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semop(2) - phpMan
SEMOP(2) Linux Programmer's Manual SEMOP(2)
NAME
semop, semtimedop - System V semaphore operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semop(int semid, struct sembuf *sops, size_t nsops);
int semtimedop(int semid, struct sembuf *sops, size_t nsops,
const struct timespec *timeout);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
semtimedop(): _GNU_SOURCE
DESCRIPTION
Each semaphore in a System V 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; /* ID of 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, contain‐
ing 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 pres‐
ence 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, distin‐
guished 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 subtracts
the value sem_op from the semaphore adjustment (semadj) value for this semaphore. This
operation can always proceed—it never forces a thread 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. Oth‐
erwise, 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 threads
waiting until this semaphore's value becomes zero) is incremented by one and the thread
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 thread 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 pro‐
ceed immediately: the absolute value of sem_op is subtracted from semval, and, if SEM_UNDO
is specified for this operation, the system adds the absolute value of sem_op to the sema‐
phore adjustment (semadj) value 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 threads waiting for this semaphore's value to increase) is incremented by one
and the thread sleeps until one of the following occurs:
· semval becomes greater than or equal to the absolute value of sem_op: the operation now
proceeds, as described above.
· The semaphore set is removed from the system: semop() fails, with errno set to EIDRM.
· The calling thread 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 caller's process ID. In addition, the sem_otime is set
to the current time.
semtimedop() behaves identically to semop() except that in those cases where the calling
thread 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.
(This sleep interval will be rounded up to the system clock granularity, and kernel sched‐
uling delays mean that the interval may overrun by a small amount.) If the specified time
limit has been reached, semtimedop() fails with errno set to EAGAIN (and none of the oper‐
ations 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 thread caught a signal; see signal(7).
EINVAL The semaphore set doesn't exist, or semid is less than zero, or nsops has a nonpos‐
itive 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 depen‐
dent 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 inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux or by any version
of POSIX. However, some old implementations required the inclusion of these header files,
and the SVID also documented their inclusion. Applications intended to be portable to
such old systems may need to include these header files.
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.
A semaphore adjustment (semadj) value is a per-process, per-semaphore integer that is the
negated sum of all operations performed on a semaphore specifying the SEM_UNDO flag. Each
process has a list of semadj values—one value for each semaphore on which it has operated
using SEM_UNDO. When a process terminates, each of its per-semaphore 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
SETVAL or SETALL request to semctl(2), the corresponding semadj values in all processes
are cleared. The clone() CLONE_SYSVSEM flag allows more than one process to share a
semadj list; see clone(2) for details.
The semval, sempid, semzcnt, and semnct values for a semaphore can all be retrieved using
appropriate semctl(2) calls.
Semaphore limits
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 per‐
formed. 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 thread
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
clone(2), semctl(2), semget(2), sigaction(2), capabilities(7), sem_overview(7), svipc(7),
time(7)
COLOPHON
This page is part of release 3.74 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at http://www.kernel.org/doc/man-pages/.
Linux 2014-09-21 SEMOP(2)
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