If the size of the mapped file is decreased after mmap(),attempts to reference beyond the end of the file are undefined@H_502_12@ and may result in an MCH0601 exception.
If the size of the file increases after the mmap() function completes,then the whole pages beyond the original end of file will not be accessible via the mapping.
在SingleUnixSpecification中也是如此：http://pubs.opengroup.org/onlinepubs/7908799/xsh/mmap.html
If the size of the mapped file changes after the call to mmap() as a result of some other operation on the mapped file,the effect of references to portions of the mapped region that correspond to added or removed portions of the file is unspecified@H_502_12@.
‘undefined’或’unspecified’表示 – 允许操作系统开始格式化磁盘或任何东西.最可能的是SIGSEGV – 杀死你的应用程序.
