操作系统学习之进程通信（PCI）
mmap将一个文件或者其它对象映射进内存。文件被映射到多个页上，如果文件的大小不是所有页的大小之和，最后一个页不被使用的空间将会清零。
http://kenby.iteye.com/blog/1164700
4、Pips
管道两端可分别用描述字fd[0]以及fd[1]来描述，需要注意的是，管道的两端是固定了任务的。即一端只能用于读，由描述字fd[0]表示，称其为管道读端；另一端则只能用于写，由描述字fd[1]来表示，称其为管道写端。如果试图从管道写端读取数据，或者向管道读端写入数据都将导致错误发生。一般文件的I/O函数都可以用于管道，如close、read、write等等。
从管道中读取数据：
- 如果管道的写端不存在，则认为已经读到了数据的末尾，读函数返回的读出字节数为0；
5、Socket
socket的英文原义是“孔”或“插座”。作为BSD UNIX的进程通信机制，取后一种意思。通常也称作"套接字"，用于描述IP地址和端口，是一个通信链的句柄。在Internet上的主机一般运行了多个服务软件，同时提供几种服务。每种服务都打开一个Socket，并绑定到一个端口上，不同的端口对应于不同的服务。Socket正如其英文原意那样，像一个多孔插座。一台主机犹如布满各种插座的房间，每个插座有一个编号，有的插座提供220伏交流电， 有的提供110伏交流电，有的则提供有线电视节目。 客户软件将插头插到不同编号的插座，就可以得到不同的服务。
The domain parameter specifies a communication domain; this selects the protocol family which will be used for communication. These families are defined in <sys/socket.h>. The currently understood formats include:
| Name | Purpose | Man page |
| PF_UNIX,PF_LOCAL | Local communication | unix(7) |
| PF_INET | IPv4 Internet protocols | ip(7) |
| PF_INET6 | IPv6 Internet protocols | |
| PF_IPX | IPX - Novell protocols | |
| PF_NETLINK | Kernel user interface device | netlink(7) |
| PF_X25 | ITU-T X.25 / ISO-8208 protocol | x25(7) |
| PF_AX25 | Amateur radio AX.25 protocol | |
| PF_ATMPVC | Access to raw ATM PVCs | |
| PF_APPLETALK | Appletalk | ddp(7) |
| PF_PACKET | Low level packet interface | packet(7) |
The socket has the indicated type, which specifies the communication semantics. Currently defined types are:
- SOCK_STREAM
- Provides sequenced, reliable, two-way, connection-based byte streams. An out-of-band data transmission mechanism may be supported.
- SOCK_DGRAM
- Supports datagrams (connectionless, unreliable messages of a fixed maximum length).
- SOCK_SEQPACKET
- Provides a sequenced, reliable, two-way connection-based data transmission path for datagrams of fixed maximum length; a consumer is required to read an entire packet with each read system call.
- SOCK_RAW
- Provides raw network protocol access.
- SOCK_RDM
- Provides a reliable datagram layer that does not guarantee ordering.
- SOCK_PACKET
- Obsolete and should not be used in new programs; see packet(7).
Some socket types may not be implemented by all protocol families; for example,SOCK_SEQPACKET is not implemented for AF_INET.