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.
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:
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:
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
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:
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:
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
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:
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);
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