使用羊群，打开和关闭文件来实现许多读卡器单写入锁

Question

我有一个项目，包括多个进程，可以读取或写入到一个单一的数据库。我希望使用系统调用flock/open/close来实现由锁文件同步的单写/多读者锁定。

一旦发生锁定失败，任何重新尝试再次锁定锁定的操作都将由请求锁定的较高级别进行（与自旋锁定不同）。

不幸的是，在测试这个模型时，它在解锁之前未被锁定的情况下失败。 也许你可以帮我找到我干了什么错在这里做的事：

// keep read and write file descriptors as global variables. 
// assuming no more than 1 thread can access db on each process. 

int write_descriptor=0; 
int read_descriptor=0; 

int lock_write() { 
    if((write_descriptor = open(LOCKFILE, O_RDWR|O_CREAT,0644))<0) { 
     return LOCK_FAIL; 
    } 

    if(flock(write_descriptor, LOCK_EX)<0) { 
     close(write_descriptor); 
     write_descriptor = 0; 
     return LOCK_FAIL; 
    } 
    return LOCK_SUCCESS; 
} 

int unlock_write() { 
    if(!write_descriptor) { 
     // sanity: try to unlock before lock. 
     return LOCK_FAIL; 
    } 

    if(flock(write_descriptor,LOCK_UN)<0) { 
     // doing nothing because even if unlock failed, we 
     // will close the fd anyway to release all locks. 
    } 
    close(write_descriptor); 
    write_descriptor = 0; 
    return LOCK_SUCCESS; 
} 


int lock_read() { 
    if((read_descriptor = open(LOCKFILE,O_RDONLY))<0) { 
     return LOCK_FAIL; 
    } 

    if(flock(read_descriptor, LOCK_SH)<0) { 
     close(read_descriptor); 
     return LOCK_FAIL; 
    } 
    return LOCK_SUCCESS; 
} 

int unlock_read() { 
    if(!read_descriptor) { 
     // sanity : try to unlock before locking first. 
     return LOCK_FAIL; 
    } 

    if(flock(read_descriptor, LOCK_UN)<0) { 
     // doing nothing because even if unlock failed, we 
     // will close the fd anyway to release all locks. 
    } 
    close(read_descriptor); 
    read_descriptor = 0; 
    return LOCK_SUCCESS; 
} 


int read_db() { 
    if(lock_read() != LOCK_SUCCESS) { 
     return DB_FAIL; 
    } 
    // read from db 
    if(unlock_read() != LOCK_SUCCESS) { 
     // close fd also unlock - so we can fail here (can i assume that ?) 
    } 
} 

int write_db() { 
    if(lock_write() != LOCK_SUCCESS) { 
     return DB_FAIL; 
    } 
    //write to db. 
    if(unlock_write() != LOCK_SUCCESS) { 
     // close fd also unlock - so we can fail here (can i assume that ?) 
    } 
} 

Answer 1

在这两个lock_read和lock_write添加为第一行：

assert ((read_descriptor == 0) && (write_descriptor == 0)); 

在unlock_read，补充一点：

assert (read_descriptor != 0); 

并在unlock_write中加入：

assert (write_descriptor != 0); 

而变化这样的代码：

if(flock(read_descriptor, LOCK_SH)<0) { 
    close(read_descriptor); 
    return LOCK_FAIL; 
} 

到：

if(flock(read_descriptor, LOCK_SH)<0) { 
    close(read_descriptor); 
    read_descriptor = 0; 
    return LOCK_FAIL; 
} 

执行相同的，这样的描述符被关闭的任何时间，相应的全局设置为零写代码。 （你真的应该使用-1来表示无效的文件描述符，因为零是合法的。）

进行调试构建并运行它。当assert旅行时，你会有你的罪魁祸首。