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semop.2: Clarify the discussion of 'semadj' 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2013-03-23 15:50:56 +01:00
parent 73f4bf1eab
commit b41a5d542d
1 changed files with 24 additions and 20 deletions
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man2/semop.2
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@ -126,9 +126,11 @@ the semaphore value
.RI ( semval ).
Furthermore, if
.B SEM_UNDO
is specified for this operation, the system updates the undo count
is specified for this operation, the system subtracts the value
.I sem_op
from the semaphore adjustment
.RI ( semadj )
for this semaphore.
value for this semaphore.
This operation can always proceed\(emit never forces a thread to wait.
The calling process must have alter permission on the semaphore set.
.PP
@ -206,9 +208,11 @@ is subtracted from
.IR semval ,
and, if
.B SEM_UNDO
is specified for this operation, the system updates the undo count
is specified for this operation, the system adds the absolute value of
.I sem_op
to the semaphore adjustment
.RI ( semadj )
for this semaphore.
value for this semaphore.
If the absolute value of
.I sem_op
is greater than
@ -233,18 +237,8 @@ one of the following occurs:
.IP \(bu 3
.I semval
becomes greater than or equal to the absolute value of
.IR sem_op ,
at which time the value of
.I semncnt
is decremented, the absolute value of
.I sem_op
is subtracted from
.I semval
and, if
.B SEM_UNDO
is specified for this operation, the system updates the undo count
.RI ( semadj )
for this semaphore.
.IR sem_op :
the operation now proceeds, as described above.
.IP \(bu
The semaphore set is removed from the system:
.BR semop ()
@ -430,12 +424,22 @@ is never automatically restarted after being interrupted by a signal handler,
regardless of the setting of the
.B SA_RESTART
flag when establishing a signal handler.
.PP
.I semadj
is a per-process integer which is simply the (negative) count
of all semaphore operations performed specifying the
A semaphore adjustment
.RI ( semadj )
value is a per-process, per-semaphore integer that is the negated sum
of all operations performed on a semaphore specifying the
.B SEM_UNDO
flag.
Each process has a list of
.I semadj
values\(emone value for each semaphore on which it has operated using
.BR SEM_UNDO .
When a process terminates, each of its per-semaphore
.I semadj
values is added to the corresponding semaphore,
thus undoing the effect of that process's operations on the semaphore
(but see BUGS below).
When a semaphore's value is directly set using the
.B SETVAL
or