与应用程序使用文件描述符（fd）访问文件一样，访问套接字也需要使用套接字描述符，其在unix系统中是用文件描述符实现的。
在AF_INET通信域中套接字SOCK_STREAM的默认协议是TCP，SOCK_DGRAM的默认协议是UDP。
不同处理器架构支持不同的字节序，小端（little-endian）freebsd linux on intel pentium，大端（big-endian）mac os on power pc solaris on sun sparc，有些处理器可以配置大端和小端，更加混乱。
TCP/IP 采用大端字节序。对于TCP/IP，地址是使用网络字节序来表示，所以应用程序有时候需要在处理器的字节序和网络字节序之间转换。
uint32_t htonl(uint32_t hostint32);
uint16_t htons(uint16_t hostint16);
uint32_t ntohl(uint32_t netint32);
uint16_t ntohsl(uint16_t netint16);
h 代表host
n 代表network
l 代表 long
s 代表 short (查看原文)
Although sbrk can expand or contract the memory of a process, most versions of malloc and free never decrease their memory size. The space that we free is available for a later allocation, but the freed space is not usually returned to the kernel; that space is kept in the malloc pool. (查看原文)
Pipes have two limitations.
1. Historically, they have been half duplex (i.e., data flows in only one direction). Some systems now provide full-duplex pipes, but for maximum portability, we should never assume that this is the case.
2. Pipes can be used only between processes that have a common ancestor. Normally, a pipe is created by a process, that process calls fork, and the pipe is used between the parent and the child. (查看原文)
In most job-control shells, this function is called after a fork to have the parent set the process group ID of the child, and to have the child set its own process group ID. One of these calls is redundant, but by doing both, we are guaranteed that the child is placed into its own process group before either process assumes that this has happened. If we didn't do this, we would have a race condition, since the child's process group membership would depend on which process executes first. (查看原文)
作者: W. Richard Stevens, Stephen A. Rago 副标题: Rough Cuts isbn: 0321638026 书名: Advanced Programming in the UNIX Environment, 3rd Edition 页数: 960 出版社: Addison-Wesley Professional 出版年: 2013-1-24 装帧: 平装
