我在尝试写入服务器上的文件时遇到了一些问题。不同的客户端可以执行读/写请求。我在写请求时使用fcntl（）来锁定文件。但是，当进程（客户端）获取锁并且另一个进程尝试获取锁时，将删除该文件的内容。知道为什么会这样吗？以下是我正在谈论的代码：
{
FILE *file = fopen("file.txt", "w+");
if (file == NULL)
printf("File cannot be opened");
fd = fileno(file);
printf("\nThis is the file descriptor : %d\n", fd);
if(file == NULL)
printf("File cannot be opened");
printf("\nLocking!!!!");
//initliazing the flock structure
if(memset(&lock, 0, sizeof(lock))== NULL) //setting 0 as a value
{
perror("Memset Error : Should return a pointer to memory area");
}
lock.l_type = F_WRLCK; //F_RDLCK, F_WRLCK, F_UNLCK
lock.l_whence = SEEK_SET; //SEEK_SET, SEEK_CUR, SEEK_END
lock.l_start = 0; //offset from l_whence
lock.l_len = 0; //length, 0 = to EOF
lock.l_pid = getpid(); //the processes's PID
//placing a write lock on the file
if((flags = fcntl(fd, F_SETLKW, &lock)) < 0)
{
perror("fnctl: ");
}
printf("\nFlag : %d \n", flags);
printf("\nLocked-------");
if(fwrite(buff + 1, 1, strlen(buff) - 1, file) == 0)
{
perror("Error or EOF reached while fwrite");
}
//lock_realease(&l);
printf("\nHit enter to unlock the file !");
getchar();
printf("\nFinished writing so we can unlock file !");
//Releasing lock
lock.l_type = F_UNLCK; //unlocks the region of the file
if((flags = fcntl(fd, F_SETLKW,&lock)) < 0)
{
perror("fnctl: ");
}
printf("\nFlag : %d \n", flags);
fclose(file); //some say it is better to close the file and let it implicitly be unlocked when it is actually closed.
//Otherwise the lock can be release while unbuffered data is still unwritten
printf("\nFile unlocked!");
}