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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON |
SEMOP(2) Linux Programmer's Manual SEMOP(2)
semop, semtimedop - System V semaphore operations
#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
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 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.
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 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.
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.
POSIX.1-2001, POSIX.1-2008, SVr4.
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(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 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.
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);
}
clone(2), semctl(2), semget(2), sigaction(2), capabilities(7),
sem_overview(7), svipc(7), time(7)
This page is part of release 4.12 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
https://www.kernel.org/doc/man-pages/.
Linux 2016-10-08 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(5), pthreads(7), sem_overview(7), signal(7), svipc(7)