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getrandom.2: Rework paragraphs marked with FIXME 

The patch clarifies when blocking may occur while calling
getrandom().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Heinrich Schuchardt 2015-01-22 20:30:46 +01:00 committed by Michael Kerrisk
parent 16199698a6
commit e14ee946e8
1 changed files with 20 additions and 15 deletions
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35
man2/getrandom.2
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@ -55,20 +55,17 @@ Therefore
should not be used for Monte Carlo simulations or other
programs/algorithms which are doing probabilistic sampling.
.\" FIXME is the following paragraph correct?
By default,
.BR getrandom ()
draws entropy from the
.IR /dev/urandom
pool, and, if that pool has been initialized and
.IR buflen
is less than or equal to 256 (see NOTES, below),
then the call never blocks when drawing from that pool
and always returns the number of bytes requested in
.IR buflen .
pool.
This behavior can be changed via the
.I flags
argument.
If the
.IR /dev/urandom
pool has been initialized, reading from that pool never blocks.
The
.I flags
@ -157,17 +154,27 @@ was introduced in version 3.17 of the Linux kernel.
This system call is Linux-specific.
.SH NOTES
.SS Interruption by a signal handler
.\" FIXME Here, I think there needs to be an opening paragraph that describes
.\" the cases where getrandom() can block. This should cover the cases with
.\" GRND_RANDOM and without GRND_RANDOM. Reading the existing page, I am
.\" still not completely confident that I know what the cases are.
A call to
.BR getrandom ()
can only block when called without the
.B GRND_NONBLOCK
flag.
When reading from
.I /dev/urandom
.RB ( GRND_RANDOM
is not set)
blocking only occurs if the entropy pool has not been initialized yet.
When reading from
.I /dev/random
.RB ( GRND_RANDOM
is set)
blocking occurs if not enough random bytes are available.
The reaction of
.BR getrandom ()
in case of an interruption of a blocking call by a signal
when reading from
.I /dev/urandom
.RB ( GRND_RANDOM
is not set)
depends on the initialization state of the entropy buffer
and on the request size
.IR buflen .
@ -184,8 +191,6 @@ Instead, it will return all of the bytes that have been requested.
.PP
When reading from
.I /dev/random
.RB ( GRND_RANDOM
is set)
these guarantees do
.I not
apply.