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
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() 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.
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);
}
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)