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The eCos HTTPD package provides a simple HTTP

server for use with applications in eCos. This server is specifically

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 

url="www.goahead.com">www.goahead.com.

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-".

When a pattern is matched, the hander function is called. It has the

following prototype:

cyg_bool cyg_httpd_handler(FILE *client,

                           char *filename,

                           char *formdata,

                           void *arg);

The client argument is the TCP connection to

the client: anything output through this stream will be returned to

the browser. The filename argument is the

filename from the HTTP request and the formdata

argument is any form response data, or NULL if none was sent. The

arg argument is the user defined value from the

table entry.

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

false, 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.

Finally, the server thread closes the connection to the client and

loops back to accept a new connection.

The HTTP server has a number of configuration options:

This option defines the TCP port that the server will listen on. It

defaults to the standard HTTP port number 80. It may be changed to a

different number if, for example, another HTTP server is using the

main HTTP port.

This is the string that is reported to the client in the

"Server:" field of the HTTP header.

The HTTP server can be configured to use more than one thread to

service HTTP requests. If you expect to serve complex pages with many

images or other components that are fetched separately, or if any

pages may take a long time to send, then it may be useful to increase

the number of server threads. For most uses, however, the connection

queuing in the TCP/IP stack and the speed with which each page is

generated, means that a single thread is usually adequate.

The HTTP server threads can be run at any priority. The exact priority

depends on the importance of the server relative to the rest of the

system. The default is to put them in the middle of the priority range

to provide reasonable response without impacting genuine high priority

threads.

This is the amount of stack to be allocated for each of the HTTPD

threads. The actual stack size allocated will be this value plus the

values of CYGNUM_HAL_STACK_SIZE_MINIMUM and

CYGNUM_HTTPD_SERVER_BUFFER_SIZE.

This defines the size of the buffer used to receive the first line of

each HTTP request. If you expect to use particularly long URLs or have

very complex forms, this should be increased.

This defines the number of system clock ticks that the HTTP server

will wait before initializing itself and spawning any extra server

threads. This is to give the application a chance to initialize

properly without any interference from the HTTPD.

The emphasis of this server is on dynamically generated content,

rather than fetching it from a filesystem. To do this the handler

functions make calls to fprintf() and

fputs(). Such handler functions would end up a

mass of print calls, with the actual structure of the HTML page hidden

in the format strings and arguments, making maintenance and debugging

very difficult. Such an approach would also result in the definition

of many, often only slightly different, format strings, leading to

unnecessary bloat.

In an effort to expose the structure of the HTML in the structure of

the C code, and to maximize the sharing of string constants, the

cyg/httpd/httpd.h header file defines a set of

helper functions and macros. Most of these are wrappers for predefined

print calls on the client stream passed to the

hander function. For examples of their use, see the System Monitor

example.

All arguments to macros are pointers to strings, unless otherwise

stated. In general, wherever a function or macro has an

attr or __attr

parameter, then the contents of this string will be inserted into the

tag being defined as HTML attributes. If it is a NULL or empty string

it will be ignored.

void cyg_http_start( FILE *client, char *content_type, int content_length );

void cyg_http_finish( FILE *client );

#define html_begin(__client)

#define html_end( __client )

The function cyg_http_start() generates a simple

HTTP response header containing the value of

CYGDAT_HTTPD_SERVER_ID in the "Server" field, and the

values of content_type and

content_length in the "Content-type"

and "Content-length" field respectively. The function

cyg_http_finish() just adds an extra newline to

the end of the output and then flushes it to force the data out to the

client.

The macro html_begin() generates an HTTP header

with a "text/html" content type followed by an opening

"<html>" tag. html_end() generates

a closing "</html>" tag and calls

cyg_http_finish().

void cyg_html_tag_begin( FILE *client, char *tag, char *attr );

void cyg_html_tag_end( FILE *client, char *tag );

#define html_tag_begin( __client, __tag, __attr )

#define html_tag_end( __client, __tag )

#define html_head( __client, __title, __meta )

#define html_body_begin( __client, __attr )

#define html_body_end( __client )

#define html_heading( __client, __level, __heading )

#define html_para_begin( __client, __attr )

#define html_url( __client, __text, __link )

#define html_image( __client, __source, __alt, __attr )

The function cyg_html_tag_begin() generates an

opening tag with the given name. The function

cyg_html_tag_end() generates a closing tag with

the given name. The macros html_tag_begin() and

html_tag_end are just wrappers for these functions.

The macro html_head() generates an HTML header

section with __title as the title. The

__meta argument defines any meta tags that will

be inserted into the header. html_body_begin() and

html_body_end generate HTML body begin and end

tags.

html_heading() generates a complete HTML header

where __level is a numerical level, between 1

and 6, and __heading is the heading

text. html_para_begin() generates a paragraph

break.

html_url() inserts a URL where

__text is the displayed text and

__link is the URL of the linked

page. html_image() inserts an image tag where

__source is the URL of the image to be

included and __alt is the alternative text for

when the image is not displayed.

#define html_table_begin( __client, __attr )

#define html_table_end( __client )

#define html_table_header( __client, __content, __attr )

#define html_table_row_begin( __client, __attr )

#define html_table_row_end( __client )

#define html_table_data_begin( __client, __attr )

#define html_table_data_end( __client )

html_table_begin() starts a table and

html_table_end() end

it. html_table_header() generates a simple table

column header containg the string __content.

html_table_row_begin() and

html_table_row_end() begin and end a table row,

and similarly html_table_data_begin() and

html_table_data_end() begin and end a table

entry.

#define html_form_begin( __client, __url, __attr )

#define html_form_end( __client )

#define html_form_input( __client, __type, __name, __value, __attr )

#define html_form_input_radio( __client, __name, __value, __checked )

#define html_form_input_checkbox( __client, __name, __value, __checked )

#define html_form_input_hidden( __client, __name, __value )

#define html_form_select_begin( __client, __name, __attr )

#define html_form_option( __client, __value, __label, __selected )

#define html_form_select_end( __client )

void cyg_formdata_parse( char *data, char *list[], int size );

char *cyg_formlist_find( char *list[], char *name );

html_form_begin() begins a form, the

__url argument is the value for the

action

attribute. html_form_end() ends the form.

html_form_input() defines a general form input

element with the given type, name and

value. html_form_input_radio creates a radio button

with the given name and value; the __checked

argument is a boolean expression that is used to determine whether the

checked attribute is added to the tag. Similarly

html_form_input_checkbox() defines a checkbox

element. html_form_input_hidden() defines a hidden

form element with the given name and value.

html_form_select_begin() begins a multiple choice

menu with the given name. html_form_select_end()

end it. html_form_option() defines a menu entry

with the given value and label; the __selected

argument is a boolean expression controlling whether the

selected attribute is added to the tag.

cyg_formdata_parse() converts a form response

string into an NULL-terminated array of

"name=value" entries. The data

argument is the string as passed to the handler function; note that

this string is not copied and will be updated in place to form the

list entries.  list is a pointer to an array of

character pointers, and is size elements long.

cyg_formlist_find() searches a list generated by

cyg_formdata_parse() and returns a pointer to the

value part of the string whose name part matches

name; if there is no match it will return

NULL.

int cyg_httpd_send_html( FILE *client, char *filename, char *request, void *arg );

typedef struct

{

    char        *content_type;

    cyg_uint32  content_length;

    cyg_uint8   *data;

} cyg_httpd_data;

#define CYG_HTTPD_DATA( __name, __type, __length, __data )

int cyg_httpd_send_data( FILE *client, char *filename, char *request, void *arg );

The HTTP server defines a couple of predefined handers to make it

easier to deliver simple, static content.

cyg_httpd_send_html() takes a

NULL-terminated string as the argument and sends it

to the client with an HTTP header indicating that it is HTML. The

following is an example of its use:

char cyg_html_message[] = "<head><title>Welcome</title></head>\n"

                          "<body><h2>Welcome to my Web Page</h2></body>\n"

CYG_HTTPD_TABLE_ENTRY( cyg_html_message_entry,

                       "/message.html",

                       cyg_httpd_send_html,

                       cyg_html_message );

cyg_httpd_send_data() Sends arbitrary data to the

client. The argument is a pointer to a cyg_httpd_data

structure that defines the content type and length of the data, and a

pointer to the data itself. The CYG_HTTPD_DATA()

macro automates the definition of the structure. Here is a typical

example of its use:

static cyg_uint8 ecos_logo_gif[] = {

    ...

};

CYG_HTTPD_DATA( cyg_monitor_ecos_logo_data,

                "image/gif",

                sizeof(ecos_logo_gif),

                ecos_logo_gif );

CYG_HTTPD_TABLE_ENTRY( cyg_monitor_ecos_logo,

                       "/monitor/ecos.gif",

                       cyg_httpd_send_data,

                       &cyg_monitor_ecos_logo_data );

Included in the HTTPD package is a simple System Monitor that is

intended to act as a test and an example of how to produce servers.

It is also hoped that it might be of some use in and of itself.

The System Monitor is intended to work in the background of any

application. Adding the network stack and the HTTPD package to any

configuration will enable the monitor by default. It may be disabled

by disabling the CYGPKG_HTTPD_MONITOR option.

The monitor is intended to be simple and self-explanatory in use. It

consists of four main pages. The thread monitor page presents a table

of all current threads showing such things as id, state, priority,

name and stack dimensions. Clicking on the thread ID will link to a

thread edit page where the thread's state and priority may be

manipulated. The interrupt monitor just shows a table of the current

interrupts and indicates which are active. The memory monitor shows a

256 byte page of memory, with controls to change the base address and

display element size. The network monitor page shows

information extracted from the active network interfaces and

protocols. Finally, if kernel instrumentation is enabled, the

instrumentation page provides some controls over the instrumentation

mechanism, and displays the instrumentation buffer.