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>Server Organization</TITLE
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><H1
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><A
NAME="NET-HTTPD-ORGANIZATION">Server Organization</H1
><P
>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.</P
><P
>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 <TT
CLASS="FUNCTION"
>cyg_scheduler_start()</TT
>.</P
><P
>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.</P
><P
>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.</P
><P
>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 <TT
CLASS="LITERAL"
>CYG_HTTPD_TABLE_ENTRY()</TT
> macro which has the
following format:</P
><TABLE
BORDER="5"
BGCOLOR="#E0E0F0"
WIDTH="70%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
> #include <cyg/httpd/httpd.h>
CYG_HTTPD_TABLE_ENTRY( __name, __pattern, __handler, __arg ) </PRE
></TD
></TR
></TABLE
><P
>The <TT
CLASS="PARAMETER"
><I
>__name</I
></TT
> 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 <TT
CLASS="PARAMETER"
><I
>__pattern</I
></TT
>
argument is the match pattern. The <TT
CLASS="PARAMETER"
><I
>__handler</I
></TT
>
argument is a pointer to the handler function and
<TT
CLASS="PARAMETER"
><I
>__arg</I
></TT
> the user defined value.</P
><P
>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.</P
><P
>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-".</P
><P
>When a pattern is matched, the hander function is called. It has the
following prototype:</P
><TABLE
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>cyg_bool cyg_httpd_handler(FILE *client,
char *filename,
char *formdata,
void *arg);</PRE
></TD
></TR
></TABLE
><P
>The <TT
CLASS="PARAMETER"
><I
>client</I
></TT
> argument is the TCP connection to
the client: anything output through this stream will be returned to
the browser. The <TT
CLASS="PARAMETER"
><I
>filename</I
></TT
> argument is the
filename from the HTTP request and the <TT
CLASS="PARAMETER"
><I
>formdata</I
></TT
>
argument is any form response data, or NULL if none was sent. The
<TT
CLASS="PARAMETER"
><I
>arg</I
></TT
> argument is the user defined value from the
table entry.</P
><P
>The handler is entirely responsible for generating the response to the
client, both HTTP header and content. If the handler decides that it
does not want to generate a response it can return
<TT
CLASS="LITERAL"
>false</TT
>, in which case the table scan is resumed for
another match. If no match is found, or no handler returns true, then
a default response page is generated indicating that the requested
page cannot be found.</P
><P
>Finally, the server thread closes the connection to the client and
loops back to accept a new connection.</P
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