semop() - Unix, Linux System Call
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NAME
semop, semtimedop - semaphore operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
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int semop(int semid, struct sembuf *sops, unsigned nsops);
int semtimedop(int semid, struct sembuf *sops, unsigned nsops, struct timespec *timeout); DESCRIPTION
Each semaphore in a 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; /* process that did last op */
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semop() performs operations on selected semaphores in the set indicated by
semid. Each of the
nsops elements in the array pointed to by
sops 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 */
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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
atomically, that is, the operations are performed at the same time, and only
if they can all be simultaneously performed.
The behaviour 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 updates the process undo count
(semadj) for this semaphore.
This operation can always proceed it never forces a process 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 processes waiting until this semaphores value becomes zero)
is incremented by one and the process sleeps until
one of the following occurs:
Tag | Description |
o
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semval becomes 0, at which time the value of
semzcnt is decremented.
|
o
|
The semaphore set
is removed:
semop() fails, with
errno set to
EIDRM. |
o
|
The calling process catches a signal:
the value of
semzcnt is decremented and
semop() fails, with
errno set to
EINTR. |
o
|
The time limit specified by
timeout in a
semtimedop() call expires:
semop() fails, with
errno set to
EAGAIN. |
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 updates the process undo count
(semadj) 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 processes waiting for this semaphores value to increase)
is incremented by one and the process sleeps until
one of the following occurs:
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o
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semval becomes greater than or equal to the absolute value of
sem_op, at which time the value of
semncnt is decremented, the absolute value of
sem_op is subtracted from
semval and, if
SEM_UNDO is specified for this operation, the system updates the process undo count
(semadj) for this semaphore.
|
o
|
The semaphore set is removed from the system:
semop() fails, with
errno set to
EIDRM. |
o
|
The calling process catches a signal:
the value of
semncnt is decremented and
semop() fails, with
errno set to
EINTR. |
o
|
The time limit specified by
timeout in a
semtimedop() call expires: the system call fails, with
errno set to
EAGAIN. |
On successful completion, the
sempid value for each semaphore specified in the array pointed to by
sops is set to the process ID of the calling process.
In addition, the
sem_otime is set to the current time.
semtimedop() behaves identically to
semop() except that in those cases were the calling process 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 parameter. 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 parameter is NULL,
then
semtimedop() behaves exactly like
semop(). RETURN VALUE
If successful
semop() and
semtimedop() return 0;
otherwise they return -1
with
errno indicating the error.
ERRORS
On failure,
errno is set to one of the following:
Tag | Description |
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.
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EAGAIN |
An operation could not proceed immediately and either
IPC_NOWAIT was specified in
sem_flg or the time limit specified in
timeout expired.
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EFAULT |
An address specified in either the
sops or
timeout parameters isnt accessible.
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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.
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EIDRM |
The semaphore set was removed.
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EINTR |
While blocked in this system call, the process caught a signal.
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EINVAL |
The semaphore set doesnt exist, or
semid is less than zero, or
nsops has a non-positive value.
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ENOMEM |
The
sem_flg of some operation specified
SEM_UNDO and the system does not have enough memory to allocate the undo
structure.
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ERANGE |
For some operation
sem_op+semval is greater than
SEMVMX, the implementation dependent maximum value for
semval. |
NOTES
The
sem_undo structures of a process arent inherited across a
fork(2)
system call, 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.
semadj is a per-process integer which is simply the (negative) count
of all semaphore operations performed specifying the
SEM_UNDO flag.
When a semaphores value is directly set using the
SETVAL or
SETALL request to
semctl(2),
the corresponding
semadj values in all processes are cleared.
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:
Tag | Description |
SEMOPM |
Maximum number of operations allowed for one
semop() call (32)
(on Linux, this limit can be read and modified via the third field of
/proc/sys/kernel/sem). |
SEMVMX |
Maximum allowable value for
semval: implementation dependent (32767).
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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.
semtimedop() first appeared in Linux 2.5.52,
and was subsequently backported into kernel 2.4.22.
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 semaphores 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 process 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.
CONFORMING TO
SVr4, POSIX.1-2001.
SEE ALSO
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