Chapter 172. Embedded HTTP Server

172.1. Introduction

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 www.goahead.com.

This server is also capable of serving content using IPv6 when the eCos configuration contains IPv6.

172.2. 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, by default, 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(). The server thread can also be started explicitly by the application, see the CYGNUM_HTTPD_SERVER_AUTO_START option for details.

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.

172.3. Server Configuration

The HTTP server has a number of configuration options:

CYGNUM_HTTPD_SERVER_PORT
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.
CYGDAT_HTTPD_SERVER_ID
This is the string that is reported to the client in the "Server:" field of the HTTP header.
CYGNUM_HTTPD_THREAD_COUNT
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.
CYGNUM_HTTPD_THREAD_PRIORITY
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.
CYGNUM_HTTPD_THREAD_STACK_SIZE
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.
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.
CYGNUM_HTTPD_SERVER_AUTO_START
This option causes the HTTP Daemon to be started automatically during system initialization. If this option is not set then the application must start the daemon explicitly by calling cyg_httpd_startup(). This option is set by default.
CYGNUM_HTTPD_SERVER_DELAY
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.

172.4. Support Functions and Macros

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.

[Note]Note

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.

172.4.1. HTTP Support

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().

172.4.2. General HTML Support

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.

172.4.3. Table Support

#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.

172.4.4. Forms Support

#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.

172.4.5. Predefined Handlers

cyg_bool 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 )

cyg_bool 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 );

172.5. System Monitor

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. Note: Accessing invalid memory locations can cause memory exceptions and the program to crash. 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.