file lock (record lock)

1. The file lock can lock a certain part of the file to be modified, and precisely control it to the byte

Set file lock through fcntl() function

　　fcntl(int fd,int cmd,.........);

　　Parameters: fd: file descriptor

　　　　    cmd:F_GETLK: Test whether the lock can be added (but it may not be able to be added, non-atomic operation. Generally not used)

　　　　　　　F_SETLK: lock the file, then return immediately with an error

　　　　　　　F_SETLKW: lock the file, block if it cannot be added

　　　　The third parameter is a structure of type strcuct flock such as struct folct  lock;

　　　　　

1  lock .l_type = F_WRLCK;         // Add a write lock    // F_RDLCK read lock, F_UNLCK release lock 
2  lock .l_whence=SEEK_SET;    // relative head offset      // SEEK_END SEEK_CUR    
3  lock .l_start = 0 ;   // relative Header offset is 0    
 4  // lock.l_start=-5;   // The pointer moves 5 bytes to the left to start 
5  
6  lock.l_len = 5 ;   // The number of bytes to lock
 7  // lock.l_len = 0;   // Everything from start is locked, including newly written ones. (locked throughout)

　　　　fctnl(fd,F_SETLKW,&lock);

　　2. Unlock

　　　　lock.l_type=F_UNLCK;

　　　　fcntl(fd,F_SETLKW,&lock);
　　Closing the file will release all locks the process has placed on the file.

　　Note the implicit release, such as:

　　　　newfd=dup (fd);

　　　　close( newfd )  //All locks added to the process will still be released
　　Reason: record locks are marked with process pid, not file descriptors. Once a close function is detected, five file locks corresponding to the process will be checked. and close.