ftell 函数， 运用实例：ftell(fp) >= size
long ftell(FILE *stream);
The ftell() function obtains the current value of the file position
indicator for the stream pointed to by stream.
unlink 函数： 应用实例：unlink(bakfile)
unlink - call the unlink function to remove the specified file
unlink FILE
unlink OPTION --------这里是断开备份文件
原文：http://www.cnblogs.com/the-tops/p/5583236.html