Linux系統編程之struct flock 結構體

該結構是在lock.h文件中定義。

lock.h File

功能

定義一些文件的鎖的選項

Description

The flock structure in the /usr/include/sys/flock.h file, which describes a lock, contains the following fields:

l_type Describes the type of lock. If the value of the Command parameter to the fcntl subroutine is F_SETLK orF_SETLKW, the l_type field indicates the type of lock to be created. Possible values are:

F_RDLCK

A read lock is requested.

F_WRLCK

A write lock is requested.

F_UNLCK

Unlock. An existing lock is to be removed.

If the value of the Command parameter to the fcntl subroutine is F_GETLK, the l_type field describes an existing lock. Possible values are:

F_RDLCK

A conflicting read lock exists.

F_WRLCK

A conflicting write lock exists.

F_UNLCK

No conflicting lock exists.

l_whence Defines the starting offset. The value of this field indicates the point from which the relative offset, the l_startfield, is measured. Possible values are:

SEEK_SET

The relative offset is measured from the start of the file.

SEEK_CUR

The relative offset is measured from the current position.

SEEK_END

The relative offset is measured from the end of the file.

These values are defined in the unistd.h file.

l_start Defines the relative offset in bytes, measured from the starting point in the l_whence field. l_len Specifies the number of consecutive bytes to be locked. l_sysid Contains the ID of the node that already has a lock placed on the area defined by the fcntl subroutine. This field is returned only when the value of the Command parameter is F_GETLK. l_pid Contains the ID of a process that already has a lock placed on the area defined by the fcntl subroutine. This field is returned only when the value of the Command parameter is F_GETLK.

l_vfs

Specifies the file system type of the node identified in the l_sysid field.

看一下示例吧！

1. int
2. waldirlock(Wal *w)
3. {
4. int r;
5. int fd;
6. struct flock lk;
7. char path[PATH_MAX];
9. r = snprintf(path, PATH_MAX, "%s/lock", w->dir);
10. if (r > PATH_MAX) {
11. twarnx("path too long: %s/lock", w->dir);
12. return 0;
13. }
15. fd = open(path, O_WRONLY|O_CREAT, 0600);
16. if (fd == -1) {
17. twarn("open");
18. return 0;
19. }
21. lk.l_type = F_WRLCK;
22. lk.l_whence = SEEK_SET;
23. lk.l_start = 0;
24. lk.l_len = 0;
25. r = fcntl(fd, F_SETLK, &lk);
26. if (r) {
27. twarn("fcntl");
28. return 0;
29. }
31. // intentionally leak fd, since we never want to close it
32. // and we'll never need it again
33. return 1;
34. }

struct flock 作為fcntl函數的第三個參數，使用F_SETLK,設置了其參數。