文件锁

最新推荐文章于 2020-04-02 13:29:22 发布

wangst4321

最新推荐文章于 2020-04-02 13:29:22 发布

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分类专栏： linux编程 c语言学习

本文链接：https://blog.youkuaiyun.com/wangst4321/article/details/8729204

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本文介绍了一个使用C语言实现的文件锁机制示例代码，并探讨了读写锁的实现原理及应用。通过具体代码展示了如何获取文件的读锁和写锁，并讨论了在实际操作中可能遇到的问题，例如在已获得写锁的情况下其他程序仍能修改文件的现象。

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1. sample学习代码

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>

#define read_lock(fd, offset, whence, len) \
            lock_reg((fd), F_SETLK, F_RDLCK, (offset), (whence), (len))
#define readw_lock(fd, offset, whence, len) \
            lock_reg((fd), F_SETLKW, F_RDLCK, (offset), (whence), (len))
#define write_lock(fd, offset, whence, len) \
            lock_reg((fd), F_SETLK, F_WRLCK, (offset), (whence), (len))
#define writew_lock(fd, offset, whence, len) \
            lock_reg((fd), F_SETLKW, F_WRLCK, (offset), (whence), (len))
#define un_lock(fd, offset, whence, len) \
            lock_reg((fd), F_SETLK, F_UNLCK, (offset), (whence), (len))

#define is_read_lockable(fd, offset, whence, len) \
            (lock_test((fd), F_RDLCK, (offset), (whence), (len)) == 0)
#define is_write_lockable(fd, offset, whence, len) \
            (lock_test((fd), F_WRLCK, (offset), (whence), (len)) == 0)

pid_t
lock_test(int fd, 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;

    if (fcntl(fd, F_GETLK, &lock) < 0) {
        printf("fcntl error\n");
        exit(1);
    }

    if (lock.l_type == F_UNLCK)
        return (0);
    return (lock.l_pid);
}

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
main(int argc, char *argv[])
{
    int fd;
    int flag;
    int result;

    if (argc != 3) {
        printf("usage %s -[rw] <filename>\n", argv[0]);
        exit(1);
    }

    if (argv[1][0] == 'r')
        flag = O_RDONLY;
    else if (argv[1][0] == 'w')
        flag = O_WRONLY;
    else {
        printf("invalid flag: 'r' or 'w'\n");
        exit(1);
    }

    if ((fd = open(argv[2], flag)) < 0) {
        printf("cannot open %s, %s\n", argv[2], strerror(errno));
        exit(2);
    }

    printf("open successfully\n");
    // test if write lock work.

    if (flag == O_RDONLY) {
        printf("read lock, ");
        result = read_lock(fd, 0, SEEK_SET, 0);
    }
    else {
        printf("write lock, ");
        result = write_lock(fd, 0, SEEK_SET, 0);
    }

    if (result < 0) {
        printf("lock fail, %s\n", strerror(errno));
        exit(3);
    }
    printf("success!\n");
    while (1) {
        //# loop forever;
    }
    exit(0);
}

当对整个文件获得写入锁时，其他的程序（vim）还是可以对文件进行写入，并且保存成功。为什么？