Dr. Dobb's is part of the Informa Tech Division of Informa PLC

This site is operated by a business or businesses owned by Informa PLC and all copyright resides with them. Informa PLC's registered office is 5 Howick Place, London SW1P 1WG. Registered in England and Wales. Number 8860726.


Channels ▼
RSS

C/C++

APR Networking & the Reactor Pattern


Socket Programming Basics

At a high level, a socket represents one endpoint of a communications channel. Network-based sockets transmit data across the wire, perhaps between two machines. Some operating systems support socket operations via the filesystem using a special file type. Streams-oriented, TCP socket communications involve steady connections between the two socket endpoints. Datagram-oriented, UDP connections pass data in minute packets, the order and delivery of which may be unreliable.

Typically, one socket represents the server side and the other is the client side. (I prefer the term "service" to "server," since the latter also refers to the machine on which the service runs.) The server listens for a client connection, exchanges some data with the client, and the two disconnect.

In code, the notion of a socket differs by the implementation. Java and ACE wrap it in an object. Traditional UNIX sockets are a combination of a file descriptor, represented by a numeric primitive, and a data structure that contains network information, such as IP addresses.

Merely creating a socket variable doesn't do anything. You have to tell it to wait for a connection (server-side) or initiate a connection to something (client-side). To wait for a connection, you first bind the socket to an address and port. Only one entity may listen on a given address:port pair at a time; binding is a way an entity stakes its claim. To describe networking in restaurant terms, for example, binding is the equivalent of renting out a space—nothing's there yet, but soon will be.

A bound socket is simply held so no one else can use it. To be available for connections, you must then listen on the socket. This means opening up your restaurant's doors for diners.

Finally, you accept client connections to perform a data exchange. Accepting clients means your host or hostess is waiting by the restaurant's door to admit and seat clients. For a blocking accept call, your host does nothing else until a client arrives. In a nonblocking accept call, the host periodically checks for clients and tends to other matters.


Related Reading


More Insights






Currently we allow the following HTML tags in comments:

Single tags

These tags can be used alone and don't need an ending tag.

<br> Defines a single line break

<hr> Defines a horizontal line

Matching tags

These require an ending tag - e.g. <i>italic text</i>

<a> Defines an anchor

<b> Defines bold text

<big> Defines big text

<blockquote> Defines a long quotation

<caption> Defines a table caption

<cite> Defines a citation

<code> Defines computer code text

<em> Defines emphasized text

<fieldset> Defines a border around elements in a form

<h1> This is heading 1

<h2> This is heading 2

<h3> This is heading 3

<h4> This is heading 4

<h5> This is heading 5

<h6> This is heading 6

<i> Defines italic text

<p> Defines a paragraph

<pre> Defines preformatted text

<q> Defines a short quotation

<samp> Defines sample computer code text

<small> Defines small text

<span> Defines a section in a document

<s> Defines strikethrough text

<strike> Defines strikethrough text

<strong> Defines strong text

<sub> Defines subscripted text

<sup> Defines superscripted text

<u> Defines underlined text

Dr. Dobb's encourages readers to engage in spirited, healthy debate, including taking us to task. However, Dr. Dobb's moderates all comments posted to our site, and reserves the right to modify or remove any content that it determines to be derogatory, offensive, inflammatory, vulgar, irrelevant/off-topic, racist or obvious marketing or spam. Dr. Dobb's further reserves the right to disable the profile of any commenter participating in said activities.

 
Disqus Tips To upload an avatar photo, first complete your Disqus profile. | View the list of supported HTML tags you can use to style comments. | Please read our commenting policy.