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SEMOP
Section: Linux Programmer's Manual (2)
Updated: 2008-10-04
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NAME
semop, semtimedop - semaphore operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semop(int semid, struct sembuf *sops, unsigned nsops);
int semtimedop(int semid, struct sembuf *sops, unsigned nsops,
struct timespec *timeout);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
semtimedop():
_GNU_SOURCE
DESCRIPTION
Each semaphore in a semaphore set has the following associated values:
unsigned short semval; /* semaphore value */
unsigned short semzcnt; /* # waiting for zero */
unsigned short semncnt; /* # waiting for increase */
pid_t sempid; /* process that did last op */
semop()
performs operations on selected semaphores in the set indicated by
semid.
Each of the
nsops
elements in the array pointed to by
sops
specifies an operation to be performed on a single semaphore.
The elements of this structure are of type
struct sembuf,
containing the following members:
unsigned short sem_num; /* semaphore number */
short sem_op; /* semaphore operation */
short sem_flg; /* operation flags */
Flags recognized in
sem_flg
are
IPC_NOWAIT
and
SEM_UNDO.
If an operation specifies
SEM_UNDO,
it will be automatically undone when the process terminates.
The set of operations contained in
sops
is performed in
array order,
and
atomically,
that is, the operations are performed either as a complete unit,
or not at all.
The behavior of the system call if not all operations can be
performed immediately depends on the presence of the
IPC_NOWAIT
flag in the individual
sem_flg
fields, as noted below.
Each operation is performed on the
sem_num-th
semaphore of the semaphore set, where the first semaphore of the set
is numbered 0.
There are three types of operation, distinguished by the value of
sem_op.
If
sem_op
is a positive integer, the operation adds this value to
the semaphore value
(semval).
Furthermore, if
SEM_UNDO
is specified for this operation, the system updates the process undo count
(semadj)
for this semaphore.
This operation can always proceed --- it never forces a process to wait.
The calling process must have alter permission on the semaphore set.
If
sem_op
is zero, the process must have read permission on the semaphore
set.
This is a "wait-for-zero" operation: if
semval
is zero, the operation can immediately proceed.
Otherwise, if
IPC_NOWAIT
is specified in
sem_flg,
semop()
fails with
errno
set to
EAGAIN
(and none of the operations in
sops
is performed).
Otherwise
semzcnt
(the count of processes waiting until this semaphore's value becomes zero)
is incremented by one and the process sleeps until
one of the following occurs:
- *
-
semval
becomes 0, at which time the value of
semzcnt
is decremented.
- *
-
The semaphore set
is removed:
semop()
fails, with
errno
set to
EIDRM.
- *
-
The calling process catches a signal:
the value of
semzcnt
is decremented and
semop()
fails, with
errno
set to
EINTR.
- *
-
The time limit specified by
timeout
in a
semtimedop()
call expires:
semop()
fails, with
errno
set to
EAGAIN.
If
sem_op
is less than zero, the process must have alter permission on the
semaphore set.
If
semval
is greater than or equal to the absolute value of
sem_op,
the operation can proceed immediately:
the absolute value of
sem_op
is subtracted from
semval,
and, if
SEM_UNDO
is specified for this operation, the system updates the process undo count
(semadj)
for this semaphore.
If the absolute value of
sem_op
is greater than
semval,
and
IPC_NOWAIT
is specified in
sem_flg,
semop()
fails, with
errno
set to
EAGAIN
(and none of the operations in
sops
is performed).
Otherwise
semncnt
(the counter of processes waiting for this semaphore's value to increase)
is incremented by one and the process sleeps until
one of the following occurs:
- *
-
semval
becomes greater than or equal to the absolute value of
sem_op,
at which time the value of
semncnt
is decremented, the absolute value of
sem_op
is subtracted from
semval
and, if
SEM_UNDO
is specified for this operation, the system updates the process undo count
(semadj)
for this semaphore.
- *
-
The semaphore set is removed from the system:
semop()
fails, with
errno
set to
EIDRM.
- *
-
The calling process catches a signal:
the value of
semncnt
is decremented and
semop()
fails, with
errno
set to
EINTR.
- *
-
The time limit specified by
timeout
in a
semtimedop()
call expires: the system call fails, with
errno
set to
EAGAIN.
On successful completion, the
sempid
value for each semaphore specified in the array pointed to by
sops
is set to the process ID of the calling process.
In addition, the
sem_otime
is set to the current time.
semtimedop()
behaves identically to
semop()
except that in those cases were the calling process would sleep,
the duration of that sleep is limited by the amount of elapsed
time specified by the
timespec
structure whose address is passed in the
timeout
argument.
If the specified time limit has been reached,
semtimedop()
fails with
errno
set to
EAGAIN
(and none of the operations in
sops
is performed).
If the
timeout
argument is NULL,
then
semtimedop()
behaves exactly like
semop().
RETURN VALUE
If successful
semop()
and
semtimedop()
return 0;
otherwise they return -1
with
errno
indicating the error.
ERRORS
On failure,
errno
is set to one of the following:
- E2BIG
-
The argument
nsops
is greater than
SEMOPM,
the maximum number of operations allowed per system
call.
- EACCES
-
The calling process does not have the permissions required
to perform the specified semaphore operations,
and does not have the
CAP_IPC_OWNER
capability.
- EAGAIN
-
An operation could not proceed immediately and either
IPC_NOWAIT
was specified in
sem_flg
or the time limit specified in
timeout
expired.
- EFAULT
-
An address specified in either the
sops
or the
timeout
argument isn't accessible.
- EFBIG
-
For some operation the value of
sem_num
is less than 0 or greater than or equal to the number
of semaphores in the set.
- EIDRM
-
The semaphore set was removed.
- EINTR
-
While blocked in this system call, the process caught a signal; see
signal(7).
- EINVAL
-
The semaphore set doesn't exist, or
semid
is less than zero, or
nsops
has a nonpositive value.
- ENOMEM
-
The
sem_flg
of some operation specified
SEM_UNDO
and the system does not have enough memory to allocate the undo
structure.
- ERANGE
-
For some operation
sem_op+semval
is greater than
SEMVMX,
the implementation dependent maximum value for
semval.
VERSIONS
semtimedop()
first appeared in Linux 2.5.52,
and was subsequently backported into kernel 2.4.22.
Glibc support for
semtimedop()
first appeared in version 2.3.3.
CONFORMING TO
SVr4, POSIX.1-2001.
NOTES
The
sem_undo
structures of a process aren't inherited by the child produced by
fork(2),
but they are inherited across an
execve(2)
system call.
semop()
is never automatically restarted after being interrupted by a signal handler,
regardless of the setting of the
SA_RESTART
flag when establishing a signal handler.
semadj
is a per-process integer which is simply the (negative) count
of all semaphore operations performed specifying the
SEM_UNDO
flag.
When a semaphore's value is directly set using the
SETVAL
or
SETALL
request to
semctl(2),
the corresponding
semadj
values in all processes are cleared.
The semval, sempid, semzcnt, and semnct values
for a semaphore can all be retrieved using appropriate
semctl(2)
calls.
The following limits on semaphore set resources affect the
semop()
call:
- SEMOPM
-
Maximum number of operations allowed for one
semop()
call (32)
(on Linux, this limit can be read and modified via the third field of
/proc/sys/kernel/sem).
- SEMVMX
-
Maximum allowable value for
semval:
implementation dependent (32767).
The implementation has no intrinsic limits for
the adjust on exit maximum value
(SEMAEM),
the system wide maximum number of undo structures
(SEMMNU)
and the per-process maximum number of undo entries system parameters.
BUGS
When a process terminates, its set of associated
semadj
structures is used to undo the effect of all of the
semaphore operations it performed with the
SEM_UNDO
flag.
This raises a difficulty: if one (or more) of these semaphore adjustments
would result in an attempt to decrease a semaphore's value below zero,
what should an implementation do?
One possible approach would be to block until all the semaphore
adjustments could be performed.
This is however undesirable since it could force process termination to
block for arbitrarily long periods.
Another possibility is that such semaphore adjustments could be ignored
altogether (somewhat analogously to failing when
IPC_NOWAIT
is specified for a semaphore operation).
Linux adopts a third approach: decreasing the semaphore value
as far as possible (i.e., to zero) and allowing process
termination to proceed immediately.
In kernels 2.6.x, x <= 10, there is a bug that in some circumstances
prevents a process that is waiting for a semaphore value to become
zero from being woken up when the value does actually become zero.
This bug is fixed in kernel 2.6.11.
EXAMPLE
The following code segment uses
semop()
to atomically wait for the value of semaphore 0 to become zero,
and then increment the semaphore value by one.
struct sembuf sops[2];
int semid;
/* Code to set semid omitted */
sops[0].sem_num = 0; /* Operate on semaphore 0 */
sops[0].sem_op = 0; /* Wait for value to equal 0 */
sops[0].sem_flg = 0;
sops[1].sem_num = 0; /* Operate on semaphore 0 */
sops[1].sem_op = 1; /* Increment value by one */
sops[1].sem_flg = 0;
if (semop(semid, sops, 2) == -1) {
perror("semop");
exit(EXIT_FAILURE);
}
SEE ALSO
semctl(2),
semget(2),
sigaction(2),
capabilities(7),
sem_overview(7),
svipc(7),
time(7)
COLOPHON
This page is part of release 3.27 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- BUGS
-
- EXAMPLE
-
- SEE ALSO
-
- COLOPHON
-
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