linux系统编程之struct flock 构造体

linux系统编程之struct flock 结构体

该结构是在lock.h文件中定义。

lock.h File

 

功能

定义一些文件的锁的选项

Description

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

 

l_typeDescribes the type of lock. If the value of the Commandparameter to the fcntl subroutine isF_SETLK orF_SETLKW, thel_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 theCommand parameter to the fcntlsubroutine 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_whenceDefines the starting offset. The value of this field indicates the point from which the relative offset, thel_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 theunistd.h file.

l_startDefines the relative offset in bytes, measured from the starting point in thel_whence field.l_lenSpecifies the number of consecutive bytes to be locked.l_sysidContains the ID of the node that already has a lock placed on the area defined by thefcntl subroutine. This field is returned only when the value of theCommand parameter is F_GETLK.l_pidContains the ID of a process that already has a lock placed on the area defined by thefcntl subroutine. This field is returned only when the value of theCommand parameter is F_GETLK.

l_vfs

Specifies the file system type of the node identified in the l_sysidfield.

 

 

看一下示例吧！

intwaldirlock(Wal *w){    int r;    int fd;    struct flock lk;    char path[PATH_MAX];    r = snprintf(path, PATH_MAX, "%s/lock", w->dir);    if (r > PATH_MAX) {        twarnx("path too long: %s/lock", w->dir);        return 0;    }    fd = open(path, O_WRONLY|O_CREAT, 0600);    if (fd == -1) {        twarn("open");        return 0;    }    lk.l_type = F_WRLCK;    lk.l_whence = SEEK_SET;    lk.l_start = 0;    lk.l_len = 0;    r = fcntl(fd, F_SETLK, &lk);    if (r) {        twarn("fcntl");        return 0;    }    // intentionally leak fd, since we never want to close it    // and we'll never need it again    return 1;}

 

struct flock 作为fcntl函数的第三个参数，使用F_SETLK,设置了其参数。

 

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