Linux系统编程7:读写锁

1. 接口

int fcntl(int filedes, int cmd,...)

1.1 锁操作

int fcntl(int fd, int cmd, struct flock *lock)
  • 参数
No. 参数 含义
1 fd 文件描述符
2 cmd 命令。F_GETLK:获取锁;F_SETLK:设置锁;F_SETLKW:设置阻塞锁。
3 struct flock 锁信息
  • 锁信息
No. 参数 含义
1 l_type F_RDLCK:读取锁(共享锁);F_WRLCK:写入锁(排斥锁);F_UNLCK:解锁
2 l_whence SEEK_SET:以文件开头为锁定的起始位置;SEEK_CUR:以目前文件读写位置为锁定的起始位置;SEEK_END:以文件结尾为锁定的起始位置
3 l_start 相对l_whence位置的偏移量
4 l_len 锁定区域的长度。0表示到整个文件的结束
5 l_pid 当前占用锁的PID,只对F_GETLK命令有效
  • 返回值
No. 返回值 含义
1 -1 失败
2 0 SET相关命令成功
  • 给指定文件添加读锁
#include 
#include 
 
int main(int argc, char* argv[]){
    if(2!=argc){
        printf("usage:%s \n",argv[0]);
        return 1;
    }
    int fd = open(argv[1],O_RDONLY);
    if(-1 == fd){
        perror("open error");
        return 1;
    }
    struct flock lock;
    lock.l_type = F_RDLCK;
    lock.l_whence = SEEK_SET;
    lock.l_start = 0;
    lock.l_len = 0; 
    if(-1 == fcntl(fd,F_SETLK,&lock)){
        perror("fcntl error");
        return 1;
    }
    pause();
    close(fd);
}
  • 查看当前锁的状态
#include 
#include 
#include 
 
int main(int argc, char* argv[]){
    if(2!=argc){
        printf("usage:%s \n",argv[0]);
        return 1;
    }
    int fd = open(argv[1],O_RDWR);
    if(-1 == fd){
        perror("open error");
        return 1;
    }
    struct flock lock;
    bzero(&lock,sizeof(lock));
    if(-1 == fcntl(fd,F_GETLK,&lock)){
        perror("fcntl error");
        return 1;
    }
    printf("file:%s,lock type:%d,start:%d,len:%d,by %d\n",argv[1],lock.l_type,lock.l_start,lock.l_len,lock.l_pid);
 
}
  • 给指定文件添加写锁
#include 
#include 
#include 
#include 
#include 
int fd;
 
void handler(int sig){
    struct flock lock;
    lock.l_type = F_UNLCK;
    lock.l_whence = SEEK_SET;
    lock.l_start = 0;
    lock.l_len = 0; 
    if(-1 == fcntl(fd,F_SETLKW,&lock)){
        perror("fcntl error");
        exit(1);
    }
}
 
int main(int argc, char* argv[]){
    signal(SIGUSR1,handler);
    int c,start = 0,len = 0;
    while((c = getopt(argc,argv,"s:l:"))!=-1){
        switch(c){
        case 's':
            start = atoi(optarg);
            break;
        case 'l':
            len = atoi(optarg);
            break;
        }
    }
 
    if(optind != argc -1){
        printf("usage:%s [-s ] [-l ] \n",argv[0]);
        return 1;
    }
    fd = open(argv[optind],O_WRONLY);
    if(-1 == fd){
        perror("open error");
        return 1;
    }
    struct flock lock;
    lock.l_type = F_WRLCK;
    lock.l_whence = SEEK_SET;
    lock.l_start = start;
    lock.l_len = len; 
    if(-1 == fcntl(fd,F_SETLKW,&lock)){
        perror("fcntl error");
        return 1;
    }
    pause();
    for(;;);
    close(fd);
}
  • 给指定文件解锁
// 本程序不能解锁,因为只有加锁进程才能执行解锁操作
#include 
#include 
 
int main(int argc, char* argv[]){
    if(2!=argc){
        printf("usage:%s \n",argv[0]);
        return 1;
    }
    int fd = open(argv[1],O_WRONLY);
    if(-1 == fd){
        perror("open error");
        return 1;
    }
    struct flock lock;
    lock.l_type = F_UNLCK;
    lock.l_whence = SEEK_SET;
    lock.l_start = 0;
    lock.l_len = 0; 
    if(-1 == fcntl(fd,F_SETLKW,&lock)){
        perror("fcntl error");
        return 1;
    }
    pause();
    close(fd);
}

1.2 死锁

交通堵塞
死锁
#include 
#include 
#include 
#include 
void lock(const char* pathname){
 
    int fd = open(pathname,O_WRONLY);
    if(-1 == fd){
        perror("open error");
        exit(1);
    }
    struct flock lock;
    lock.l_type = F_WRLCK;
    lock.l_whence = SEEK_SET;
    lock.l_start = 0;
    lock.l_len = 0; 
    if(-1 == fcntl(fd,F_SETLKW,&lock)){
        perror("fcntl error");
        exit(1);
    }
    //close(fd);
}
int main(int argc, char* argv[]){
    if(3!=argc){
        printf("usage:%s  \n",argv[0]);
        return 1;
    }
    printf("PID:%d lock file %s\n",getpid(),argv[1]);
    lock(argv[1]);
    printf("sleep 1s\n");
    sleep(1);
    printf("PID:%d lock file %s\n",getpid(),argv[2]);
    lock(argv[2]);
    pause();
}

1.3 记录锁

  • 作用:文件读与写的同步
  • 本质:当一个进程正在读或修改文件的某个部分时,阻止其他进程修改同一文件区。

1.2.1 分类

  • 访问操作
    写入锁(排他锁)/读取锁(共享锁)
  • 加锁区域
    文件锁(整个文件上锁)/记录锁(文件部分内容上锁)

1.2.2 操作

1.2.2.1 获取
fcntl(fd,F_GETLK)

1.2.2.2 设置

fcntl(fd,F_SETLK,arg)
fcntl(fd,F_SETLKW,arg)

辅助函数

  • lseek_append.cpp
#include 
#include 
#include 
#include 
#include 
 
int main(int argc,char* argv[]){
    if(4 != argc){
        printf("usage:%s   \n",argv[0]);
        return 1;
    }
    int fd = open(argv[1],O_RDWR);
    if(-1 == fd){
        return 1;
    }
    off_t off = lseek(fd,atoi(argv[2]),SEEK_SET);
    if(-1 == off){
        return 1;
    }
    struct stat stat_buf;
    fstat(fd,&stat_buf);
    size_t save_size = stat_buf.st_size - off;
    //char buf[save_size];
    char *buf = malloc(save_size);
    if(-1 == read(fd,buf,save_size)){
        return 1;
    }
     
    off = lseek(fd,atoi(argv[2]),SEEK_SET);
    if(-1 == off){
        return 1;
    }
    if(-1 == write(fd,argv[3],strlen(argv[3]))){
        return 1;
    }
    if(-1 == write(fd,buf,save_size)){
        return 1;
    }
 
    free(buf);
    buf = NULL;
}
  • lseek_read.cpp
#include 
#include 
#include 
#include 
#include 
 
int main(int argc,char* argv[]){
    if(4 != argc){
        printf("usage:%s   \n",argv[0]);
        return 1;
    }
    int fd = open(argv[1],O_RDONLY);
    if(-1 == fd){
        return 1;
    }
    off_t off = lseek(fd,atoi(argv[2]),SEEK_SET);
    if(-1 == off){
        return 1;
    }
    size_t len = atoi(argv[3]);
    char buf[len+1];
    bzero(buf,len+1);
    if(-1 == read(fd,buf,len)){
        return 1;
    }
    printf("%s\n",buf);
}
  • lseek_write.cpp
#include 
#include 
#include 
#include 
#include 
 
int main(int argc,char* argv[]){
    if(4 != argc){
        printf("usage:%s   \n",argv[0]);
        return 1;
    }
    int fd = open(argv[1],O_WRONLY);
    if(-1 == fd){
        return 1;
    }
    off_t off = lseek(fd,atoi(argv[2]),SEEK_SET);
    if(-1 == off){
        return 1;
    }
    if(-1 == write(fd,argv[3],strlen(argv[3]))){
        return 1;
    }
}

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