说明：   UNIX提供了程序共享内存的机制，Linux内核从2.0开始也加入了这一功能。mmap(memory map)函数用于建立一段内存，使之能够被多个程序读写。它创建一个指向一段内存区域的指针并将其与特定的文件内容关联。
(UNIX provides a mechanism for sharing memory, Linux kernel 2.0 from the beginning of this feature is also added. mmap (memory map) for the establishment of a section of memory function, so that multiple programs can be read and write. Point to a section of it to create a pointer memory area and the content of the document with specific relevance.)