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Chapter 1. Introduction


The client and server organization is used by most network-awared applications. Some complex applications also require asynchronous callback communication, where the server initiates a message to the client.

unp.h header

A Simple Daytime Client


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Create TCP socket

The socket function creates an Internet (AF_INET) stream (SOCK_STREAM) socket, which is a fancy name for a TCP socket. The function returns a small integer descriptor to identify the socket.

Specify server's IP address and port

The IP address (sin_addr) and port number (sin_port) fields in the Internet socket address structure (sockaddr_in) must be in specific formats:

  • htons (host to network short): converts the binary port number
  • inet_pton (presentation to numeric): convert the ASCII command-line argument (such as when we ran this example) into the proper format.

bzero is not an ANSI C function, but is used in this book instead of the ANSI C memset function, because bzero is easier to remember (with only two arguments) than memset (with three arguments).

Establish connection with server

  • connect

In the unp.h header, SA is defined to be struct sockaddr, a generic socket address structure.

Read and display server's reply

We must be careful when using TCP because it is a byte-stream protocol with no record boundaries. Since we cannot assume that the server's reply will be returned by a single read, we always need to code the read in a loop when reading from a TCP socket.

Terminate program

exit terminates the program. Unix always closes all open descriptors when a process terminates.

Protocol Independence

The above program is protocol-depdent on IPv4.

It is better to make a program protocol-independent by using the getaddrinfo function.

Error Handling: Wrapper Functions

We can shorten our programs by defining a wrapper function that performs the actual function call, tests the return value, and terminates on an error.

sockfd = Socket(AF_INET, SOCK_STREAM, 0);

With careful C coding, we could use macros instead of functions, providing a little run-time efficiency, but these wrapper functions are rarely the performance bottleneck of a program. This book uses these wrapper functions unless otherwise explicit error needs handling.

Unix errno Value

The value of errno is set by a function only if an error occurs. All of the positive error values are constants with all-uppercase names beginning with "E," and are normally defined in the <sys/errno.h> header. No error has a value of 0.

Storing errno in a global variable does not work with multiple threads that share all global variables.

A Simple Daytime Server


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Create a TCP socket

Identical to the client code.

Bind server's well-known port to socket

  • bind: the server's well-known port (13) is bound to the socket by calling bind
  • INADDR_ANY allows the server to accept a client connection on any interface

Convert socket to listening socket

  • listen: converts the socket into a listening socket, on which incoming connections from clients will be accepted by the kernel
  • listenfd in the code is called a listening descriptor

Accept client connection, send reply

  • accept
  • connfd in the code is called a connected descriptor for communication with the client. A new descriptor is returned by accept for each client that connects to our server.

This book uses this code style for infinite loop:

for ( ; ; ) {
    // . . .

snprintf function

  • snprintf instead of sprintf


  • fgets instead of gets
  • strncat or strlcat instead of strcat
  • strncpy or strlcpy instead of a strcpy

Terminate connection

close initiates the normal TCP connection termination sequence: a FIN is sent in each direction and each FIN is acknowledged by the other end.

The server implemented in the above server code is:

  • Protocol-dependent on IPv4
  • Handles only one client at a time. If multiple client connections arrive at about the same time, the kernel queues them, up to some limit, and returns them to accept one at a time.
  • Called an iterative server. A concurrent server handles multiple clients at the same time.

OSI Model

Some terms mentioned:

  • Raw socket: it is possible for an application to bypass the transport layer and use IPv4 or IPv6 directly
  • [XTI](X/Open Transport Interface)

Sockets provide the interface from the upper three layers of the OSI model into the transport layer:

  • The upper three layers handle all the details of the application. The lower four layers know little about the application, but handle all the communication details
  • The upper three layers form what is called a user process while the lower four layers are normally provided as part of the operating system (OS) kernel

BSD Networking History

Linux does not fit into the Berkeley-derived classification: Its networking code and sockets API were developed from scratch.

Unix Standards

Background on POSIX

  • POSIX: Portable Operating System Interface, developed by IEEE and adopted as standards by ISO and IEC (ISO/IEC)

Background on The Open Group

  • Single UNIX Specification

Internet Engineering Task Force (IETF)

64-Bit Architectures

  • ILP32: integers (I), long integers (L), and pointers (P) occupy 32 bits.
  • LP64:only long integers (L) and pointers (P) require 64 bits.

From a programming perspective, the LP64 model means we cannot assume that a pointer can be stored in an integer. We must also consider the effect of the LP64 model on existing APIs

On a 32-bit system, size_t is a 32-bit value, but on a 64-bit system, it must be a 64-bit value, to take advantage of the larger addressing model. This means a 64-bit system will probably contain a typedef of size_t to be an unsigned long.