Linux 文件锁

系统调用fcntl

#include 
#include 

int fcntl(int fd, int cmd, struct flock *lock );

参数：
fd：文件描述符
cmd：F_GETLK, F_SETLK or F_SETLKW
lock：按照如下规则填写，可以达到相应效果

F_GETLK, F_SETLK and F_SETLKW are used to acquire, release, and test for the existence of record locks (also known as file-segment  or  file-region
       locks).  The third argument, lock, is a pointer to a structure that has at least the following fields (in unspecified order).

           struct flock {
               ...
               short 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
                                   (F_GETLK only) */
               ...
           };

文件锁的封装

filelock.h：

#ifndef _FILE_LOCK_H
#define _FILE_LOCK_H

int lock_reg(int fd, int cmd, int type, off_t offset, int whence, off_t len);

int File_ReadLck(int fd, off_t offset, int whence, off_t len);
int File_WriteLck(int fd, off_t offset, int whence, off_t len);

int File_ReadLckW(int fd, off_t offset, int whence, off_t len);
int File_WriteLckW(int fd, off_t offset, int whence, off_t len);


int File_Unlock(int fd, off_t offset, int whence, off_t len);

#endif

filelock.c：

#include 
#include 
#include 

#include "filelock.h"

int lock_reg(int fd, int cmd, int type, off_t offset, int whence, off_t len)
{
    struct flock lock;

    lock.l_type = type;
    lock.l_start = offset;
    lock.l_whence = whence;
    lock.l_len = len;

    return (fcntl(fd, cmd, &lock));
}


int File_ReadLck(int fd, off_t offset, int whence, off_t len)
{
    return lock_reg(fd, F_SETLK, F_RDLCK, offset, whence, len);
}


int File_WriteLck(int fd, off_t offset, int whence, off_t len)
{
    return lock_reg(fd, F_SETLK, F_WRLCK, offset, whence, len);
}


int File_ReadLckW(int fd, off_t offset, int whence, off_t len)
{
    return lock_reg(fd, F_SETLKW, F_RDLCK, offset, whence, len);
}


int File_WriteLckW(int fd, off_t offset, int whence, off_t len)
{
    return lock_reg(fd, F_SETLKW, F_WRLCK, offset, whence, len);    
}


int File_Unlock(int fd, off_t offset, int whence, off_t len)
{
    return lock_reg(fd, F_SETLK, F_UNLCK, offset, whence, len);
}

例程

#include 
#include 

#include "filelock.h"

int main(void)
{
    int fd = 0;
    pid_t pid = 0;

    fd = open("tmp", O_RDWR|O_CREAT|O_TRUNC, 0666);
    unlink("tmp");

    pid = fork();

    if(0 == pid)
    {
        while(1)
        {
            File_WriteLckW(fd, 0, SEEK_SET, 1);
            printf("====> child process\n\n");
            usleep(5000000);
            File_Unlock(fd, 0, SEEK_SET, 1);
            usleep(500000);
        }
    }

    while(1)
    {
        File_WriteLckW(fd, 0, SEEK_SET, 1);
        printf("#### parent process\n\n");
        usleep(5000000);
        File_Unlock(fd, 0, SEEK_SET, 1);
        usleep(500000);
    }
    
    return 0;
}

文件锁的用法：

• 不同的对象访问同一文件，可以使用文件锁达到读写同步，不会导致文件写入混乱。
• 进程间访问同一资源，使用文件锁作为进程间的锁达到访问同步；用法：创建一个空文件，并使用文件的第一字节（无需存在）作为锁字节，然后unlink文件；获取资源之前，先获取该字节的写锁；释放资源时，对该字节解锁。记录锁的一个优点是：持有锁的进程终止退出时，内核会自动释放该锁，所以进程间就不会因为进程崩溃导致死锁。