Comtrol eCos - Manual (Page 633)

Chapter 48. Embedded HTTP Server
Intrduction
The eCos HTTPD package provides a simple HTTP server for use with applications in eCos. This server is specif-
ically aimed at the remote control and monitoring requirements of embedded applications. For this reason the
emphasis is on dynamically generated content, simple forms handling and a basic CGI interface. It is not intended
to be a general purpose server for delivering arbitrary web content. For these purposes a port of the GoAhead web
server is available from www.goahead.com.
Server Organization
The server consists of one or more threads running in parallel to any application threads and which serve web pages
to clients. Apart from defining content, the application does not need to do anything to start the HTTP server.
The HTTP server is started by a static constructor. This simply creates an initial thread and sets it running. Since this
is called before the scheduler is started, nothing will happen until the application calls
cyg_scheduler_start() .
When the thread gets to run it first optionally delays for some period of time. This is to allow the application to
perform any initialization free of any interference from the HTTP server. When the thread does finally run it creates
a socket, binds it to the HTTP server port, and puts it into listen mode. It will then create any additional HTTPD
server threads that have been configured before becoming a server thread itself.
Each HTTPD server thread simply waits for a connection to be made to the server port. When the connection is
made it reads the HTTP request and extracts the filename being accessed. If the request also contains form data,
this is also preserved. The filename is then looked up in a table.
Each table entry contains a filename pattern string, a pointer to a handler function, and a user defined argument for
the function. Table entries are defined using the same link-time table building mechanism used to generate device
tables. This is all handled by the
CYG_HTTPD_TABLE_ENTRY() macro which has the following format:
#include < cyg/httpd/httpd.h >
CYG_HTTPD_TABLE_ENTRY( __name, __pattern, __handler, __arg )
The __name argument is a variable name for the table entry since C does not allow us to define anonymous data
structures. This name should be chosen so that it is unique and does not pollute the name space. The __pattern
argument is the match pattern. The __handler argument is a pointer to the handler function and __arg the user
defined value.
The link-time table building means that several different pieces of code can define server table entries, and so long
as the patterns do not clash they can be totally oblivious of each other. However, note also that this mechanism does
not guarantee the order in which entries appear, this depends on the order of object files in the link, which could
vary from one build to the next. So any tricky pattern matching that relies on this may not always work.
A request filename matches an entry in the table if either it exactly matches the pattern string, or if the pattern
ends in an asterisk, and it matches everything up to that point. So for example the pattern "/monitor/threads.html"
will only match that exact filename, but the pattern "/monitor/thread-*" will match "/monitor/thread-0040.html",
"/monitor/thread-0100.html" and any other filename starting with "/monitor/thread-".
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