semop(2) BSD System Calls Manual semop(2)
NAME
semop -- atomic array of operations on a semaphore set
SYNOPSIS
#include <sys/sem.h>
int
semop(int semid, struct sembuf *sops, size_t nsops);
DESCRIPTION
The semop() system call atomically performs the array of operations indi-
cated by sops on the semaphore set indicated by semid. The length of
sops is indicated by nsops. Each operation is encoded in a struct
sembuf, which is defined as follows:
struct sembuf {
u_short sem_num; /* semaphore # */
short sem_op; /* semaphore operation */
short sem_flg; /* operation flags */
};
For each element in sops, sem_op and sem_flg determine an operation to be
performed on semaphore number sem_num in the set. The values SEM_UNDO
and IPC_NOWAIT may be OR'ed into the sem_flg member in order to modify
the behavior of the given operation.
The operation performed depends as follows on the value of sem_op:
o When sem_op is positive and the process has alter permission, the
semaphore's value is incremented by sem_op's value. If SEM_UNDO is
specified, the semaphore's adjust on exit value is decremented by
sem_op's value. A positive value for sem_op generally corresponds to
a process releasing a resource associated with the semaphore.
o The behavior when sem_op is negative and the process has alter per-
mission, depends on the current value of the semaphore:
o If the current value of the semaphore is greater than or equal to
the absolute value of sem_op, then the value is decremented by
the absolute value of sem_op. If SEM_UNDO is specified, the sem-
aphore's adjust on exit value is incremented by the absolute
value of sem_op.
o If the current value of the semaphore is less than the absolute
value of sem_op, one of the following happens:
o If IPC_NOWAIT was specified, then semop() returns immediately
with a return value of EAGAIN.
o Otherwise, the calling process is put to sleep until one of
the following conditions is satisfied:
o Some other process removes the semaphore with the
IPC_RMID option of semctl(2). In this case, semop()
returns immediately with a return value of EIDRM.
o The process receives a signal that is to be caught. In
this case, the process will resume execution as defined
by sigaction(2).
o The semaphore's value is greater than or equal to the
absolute value of sem_op. When this condition becomes
true, the semaphore's value is decremented by the abso-
lute value of sem_op, the semaphore's adjust on exit
value is incremented by the absolute value of sem_op.
A negative value for sem_op generally means that a process is waiting
for a resource to become available.
o When sem_op is zero and the process has read permission, one of the
following will occur:
o If the current value of the semaphore is equal to zero then
semop() can return immediately.
o If IPC_NOWAIT was specified, then semop() returns immediately
with a return value of EAGAIN.
o Otherwise, the calling process is put to sleep until one of the
following conditions is satisfied:
o Some other process removes the semaphore with the IPC_RMID
option of semctl(2). In this case, semop() returns immedi-
ately with a return value of EIDRM.
o The process receives a signal that is to be caught. In this
case, the process will resume execution as defined by
sigaction(2).
o The semaphore's value becomes zero.
For each semaphore a process has in use, the kernel maintains an ``adjust
on exit'' value, as alluded to earlier. When a process exits, either
voluntarily or involuntarily, the adjust on exit value for each semaphore
is added to the semaphore's value. This can be used to insure that a
resource is released if a process terminates unexpectedly.
RETURN VALUES
The semop() function returns the value 0 if successful; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error.
ERRORS
The semop() system call will fail if:
[E2BIG] Too many operations are specified. [SEMOPM]
[EACCES] Permission is denied, due to a mismatch between the
operation and the mode of the semaphore set.
[EAGAIN] The semaphore's value would result in the process
being put to sleep and IPC_NOWAIT is specified.
[EFBIG] sem_num is not in the range of valid semaphores for
the set.
[EIDRM] The semaphore set is removed from the system.
[EINTR] The semop() system call is interrupted by a signal.
[EINVAL] No semaphore set corresponds to semid, or the process
would exceed the system-defined limit for the number
of per-process SEM_UNDO structures.
[ENOSPC] The system SEM_UNDO pool [SEMMNU] is full.
[ERANGE] The requested operation would cause either the sema-
phore's current value [SEMVMX] or its adjust-on-exit
value [SEMAEM] to exceed the system-imposed limits.
LEGACY SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
The include files <sys/types.h> and <sys/ipc.h> are necessary.
SEE ALSO
semctl(2), semget(2), sigaction(2), compat(5)
BUGS
The semop() system call may block waiting for memory even if IPC_NOWAIT
was specified.
BSD September 22, 1995 BSD
Mac OS X 10.9.1 - Generated Mon Jan 6 14:08:46 CST 2014