ESP32 Socket Programming

 BSD socket API is A set of standard function calls to include internet communication capabilities in a product 

Other sockets API exists but the BSD socket is generally regarded as the standard. 

The sockets API makes use of 2 mechanisms to deliver data to the application level: ports & sockets. 

All TCP/IP stacks have 65,535 ports for both TCP & UDP 

A port is not a physical interface. Once an application has created a socket and bound it to a port, data destined for that port will be delivered to the application. 

A TCP/IP or UDP/IP packet is transmitted to a host, it eventually goes to the correct port then the socket conveys the packet’s data to the application 

Now Let’s go though some common socket calls 

socket()	Creates a socket & returns a reference number for that socket
connect()	Creates a connection with another host
send()	Send data over a connected socket
recv()	Receive data on the connected host

Now we will go through this example https://github.com/espressif/esp-idf/tree/master/examples/protocols/sockets/tcp_client 

This example makes esp32 a tcp client. At the host computer,  a script will run as a TCP server. 

ESP32 has a lwIP stack, this stack has the socket API. 

The API can be found in this file 

esp-idf/components/lwip/lwip/src/include/lwip/sockets.h

To create a socket instance we have to use the following function of sockets.h 

static inline int socket(int domain,int type,int protocol) { return lwip_socket(domain,type,protocol); }

The type argument can be from the following values 

/* Socket protocol types (TCP/UDP/RAW) */ #define SOCK_STREAM     1 #define SOCK_DGRAM      2 #define SOCK_RAW        3

domain & protocol initialized as 

int addr_family; int ip_protocol; ....... ....... addr_family = AF_INET; ip_protocol = IPPROTO_IP;

The socket() creates a socket and returns the socket number 

int sock =  socket(addr_family, SOCK_STREAM, ip_protocol);

Now we are going to understand another basic structure related to socket programming which is sockaddr_in 

/* members are in network byte order */ struct sockaddr_in {   u8_t            sin_len;          //length of this structure   sa_family_t     sin_family;       //AF_INET   in_port_t       sin_port;         //transport layer port   struct in_addr  sin_addr;         //the ip address #define SIN_ZERO_LEN 8   char            sin_zero[SIN_ZERO_LEN]; };

We have to create this structure type variable and use it to assign the server’s IP address to which we want to connect 

#define HOST_IP_ADDR CONFIG_EXAMPLE_IPV4_ADDR #define PORT CONFIG_EXAMPLE_PORT

struct sockaddr_in dest_addr; dest_addr.sin_addr.s_addr = inet_addr(HOST_IP_ADDR); dest_addr.sin_family = AF_INET; dest_addr.sin_port = htons(PORT); inet_ntoa_r(dest_addr.sin_addr, addr_str, sizeof(addr_str) - 1);

Then this variable is used at the socket connection method 

int err = connect(sock, (struct sockaddr *)&dest_addr, sizeof(dest_addr));

This above function will connect to the server 

Similarly, other function such as send() and recv() of the API is used for communication 

The example code connects to wifi using example_connect() function, this is a blocking function and the block waits until a connection is established. the default event loop must be created before as wifi connection code is event based

tcpip_adapter_init(); ESP_ERROR_CHECK(esp_event_loop_create_default()); ESP_ERROR_CHECK(example_connect());

The code creates a freeRtos task that tries to connect to the server in an infinite loop and if connected then it communicates with the server in another infinite loop 

Running the Server Script

hassin@hassin-HP:~/esp/esp-idf/examples/protocols/sockets/scripts$ ifconfig

enx34298f9117bd: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500         inet 192.168.11.96  netmask 255.255.255.0  broadcast 192.168.11.255

hassin@hassin-HP:~/esp/esp-idf/examples/protocols/sockets/scripts$ python3 tcpserver.py

Socket created Socket binded Socket listening

So the server is running on our host whose IP is 192.168.11.96 

Now we need to assign this IP address to CONFIG_EXAMPLE_IPV4_ADDR macro 

This can be done by idf.py menuconfig , in the same way, we assigned wifi SSID and Password values using the menuconfig  

After assigning the server’s IP address and Port , program esp32 . 

After getting connected to wifi , esp32 will connect to the server [host machine] and the server and client will communicate with each other 

Here on the left side ESP32 log is printed and on the right side the python server’s log 

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Reference 

http://wiki.treck.com/Introduction_to_BSD_Sockets