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futex.2: Reword paragraph describing futex word 

Reported-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2015-07-28 09:38:47 +02:00
parent 02f7b623c9
commit 7e8dcabcb1
1 changed files with 15 additions and 7 deletions
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22
man2/futex.2
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@ -50,14 +50,22 @@ a longer time until the condition becomes true.
The program uses another operation of the system call to wake
anyone waiting for a particular condition.
The condition is represented by the futex word, which is an address
in memory supplied to the
A futex is a 32-bit value\(emreferred to below as a
.IR "futex word" \(emwhose
address is supplied to the
.BR futex ()
system call, and the 32-bit value at this memory location.
(While the virtual addresses for the same physical memory address
in separate processes may be different,
the same physical address may be shared by the processes using
.BR mmap (2).)
system call.
(Futexes are 32-bits in size on all platforms, including 64-bit systems.)
All futex operations are governed by this value.
In order to share a futex between processes,
the futex is placed in a region of shared memory,
created using (for example)
.BR mmap (2)
or
.BR shmat (2).
(Thus the futex word may have different
virtual addresses in different processes,
but these addresses all refer to the same location in physical memory.)
When executing a futex operation that requests to block a thread,
the kernel will block only if the futex word has the value that the