ESP32 (20) – Webserver

luca 08/07/2017 7

One of the most popular projects among the ones included in my tutorial about the enc28j60 chip is fore sure WebRelay. This project allows to control an output pin of Arduino using a simple web page, designed to be accessed also using your smartphone. Today I’m going to show you how to implement a similar project with the esp32 chip; it’s also the opportunity to teach how to write a TCP server, especially a web server.

Netconn API

As you know, the esp-idf framework uses the lwip library to manage network communication. This library offers different abstraction levels: the programmer can decide to work on the raw packets or to leverage pre-build components.

To develop my TCP server, I decided to use one of those pre-build components, the Netconn API.

Using Netconn API it’s very easy to implement a server and the required steps are described below:

webrelay-01

The netconn_new() method creates a new connection and returns a a pointer to struct netconn which represents the connection:

struct netconn *conn;
conn = netconn_new(NETCONN_TCP);

The parameter defines the connection type… the most common ones are NETCONN_TCP for a connection using the TCP protocol and NETCONN_UDP for a connection using the UDP protocol.

To use the connection in server mode, you have then to associate (bind) it to a specific port… for example a webserver normally listens on port 80 (443 if HTTPS):

netconn_bind(conn, NULL, 80);

The second parameter (NULL above) allows to bind the connection to a specific IP address and may be useful if the device has more than a network interface. If you pass NULL (or the equivalent IP_ADDR_ANY) you ask the library to bind the connection to any available interfaces.

Now you can start listening on that port with:

netconn_listen(conn);

Working with a client connection

With the netconn_accept() method, your program can accept a new incoming connection:

struct netconn *newconn;
netconn_accept(conn, &newconn);

The method returns a pointer to a new struct netconn that represents the connection established with the client. This method is blocking: the program is stopped until a client makes a connection request.

Once the connection is established, you can use the netconn_recv() and netconn_write() methods to receive or send data to the client:

netconn_recv(struct netconn* aNetConn, struct netbuf** aNetBuf );
netconn_write(struct netconn* aNetConn, const void* aData, size_t aSize, u8_t aApiFlags);

To optimize RAM memory usage, the netconn_recv() method handles data using an internal buffer (zero-copy mode). To access the received data you therefore have to:

  • declare a new variable as a pointer to struct netbuf
  • pass the pointer address as the second parameter of the recv method
  • use the netbuf_data() method to obtain a pointer to the data within the netbuffer structure and its length

webrelay-02

struct netbuf *inbuf;
char *buf;
u16_t buflen;
netconn_recv(conn, &inbuf);
netbuf_data(inbuf, (void**)&buf, &buflen);

Likewise, the netconn_write() method accepts, as last parameter, a flag to indicate whether or not to copy the buffer content before sending it. To save memory you can therefore, if you’re sure that the buffer won’t be modified by other theads, use NETCONN_NOCOPY as flag:

netconn_write(conn, outbuff, sizeof(outbuff), NETCONN_NOCOPY);

When the communication with the client is complete, you can close the connection and free the buffer:

netconn_close(conn);
netbuf_delete(inbuf);

HTTP server

What you learned so far can be applied to every TCP server. If you want to communicate with an Internet browser, you have to “talk” the same language, that is the HTTP protocol.

In the example program (available on Github) I implemented a very minimal version of that protocol. When you type an address in your browser (for example www.google.com), the browser connects to Google server and sends a request with the form:

GET <resource>
[...]

The request can have different fields, but the first line always contains the name of the requested resource (html page, image…). In particular if you access the homepage of the website the request will simply be GET /.

The website published by the esp32 chip to control the relay consists of only two pages:

  • off.html, displayed when the relay is off
  • on.html, displayed when the relay is on

Each page contains a label (“Relay is ON|OFF“) and an image. The image itself is a link to the other page and when you click on it, the relay status is also changed:

webrelay-03

The program identifies the resource to be sent analyzing the first line of the request with strstr():

char *first_line = strtok(buf, "\n");
if(strstr(first_line, "GET / ")) [...]
else if(strstr(first_line, "GET /on.html ")) [...]
else if(strstr(first_line, "GET /on.png ")) [...]

An HTTP server responds to the browser by first indicating the result of the request. If ok, the return code is 200:

HTTP/1.1 200 OK

then it returns the media type of the resource and finally it sends the resource file. In this example, the possible media types are:

  • text/html for HTML pages
  • image/png for images

Static content

A webserver it normally stores the resouces for the website it publishes on an external device (memory card, hard drive…). For simple projects, you may consider to include all the content inside your program.

For example in my code HTML pages and HTTP protocol messages are defined as static arrays:

const static char http_html_hdr[] = "HTTP/1.1 200 OK\nContent-type: text/html\n\n";
const static char http_png_hdr[] = "HTTP/1.1 200 OK\nContent-type: image/png\n\n";
const static char http_off_hml[] = "";

To be able to include also the images, I used a feature of the framework (embedding binary data). You can indeed specify in the component.mk file the binary resources to be included:

webrelay-04

In your program you can access the content of the embedded files using the following pointers:

extern const uint8_t on_png_start[] asm("_binary_on_png_start");
extern const uint8_t on_png_end[]   asm("_binary_on_png_end");
extern const uint8_t off_png_start[] asm("_binary_off_png_start");
extern const uint8_t off_png_end[]   asm("_binary_off_png_end");

The syntax is _binary_filename_start|end, replace “.” with “_” in the filename. Having pointers to both the start and the end of the resource content, it’s easy to send it with the netconn_write() method:

netconn_write(conn, on_png_start, on_png_end - on_png_start, NETCONN_NOCOPY);

Demo

7 Comments »

  1. John K Kovach 14/07/2017 at 12:55 - Reply

    Luca,
    Great article. Thank you for your time, your generosity and the help that it gives. I have been trying to find an example that shows a web server serving a html file from memory. I would like to be able to serve a large SPA file like ReactJS from a SD memory card. Is that possible?

    • luca 18/07/2017 at 07:33 - Reply

      Hi John, yes it’s possible and I’m working on it! You may also consider (this will be the topic of a future post) to use a partition (FAT or SPIFFS) in the internal flash.

  2. Marek Hlavsa 19/07/2017 at 20:55 - Reply

    Hi Luca,
    You publish really great tutorials! Thanks. Regarding the webserver – is there a simple way how to get user input from the web (e.g. to retrieve content from a web form submitted). I am thinking about something like a “simple admin web console” to set-up / administer SSID, user credentials … where text input is necessary.
    Thanks in advance
    Marek

    • luca 20/07/2017 at 14:41 - Reply

      Hi Marek, yes it’s quite easy to receive (GET or POST methods) data from the user… you only need to parse the incoming request: GET parameters are added to the URL while POST parameters are added in the request body. One of my future tutorials will be about this!

  3. Marek Hlavsa 14/08/2017 at 16:22 - Reply

    Hi Luca,
    Thanks for the answer. One more thing regarding your demo – the web server is working perfectly fine when connected from android (e.g. from mobile phone), but since I connect from Windows (e.g. notebook running Win10) the pictures downloading is extremely slow (and in some cases even stops the web server on ESP32 to listen to requests and respond) – tested on 2 different Windows devices. Any idea what could be the problem (BTW – without the images it works fine even on Windows)?
    Thanks in advance
    Marek

  4. Tony 18/09/2017 at 13:41 - Reply

    It’s working really really well. I could not believe a http server was running on my ESP.

    The keyword “static” makes my compile fail though (Cygwin GCC, ESP SDK 2.1) –I am not entirely sure why.

Leave A Response »

Click here to cancel reply.

This website uses cookies to ensure you get the best experience on our website maggiori informazioni

Questo sito utilizza i cookie per fonire la migliore esperienza di navigazione possibile. Continuando a utilizzare questo sito senza modificare le impostazioni dei cookie o clicchi su "Accetta" permetti al loro utilizzo.

Chiudi