semop does the checks of all sops atomically? - concurrency

Question

I am implementing R/W lock with semop as follows.
My question is: semop does the checks of all sops atomically? Like, semop adds some lock firstly, and then check these three sops one by one.
If not, can the following scenario happens?
when one thread calls CSemRWLock::CWriteLock::lock(), finds there is no reader by checking the first sops, and goes to the second sops. At the same time, another thread calling CReadLock::lock() changes the first to non-zero value. Both two threads get locks, one read and one write.
If so, CWriteLock::lock() needs to exchange the first two sops, specify there is writer first, then judge no reader. Right?
bool CSemRWLock::CWriteLock::lock()
{
//judge no reader
sbuf[0].sem_num = 0;
sbuf[0].sem_op = 0;
sbuf[0].sem_flg = 0;
//specify there is writer
sbuf[1].sem_num = 1;
sbuf[1].sem_op = 1;
sbuf[1].sem_flg = SEM_UNDO;
//occupy the writter resource
sbuf[2].sem_num = 2;
sbuf[2].sem_op = -1;
sbuf[2].sem_flg = SEM_UNDO;
semop(m_iSemID, sbuf, 3)
......
}
bool CSemRWLock::CReadLock::lock()
{
//add a reader
sbuf[0].sem_num = 0;
sbuf[0].sem_op = 1;
sbuf[0].sem_flg = SEM_UNDO;
//judge no writer
sbuf[1].sem_num = 1;
sbuf[1].sem_op = 0;
sbuf[1].sem_flg = 0;
semop(m_iSemID, sbuf, 2);
......
}