SEMOP(2) FreeBSD System Calls Manual SEMOP(2)
NAME
semop - atomic array of operations on a semaphore set
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int
semop(int semid, struct sembuf *array, size_t nops);
DESCRIPTION
The semop() system call atomically performs the array of operations
indicated by array on the semaphore set indicated by semid. The length
of array is indicated by nops. 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 array, 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:
• 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.
• The behavior when sem_op is negative and the process has alter
permission, depends on the current value of the semaphore:
• 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
semaphore's adjust on exit value is incremented by the absolute
value of sem_op.
• If the current value of the semaphore is less than the absolute
value of sem_op, one of the following happens:
• If IPC_NOWAIT was specified, then semop() returns immediately
with a return value of EAGAIN.
• Otherwise, the calling process is put to sleep until one of
the following conditions is satisfied:
• 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.
• The process receives a signal that is to be caught. In
this case, the process will resume execution as defined
by sigaction(2).
• 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
absolute 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.
• When sem_op is zero and the process has read permission, one of the
following will occur:
• If the current value of the semaphore is equal to zero then
semop() can return immediately.
• If IPC_NOWAIT was specified, then semop() returns immediately
with a return value of EAGAIN.
• Otherwise, the calling process is put to sleep until one of the
following conditions is satisfied:
• 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.
• The process receives a signal that is to be caught. In this
case, the process will resume execution as defined by
sigaction(2).
• 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 ensure 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:
[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.
[EACCES] Permission denied due to mismatch between operation
and mode of semaphore set.
[EAGAIN] The semaphore's value would have resulted in the
process being put to sleep and IPC_NOWAIT was
specified.
[E2BIG] Too many operations were specified. [SEMOPM]
[EFBIG] sem_num was not in the range of valid semaphores for
the set.
[EIDRM] The semaphore set was removed from the system.
[EINTR] The semop() system call was interrupted by a signal.
[ENOSPC] The system SEM_UNDO pool [SEMMNU] is full.
[ERANGE] The requested operation would cause either the
semaphore's current value [SEMVMX] or its adjust on
exit value [SEMAEM] to exceed the system-imposed
limits.
SEE ALSO
semctl(2), semget(2), sigaction(2)
BUGS
The semop() system call may block waiting for memory even if IPC_NOWAIT
was specified.
FreeBSD 13.1-RELEASE-p6 September 22, 1995 FreeBSD 13.1-RELEASE-p6
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