使用羊羣，打開和關閉文件來實現許多讀卡器單寫入鎖

Question

我有一個項目，包括多個進程，可以讀取或寫入到一個單一的數據庫。我希望使用系統調用flock/open/close來實現由鎖文件同步的單寫/多讀者鎖定。

一旦發生鎖定失敗，任何重新嘗試再次鎖定鎖定的操作都將由請求鎖定的較高級別進行（與自旋鎖定不同）。

不幸的是，在測試這個模型時，它在解鎖之前未被鎖定的情況下失敗。 也許你可以幫我找到我幹了什麼錯在這裏做的事：

// 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

在這兩個lock_read和lock_write添加爲第一行：

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

在unlock_read，補充一點：

assert (read_descriptor != 0); 

並在unlock_write中加入：

assert (write_descriptor != 0); 

而變化這樣的代碼：

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; 
} 

執行相同的，這樣的描述符被關閉的任何時間，相應的全局設置爲零寫代碼。 （你真的應該使用-1來表示無效的文件描述符，因爲零是合法的。）

進行調試構建並運行它。當assert旅行時，你會有你的罪魁禍首。