TCP sockets are used for communication between a server and a client process. The server’s code runs first, which opens a port and listens for incoming connection requests from clients. Once a client connects to the same (server) port, the client or server may send a message. Once the message is sent, whoever receives it (server or client) will process it accordingly.
#include <stdio.h>#include <string.h>#include <sys/socket.h>#include <arpa/inet.h>int main(void){int socket_desc, client_sock, client_size;struct sockaddr_in server_addr, client_addr;char server_message[2000], client_message[2000];// Clean buffers:memset(server_message, '\0', sizeof(server_message));memset(client_message, '\0', sizeof(client_message));// Create socket:socket_desc = socket(AF_INET, SOCK_STREAM, 0);if(socket_desc < 0){printf("Error while creating socket\n");return -1;}printf("Socket created successfully\n");// Set port and IP:server_addr.sin_family = AF_INET;server_addr.sin_port = htons(2000);server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");// Bind to the set port and IP:if(bind(socket_desc, (struct sockaddr*)&server_addr, sizeof(server_addr))<0){printf("Couldn't bind to the port\n");return -1;}printf("Done with binding\n");// Listen for clients:if(listen(socket_desc, 1) < 0){printf("Error while listening\n");return -1;}printf("\nListening for incoming connections.....\n");// Accept an incoming connection:client_size = sizeof(client_addr);client_sock = accept(socket_desc, (struct sockaddr*)&client_addr, &client_size);if (client_sock < 0){printf("Can't accept\n");return -1;}printf("Client connected at IP: %s and port: %i\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));// Receive client's message:if (recv(client_sock, client_message, sizeof(client_message), 0) < 0){printf("Couldn't receive\n");return -1;}printf("Msg from client: %s\n", client_message);// Respond to client:strcpy(server_message, "This is the server's message.");if (send(client_sock, server_message, strlen(server_message), 0) < 0){printf("Can't send\n");return -1;}// Closing the socket:close(client_sock);close(socket_desc);return 0;}
Include the header files sys/socket.h
and arpa/inet.h
:
#include <sys/socket.h>
#include <arpa/inet.h>
Create a socket that returns a socket descriptor; this will be used to refer to the socket later on in the code:
int socket_desc = socket(AF_INET, SOCK_STREAM, 0);
sockaddr_in
, which is a struct
.Initialize the server address by the port and IP:
struct sockaddr_in server_addr;
server_addr.sin_family = AF_INET;
server_addr.sin_port = htons(2000);
server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
Bind the socket descriptor to the server address:
bind(socket_desc, (struct sockaddr*)&server_addr, sizeof(server_addr);
Turn on the socket to listen for incoming connections:
listen(socket_desc, 1);
Store the client’s address and socket descriptor by accepting an incoming connection:
struct sockaddr client_addr;
int client_size = sizeof(client_addr);
int client_sock = accept(socket_desc, (struct sockaddr*)&client_addr, &client_size);
accept()
until a client calls connect()
.Communicate with the client using send()
and recv()
:
recv(client_sock, client_message, sizeof(client_message), 0);
send(client_sock, server_message, strlen(server_message), 0);
recv()
is called, the code stops and waits for a message from the client.Close the server and client socket to end communication:
close(client_sock);
close(socket_desc);
#include <stdio.h>#include <string.h>#include <sys/socket.h>#include <arpa/inet.h>int main(void){int socket_desc;struct sockaddr_in server_addr;char server_message[2000], client_message[2000];// Clean buffers:memset(server_message,'\0',sizeof(server_message));memset(client_message,'\0',sizeof(client_message));// Create socket:socket_desc = socket(AF_INET, SOCK_STREAM, 0);if(socket_desc < 0){printf("Unable to create socket\n");return -1;}printf("Socket created successfully\n");// Set port and IP the same as server-side:server_addr.sin_family = AF_INET;server_addr.sin_port = htons(2000);server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");// Send connection request to server:if(connect(socket_desc, (struct sockaddr*)&server_addr, sizeof(server_addr)) < 0){printf("Unable to connect\n");return -1;}printf("Connected with server successfully\n");// Get input from the user:printf("Enter message: ");gets(client_message);// Send the message to server:if(send(socket_desc, client_message, strlen(client_message), 0) < 0){printf("Unable to send message\n");return -1;}// Receive the server's response:if(recv(socket_desc, server_message, sizeof(server_message), 0) < 0){printf("Error while receiving server's msg\n");return -1;}printf("Server's response: %s\n",server_message);// Close the socket:close(socket_desc);return 0;}
Include header files, create a socket, and initialize the server’s address information in a variable of type sockaddr_in
, similar to how it was done at the server-side:
#include <sys/socket.h>
#include <arpa/inet.h>
int socket_desc = socket(AF_INET, SOCK_STREAM, 0);
struct sockaddr_in server_addr;
server_addr.sin_family = AF_INET;
server_addr.sin_port = htons(2000);
server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
Send a connection request to the server, which is waiting at accept()
:
connect(socket_desc, (struct sockaddr*)&server_addr, sizeof(server_addr));
Communicate with the server using send()
and recv()
:
send(socket_desc, client_message, strlen(client_message),0);
recv(socket_desc, server_message, sizeof(server_message),0);
recv()
is called.Close the socket:
close(socket_desc);
A deadlock will occur if both the client and the server are waiting for each other’s message at
recv()
.
The following client-server application enables a client to connect to a server and send only one message. The server replies with "This is the server’s message" and the communication terminates.
+
button to open another terminal tab and execute the Client’s command.Server | Client |
---|---|
usercode/server |
usercode/client |
#include <stdio.h> #include <string.h> #include <sys/socket.h> #include <arpa/inet.h> int main(void) { int socket_desc, client_sock, client_size; struct sockaddr_in server_addr, client_addr; char server_message[100], client_message[100]; // Clean buffers: memset(server_message, '\0', sizeof(server_message)); memset(client_message, '\0', sizeof(client_message)); // Create socket: socket_desc = socket(AF_INET, SOCK_STREAM, 0); if(socket_desc < 0){ printf("Error while creating socket\n"); return -1; } printf("Socket created successfully\n"); // Set port and IP: server_addr.sin_family = AF_INET; server_addr.sin_port = htons(2000); server_addr.sin_addr.s_addr = inet_addr("127.0.0.1"); // Bind to the set port and IP: if(bind(socket_desc, (struct sockaddr*)&server_addr, sizeof(server_addr))<0){ printf("Couldn't bind to the port\n"); return -1; } printf("Done with binding\n"); // Listen for clients: if(listen(socket_desc, 1) < 0){ printf("Error while listening\n"); return -1; } printf("\nListening for incoming connections.....\n"); // Accept an incoming connection: client_size = sizeof(client_addr); client_sock = accept(socket_desc, (struct sockaddr*)&client_addr, &client_size); if (client_sock < 0){ printf("Can't accept\n"); return -1; } printf("Client connected at IP: %s and port: %i\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port)); // Receive client's message: if (recv(client_sock, client_message, sizeof(client_message), 0) < 0){ printf("Couldn't receive\n"); return -1; } printf("Msg from client: %s\n", client_message); // Respond to client: strcpy(server_message, "This is the server's message."); if (send(client_sock, server_message, strlen(server_message), 0) < 0){ printf("Can't send\n"); return -1; } // Closing the socket: close(client_sock); close(socket_desc); return 0; }