signals 

The text currently states that O_NONBLOCK has no effect for
/dev/urandom, which is true.  It also says that reads from
/dev/urandom are nonblocking.  This is at the least confusing.
If one attempts large reads (say 10MB) from /dev/urandom
there is an appreciable delay, and interruption by a signal
handler will result in a short read. Amend the text to
reflect this.

Reviewed-by: Laurent Georget <laurent.georget@supelec.fr>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2016-11-10 14:37:33 +01:00
parent 1d99db5f09
commit dbf63eed2d
1 changed files with 5 additions and 3 deletions
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8
man4/random.4
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@ -46,7 +46,6 @@ When read, the
.I /dev/urandom
device returns random bytes using a pseudorandom
number generator seeded from the entropy pool.
Reads from this device are nonblocking.
When read during early boot time, this device may return
data prior to the entropy pool being initialized.
If this is of concern in your application, use
@ -88,8 +87,11 @@ When calling
.BR read (2)
for the device
.IR /dev/urandom ,
signals will not be handled until after the requested random bytes
have been generated.
reads of up to 256 bytes will return as many bytes as are requested
and will not be interrupted by a signal handler.
Reads with a buffer over this limit may return less than the
requested number of bytes or fail with the error
.BR EINTR .
Since Linux 3.16,
.\" commit 79a8468747c5f95ed3d5ce8376a3e82e0c5857fc