UNIX环境高级编程：记录上锁（fcntl函数）以及死锁检测

运行结果：

huangcheng@ubuntu:~$ ./a.out
child 2 try to get read lock...  
child 1 try to get write lock...  
child 2 get read lock...  
child 2 release read lock...  
child 1 get write lock...  
child 1 release write lock...

将上面代码该成child2 sleep 3s，child1不sleep：

#include <stdio.h>  
#include <stdlib.h>  
#include <unistd.h>  
#include <fcntl.h>  
#include <sys/stat.h>  
      
#define FILE_PATH "/home/huangcheng/data.txt"  
#define FILE_MODE 0777  
      
void lock_init(struct flock *lock, short type, short whence, off_t start, off_t len)  
{  
    if (lock == NULL)  
        return;  
      
    lock->l_type = type;  
    lock->l_whence = whence;  
    lock->l_start = start;  
    lock->l_len = len;  
}  
      
int readw_lock(int fd)  
{  
    if (fd < 0)  
    {  
        return -1;  
    }  
      
    struct flock lock;  
    lock_init(&lock, F_RDLCK, SEEK_SET, 0, 0);  
      
    if (fcntl(fd, F_SETLKW, &lock) != 0)  
    {  
        return -1;  
    }  
          
    return 0;  
}  
      
int writew_lock(int fd)  
{  
    if (fd < 0)  
    {  
        return -1;  
    }  
      
    struct flock lock;  
    lock_init(&lock, F_WRLCK, SEEK_SET, 0, 0);  
      
    if (fcntl(fd, F_SETLKW, &lock) != 0)  
    {  
        return -1;  
    }  
      
    return 0;  
}  
      
int unlock(int fd)  
{  
    if (fd < 0)  
    {  
        return -1;  
    }  
      
    struct flock lock;  
    lock_init(&lock, F_UNLCK, SEEK_SET, 0, 0);  
      
    if (fcntl(fd, F_SETLKW, &lock) != 0)  
    {  
        return -1;  
    }  
      
    return 0;  
}  
      
int main()  
{   
    int fd = open(FILE_PATH, O_RDWR | O_CREAT, FILE_MODE);  
    writew_lock(fd);  
      
    //child  1  
    if (fork() == 0)  
    {  
        printf("child 1 try to get write lock...n");  
        writew_lock(fd);  
        printf("child 1 get write lock...n");  
      
        unlock(fd);  
        printf("child 1 release write lock...n");  
      
        exit(0);  
    }  
      
    //child 2  
    if (fork() == 0)  
    {  
        sleep(3);  
        printf("child 2 try to get read lock...n");  
        readw_lock(fd);  
        printf("child 2 get read lock...n");  
      
        unlock(fd);  
        printf("child 2 release read lock...n");  
      
        exit(0);  
    }  
      
    sleep(10);  
    unlock(fd);  
      
    return 0;  
}

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