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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | VERSIONS | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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semop(2) System Calls Manual semop(2)
semop, semtimedop - System V semaphore operations
Standard C library (libc, -lc)
#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
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 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 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. 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 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.
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 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.
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 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 scheduling 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
semop().
Note that if semtimedop() is interrupted by a signal, causing the
call to fail with the error EINTR, the contents of timeout are
left unchanged.
On success, semop() and semtimedop() return 0. On failure, they
return -1, and set errno to indicate the error.
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 implementation dependent maximum value for semval.
POSIX.1-2008.
Linux 2.5.52 (backported into Linux 2.4.22), glibc 2.3.3.
POSIX.1-2001, SVr4.
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(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.
Before 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
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 Linux 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 Linux 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);
}
A further example of the use of semop() can be found in shmop(2).
clone(2), semctl(2), semget(2), sigaction(2), capabilities(7),
sem_overview(7), sysvipc(7), time(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 semop(2)
Pages that refer to this page: ipcs(1), lsipc(1), pcp-ipcs(1), clone(2), fork(2), ipc(2), semctl(2), semget(2), syscalls(2), unshare(2), proc_sys_kernel(5), pthreads(7), sem_overview(7), signal(7), sysvipc(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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