semop(2) System Calls Manual semop(2)
NAME
semop, semtimedop - System V semaphore operations
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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 *_Nullable 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; /* PID of process that last
modified the semaphore value */
semop() performs operations on selected semaphores in the set indicated
by semid. Each of the nsops elements in the array pointed to by sops is
a structure that specifies an operation to be performed on a single sem-
aphore. The elements of this structure are of type struct sembuf, con-
taining 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 opera-
tion 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 opera-
tions 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 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 sema-
phore set. This is a "wait-for-zero" operation: if semval is zero, the
operation can immediately proceed. Otherwise, if IPC_NOWAIT is speci-
fied 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 incre-
mented 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 decre-
mented and semop() fails, with errno set to EINTR.
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 adds the absolute value of sem_op to the
semaphore 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 opera-
tions 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 er-
rno set to EIDRM.
• The calling thread catches a signal: the value of semncnt is decre-
mented and semop() fails, with errno set to EINTR.
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()
semtimedop() behaves identically to semop() except that in those cases
where the calling thread would sleep, the duration of that sleep is lim-
ited 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 schedul-
ing 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 operations in sops is performed). If the
timeout argument is NULL, then semtimedop() behaves exactly like se-
mop().
Note that if semtimedop() is interrupted by a signal, causing the call
to fail with the error EINTR, the contents of timeout are left un-
changed.
RETURN VALUE
On success, semop() and semtimedop() return 0. On failure, they return
-1, and set errno to indicate the error.
ERRORS
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 in the user namespace that governs its
IPC namespace.
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 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 im-
plementation dependent maximum value for semval.
STANDARDS
POSIX.1-2008.
VERSIONS
Linux 2.5.52 (backported into Linux 2.4.22), glibc 2.3.3. POSIX.1-2001,
SVr4.
NOTES
The sem_undo structures of a process aren't inherited by the child pro-
duced 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 sema-
phore 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 re-
quest to semctl(2), the corresponding semadj values in all processes are
cleared. The clone(2) 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. Be-
fore Linux 3.19, the default value for this limit was 32. Since
Linux 3.19, the default value is 500. On Linux, this limit can
be read and modified via the third field of /proc/sys/kernel/sem.
Note: this limit should not be raised above 1000, because of the
risk of that semop() fails due to kernel memory fragmentation
when allocating memory to copy the sops array.
SEMVMX Maximum allowable value for semval: implementation dependent
(32767).
The implementation has no intrinsic limits for the adjust on exit maxi-
mum value (SEMAEM), the system wide maximum number of undo structures
(SEMMNU) and the per-process maximum number of undo entries system para-
meters.
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 pos-
sibility is that such semaphore adjustments could be ignored altogether
(somewhat analogously to failing when IPC_NOWAIT is specified for a sem-
aphore operation). Linux adopts a third approach: decreasing the sema-
phore value as far as possible (i.e., to zero) and allowing process ter-
mination to proceed immediately.
In Linux 2.6.x, x <= 10, there is a bug that in some circumstances pre-
vents 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 Linux 2.6.11.
EXAMPLES
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);
}
A further example of the use of semop() can be found in shmop(2).
SEE ALSO
clone(2), semctl(2), semget(2), sigaction(2), capabilities(7),
sem_overview(7), sysvipc(7), time(7)
Linux man-pages 6.9.1 2024-05-02 semop(2)
Generated by dwww version 1.16 on Tue Dec 16 04:05:43 CET 2025.