SEMOP(2) | System Calls Manual | SEMOP(2) |
semop
— atomic
array of operations on a semaphore set
#include
<sys/sem.h>
int
semop
(int semid,
struct sembuf *sops, size_t
nsops);
The
semop
()
system call atomically performs the array of operations indicated 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:
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.SEM_UNDO
is specified, the semaphore's adjust
on exit value is incremented by the absolute value of
sem_op.IPC_NOWAIT
was specified,
then
semop
()
returns immediately with a return value of
EAGAIN
.IPC_RMID
option of
semctl(2). In this case,
semop
() returns immediately with a
return value of EIDRM
.A negative value for sem_op generally means that a process is waiting for a resource to become available.
semop
()
can return immediately.IPC_NOWAIT
was specified, then
semop
() returns immediately with a return
value of EAGAIN
.IPC_RMID
option of
semctl(2). In this case,
semop
() returns immediately with a return
value of EIDRM
.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.
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.
The semop
() system call will fail if:
E2BIG
]SEMOPM
]EACCES
]EAGAIN
]IPC_NOWAIT
is specified.EFBIG
]EIDRM
]EINTR
]semop
() system call is interrupted by a
signal.EINVAL
]SEM_UNDO
structures.ENOSPC
]SEM_UNDO
pool
[SEMMNU
] is full.ERANGE
]SEMVMX
] or its adjust-on-exit value
[SEMAEM
] to exceed the system-imposed limits.#include
<sys/types.h>
#include
<sys/ipc.h>
#include
<sys/sem.h>
The include files
<sys/types.h>
and
<sys/ipc.h>
are
necessary.
The semop
() system call may block waiting
for memory even if IPC_NOWAIT
was specified.
September 22, 1995 | macOS 14.6 |