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sched_setaffinity.2: Warn that CPU_ALLOC() may allocate a slightly CPU set than requested 

Reported-by: Florian Weimer <fweimer@redhat.com>
Reviewed-by: Florian Weimer <fweimer@redhat.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2016-03-10 16:43:52 +01:00
parent cf486e4aa9
commit 310ac27840
1 changed files with 15 additions and 0 deletions
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15
man2/sched_setaffinity.2
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@ -276,6 +276,21 @@ of the required mask using
.BR sched_getaffinity ()
calls with increasing mask sizes (until the call does not fail with the error
.BR EINVAL ).
Be aware that
.BR CPU_ALLOC (3)
may allocate a slightly larger CPU set than requested
(because CPU sets are implemented as bit masks allocated in units of
.IR sizeof(long) ).
Consequently,
.BR sched_getaffinity ()
can set bits beyond the requested allocation size, because the kernel
sees a few additional bits.
Therefore, the caller should iterate over the bits in the returned set,
counting those which are set, and stop upon reaching the value returned by
.BR CPU_COUNT (3)
(rather than iterating over the number of bits
requested to be allocated).
.SH EXAMPLE
The program below creates a child process.
The parent and child then each assign themselves to a specified CPU