semop(2) - Linux manual page (original) (raw)
semop(2) System Calls Manual semop(2)
NAME top
semop, semtimedop - System V semaphore operationsLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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_SOURCEDESCRIPTION top
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 _semflg_ 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
_semflg_ fields, as noted below.
Each operation is performed on the _semnum_-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
_semop_.
If _semop_ 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
_semop_ 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 _semop_ 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 _semflg_, **semop**() fails with _[errno](../man3/errno.3.html)_ 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](../man3/errno.3.html)_ set to
**EIDRM**.
• The calling thread catches a signal: the value of _semzcnt_ is
decremented and **semop**() fails, with _[errno](../man3/errno.3.html)_ set to **EINTR**.
If _semop_ 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 _semop_, the operation can proceed
immediately: the absolute value of _semop_ is subtracted from
_semval_, and, if **SEM_UNDO** is specified for this operation, the
system adds the absolute value of _semop_ to the semaphore
adjustment (_semadj_) value for this semaphore. If the absolute
value of _semop_ is greater than _semval_, and **IPC_NOWAIT** is
specified in _semflg_, **semop**() fails, with _[errno](../man3/errno.3.html)_ 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
_semop_: the operation now proceeds, as described above.
• The semaphore set is removed from the system: **semop**() fails,
with _[errno](../man3/errno.3.html)_ set to **EIDRM**.
• The calling thread catches a signal: the value of _semncnt_ is
decremented and **semop**() fails, with _[errno](../man3/errno.3.html)_ 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 _semotime_ 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.RETURN VALUE top
On success, **semop**() and **semtimedop**() return 0. On failure, they
return -1, and set _[errno](../man3/errno.3.html)_ to indicate the error.ERRORS top
**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 _semflg_ 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 _semnum_ 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)](../man7/signal.7.html).
**EINVAL** The semaphore set doesn't exist, or _semid_ is less than
zero, or _nsops_ has a nonpositive value.
**ENOMEM** The _semflg_ of some operation specified **SEM_UNDO** and the
system does not have enough memory to allocate the undo
structure.
**ERANGE** For some operation _semop+semval_ is greater than **SEMVMX**,
the implementation dependent maximum value for _semval_.STANDARDS top
POSIX.1-2008.VERSIONS top
Linux 2.5.52 (backported into Linux 2.4.22), glibc 2.3.3.
POSIX.1-2001, SVr4.NOTES top
The _semundo_ structures of a process aren't inherited by the child
produced by [fork(2)](../man2/fork.2.html), but they are inherited across an [execve(2)](../man2/execve.2.html)
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)](../man2/semctl.2.html), the corresponding _semadj_ values in all
processes are cleared. The [clone(2)](../man2/clone.2.html) **CLONE_SYSVSEM** flag allows
more than one process to share a _semadj_ list; see [clone(2)](../man2/clone.2.html) for
details.
The _semval_, _sempid_, _semzcnt_, and _semnct_ values for a semaphore can
all be retrieved using appropriate [semctl(2)](../man2/semctl.2.html) 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.BUGS top
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.EXAMPLES top
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)](../man2/shmop.2.html).SEE ALSO top
[clone(2)](../man2/clone.2.html), [semctl(2)](../man2/semctl.2.html), [semget(2)](../man2/semget.2.html), [sigaction(2)](../man2/sigaction.2.html), [capabilities(7)](../man7/capabilities.7.html),
[sem_overview(7)](../man7/sem%5Foverview.7.html), [sysvipc(7)](../man7/sysvipc.7.html), [time(7)](../man7/time.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 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)