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SEMOP(2) Linux Programmer's Manual SEMOP(2)
semop, semtimedop - semaphore operations
#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
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:
o semval becomes 0, at which time the value of semzcnt is decremented.
o The semaphore set is removed: semop() fails, with errno set to EIDRM.
o The calling process catches a signal: the value of semzcnt is decremented
and semop() fails, with errno set to EINTR.
o 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:
o 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.
o The semaphore set is removed from the system: semop() fails, with errno set
to EIDRM.
o The calling process catches a signal: the value of semncnt is decremented
and semop() fails, with errno set to EINTR.
o 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().
If successful semop() and semtimedop() return 0; otherwise they return -1 with
errno indicating the error.
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 non-positive 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.
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.
SVr4, POSIX.1-2001.
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.
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.
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);
}
semctl(2), semget(2), sigaction(2), capabilities(7), sem_overview(7),
svipc(7), time(7)
This page is part of release 3.23 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/.
Linux 2008-10-04 SEMOP(2)