UNIX环境高级编程：记录上锁（fcntl函数）以及死锁检测

第二个程序是在在父进程中加写锁后，然后再子进程中测试能否继续加读写锁。

#include <stdio.h>  
#include <stdlib.h>  
#include <unistd.h>  
#include <fcntl.h>  
#include <sys/stat.h>  
#include <sys/types.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 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;  
}  
      
pid_t lock_test(int fd, short type, short whence, off_t start, off_t len)  
{  
    struct flock lock;  
    lock_init(&lock, type, whence, start, len);  
      
    if (fcntl(fd, F_GETLK, &lock) == -1)  
    {  
        return -1;  
    }  
      
    if(lock.l_type == F_UNLCK)  
        return 0;  
    return lock.l_pid;  
}  
      
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);  
      
    if (fork() == 0)  
    {  
        printf("child F_WRLCK return %dn",lock_test(fd, F_WRLCK, SEEK_SET, 0, 0));  
        printf("child F_RDLCK return %dn",lock_test(fd, F_RDLCK, SEEK_SET, 0, 0));  
        printf("child pid = %d and ppid = %dn",getpid(),getppid());  
      
        exit(0);  
    }  
      
    sleep(3);  
    unlock(fd);  
      
    return 0;  
}

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