如何鎖定文件以進行讀寫訪問O_RDWR

Question

我無法找到關於如何鎖定文件以進行讀寫的答案。

lock.l_type = F_WRLCK //for write. 
lock.l_type = F_RDLCK //for read 
lock.l_type = F_RDLCK|F_WRLCK //maybe for read/write???? 

下面的代碼是否正確？

fd=MyOpenWrite(name,O_RDWR); //for read/write 
//(open file for read/write, but lock for write) 
... 
fd=MyOpenRead(name,O_RDONLY); //open and lock for read 
... 
fd=MyOpenWrite(name,O_CREAT|O_WRONLY|O_TRUNC); //for write 
... 
fd=MyOpenWrite(name,O_WRONLY|O_APPEND); //for append 

int MyOpenRead(char *name,int flags) { 
    int fd; 
    struct flock lock; 
    fd = open(name,flags); 
    if (fd<0) return -1; 
    lock.l_type = F_RDLCK; 
    lock.l_start = 0; 
    lock.l_whence = SEEK_SET; 
    lock.l_len = 0; 
    fcntl(fd,F_SETLKW,&lock); 
    return fd; 
} 

int MyOpenWrite(char *name,int flags) { 
    int fd; 
    struct stat st; 
    fd = open(name,flags,S_IREAD|S_IWRITE|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH); 
    if (fd<0) return -1; 

    lock.l_type = F_WRLCK; 
    lock.l_start = 0; 
    lock.l_whence = SEEK_SET; 
    lock.l_len = 0; 
    fcntl(fd,F_SETLKW,&lock); 
    return fd; 
} 

Answer 1

寫鎖定還阻止讀者。它是一個獨佔鎖，所以只有所有者可以訪問鎖定的字節，並且沒有其他人可以通過讀或寫來訪問這些字節。

* fcntl（）鎖是諮詢鎖。所以打開文件的其他人可以自由地讀/寫它們，如果它們不合作，並且使用fcntl（）來獲取鎖。如果您需要強制鎖定，請參閱here