文件I/O口编程 -- fcntl使用实例

//lock_set.c

#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>


void lock_set(int fd, int type)
{
    struct flock lock;
    lock.l_whence = SEEK_SET;
    lock.l_start = 0;
    lock.l_len = 0;
    while(1) {
        lock.l_type = type;
        if( (fcntl(fd, F_SETLK, &lock))==0) {
            if(lock.l_type == F_RDLCK) {
                printf("read lock set by %d \n", getpid());
            } else if (lock.l_type == F_WRLCK) {
                printf("write lock set by %d \n", getpid());
            } else if (lock.l_type == F_UNLCK) {
                printf("release lock set by %d \n", getpid());
            }
        return ;
        }
        //判断文件是否可以上锁
        fcntl(fd, F_GETLK, &lock);
        //判断不能上锁的原因
        if(lock.l_type != F_UNLCK) {
            if(lock.l_type == F_RDLCK) {        //该文件已有写入锁
                printf("read lock already set by %d \n", lock.l_pid);    
            } else if(lock.l_type == F_WRLCK) {
                printf("write lock already set by %d \n", lock.l_pid);    
            }
        getchar();
        }
    }
}

//fcntl_write.c
//测试写入锁主函数部分
#include <sys/types.h>
#include <sys/file.h>
#include <sys/stat.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>

int main(void)
{
    int fd;
    fd = open("hello.c", O_CREAT | O_RDWR, 0666);
    if(fd<0) {
        perror("open:");
        exit(1);
    } else {
        printf("Open file: hello.c %d\n", fd);
    }
    
    //给文件上写入锁
    lock_set(fd, F_WRLCK);
    getchar();
    //给文件解锁
    lock_set(fd, F_UNLCK);
    getchar();
    
    
    if(close(fd)<0) {
        perror("close:");
        exit(1);
    } else {
        printf("Close hello.c \n");
    }
    exit(0);
}

最后在linux系统下编译：
　　gcc -o fcntl_write fcntl_write.c lock_set.c

编译后,执行：

　　./fcntl_write