t l_type; /* Type of lock: F_RDLCK,
F_WRLCK, F_UNLCK */
short l_whence; /* How to interpret l_start:
SEEK_SET, SEEK_CUR, SEEK_END */
off_t l_start; /* Starting offset for lock */
off_t l_len; /* Number of bytes to lock */
pid_t l_pid; /* PID of process blocking our lock
(set by F_GETLK and F_OFD_GETLK) */
...
};
- F_SETLK:上锁。如果发现已经被别的进程上锁了,就直接返回-1,errno被设置成EACCES或者EAGAIN,不阻塞。
- F_SETLKW:上锁。阻塞等待。
- F_GETLK:得到锁的状态。
修改上面的函数my_lock,my_unlock。main函数不变。
例子2:
void my_lock(int fd){
struct flock lock;
lock.l_type = F_WRLCK;
wlock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
fcntl(fd, F_SETLKW, lock);
}
void my_unlock(int fd){
struct flock lock;
lock.l_type = F_UNLCK;
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
fcntl(fd, F_SETLK, lock);
}
执行结果如下,发现数字不乱套了。
ubuntu$ ./flockmain & ./flockmain &
[1] 4882
[2] 4883
ubuntu$ ./flockmain:pid = 4883, seq = 1
./flockmain:pid = 4883, seq = 2
./flockmain:pid = 4883, seq = 3
./flockmain:pid = 4883, seq = 4
./flockmain:pid = 4883, seq = 5
./flockmain:pid = 4883, seq = 6
./flockmain:pid = 4883, seq = 7
./flockmain:pid = 4883, seq = 8
./flockmain:pid = 4883, seq = 9
./flockmain:pid = 4883, seq = 10
./flockmain:pid = 4883, seq = 11
./flockmain:pid = 4883, seq = 12
./flockmain:pid = 4883, seq = 13
./flockmain:pid = 4883, seq = 14
./flockmain:pid = 4883, seq = 15
./flockmain:pid = 4883, seq = 16
./flockmain:pid = 4883, seq = 17
./flockmain:pid = 4883, seq = 18
./flockmain:pid = 4883, seq = 19
./flockmain:pid = 4883, seq = 20
./flockmain:pid = 4882, seq = 21
./flockmain:pid = 4882, seq = 22
./flockmain:pid = 4882, seq = 23
./flockmain:pid = 4882, seq = 24
./flockmain:pid = 4882, seq = 25
./flockmain:pid = 4882, seq = 26
./flockmain:pid = 4882, seq = 27
./flockmain:pid = 4882, seq = 28
./flockmain:pid = 4882, seq = 29
./flockmain:pid = 4882, seq = 30
./flockmain:pid = 4882, seq = 31
./flockmain:pid = 4882, seq = 32
./flockmain:pid = 4882, seq = 33
./flockmain:pid = 4882, seq = 34
./flockmain:pid = 4882, seq = 35
./flockmain:pid = 4882, seq = 36
./flockmain:pid = 4882, seq = 37
./flockmain:pid = 4882, seq = 38
./flockmain:pid = 4882, seq = 39
./flockmain:pid = 4882, seq = 40
到此为止,貌似解决了问题,但是如果同时执行例子1和例子2,结果如下,发现还是乱的。
也就是说在协作线程(cooperating processes)间,文件锁(也叫劝告性上锁)也起作用的。但是不完全不相关的进程中,文件锁也不起作用的。如何解决呢?使用强制性上锁。
ys@ys-VirtualBox:~/IPC$ ./flockmain1 & ./flockmain &
[1] 3602
[2] 3603
ys@ys-VirtualBox:~/IPC$ ./flockmain1:pid = 3602, seq = 1
./flockmain:pid = 3603, seq = 1
./flockmain:pid = 3603, seq = 2
./flockmain:pid = 3603, seq = 3
./flockmain:pid = 3603, seq = 4
./flockmain:pid = 3603, seq = 5
./flockmain:pid = 3603, seq = 6
./flockmain:pid = 3603, seq = 7
./flockmain:pid = 3603, seq = 8
./flockmain:pid = 3603, seq = 9
./flockmain:pid = 3603, seq = 10
./flockmain1:pid = 3602, seq = 2
./flockmain1:pid = 3602, seq = 3
./flockmain1:pid = 3602, seq = 4
./flockmain:pid = 3603, seq = 11
./flockmain:pid = 3603, seq = 12
./flockmain1:pid = 3602, seq = 5
./flockmain:pid = 3603, seq = 13
./flockmain1:pid = 3602, seq = 6
./flockmain1:pid = 3602, seq = 7
./flockmain1:pid = 3602, seq = 8
./flockmain:pid = 3603, seq = 14
./flockmain:pid = 3603, seq = 15
./flockmain1:pid = 3602, seq = 9
./flockmain1:pid = 3602, seq = 10
./flockmain:pid = 3603, seq = 16
./flockmain:pid = 3603, seq = 17
./flockmain1:pid = 3602, seq = 11
./flockmain:pid = 3603, seq = 18
./flockmain1:pid = 3602, seq = 12
./flockmain1:pid = 3602, seq = 13
./flockmain1:pid = 3602, seq = 1