Last Updated : 07 Aug, 2025
Socket programming is a way of connecting two nodes on a network to communicate with each other. One socket(node) listens on a particular port at an IP, while the other socket reaches out to the other to form a connection. The server forms the listener socket while the client reaches out to the server.
Socket programming is widely used in instant messaging applications, binary streaming, and document collaborations, online streaming platforms, etc.
In this C program we are exchanging one hello message between server and client to demonstrate the client/server model.
server.c C
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>
#define PORT 8080
int main(int argc, char const* argv[])
{
int server_fd, new_socket;
ssize_t valread;
struct sockaddr_in address;
int opt = 1;
socklen_t addrlen = sizeof(address);
char buffer[1024] = { 0 };
char* hello = "Hello from server";
// Creating socket file descriptor
if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
perror("socket failed");
exit(EXIT_FAILURE);
}
// Forcefully attaching socket to the port 8080
if (setsockopt(server_fd, SOL_SOCKET,
SO_REUSEADDR | SO_REUSEPORT, &opt,
sizeof(opt))) {
perror("setsockopt");
exit(EXIT_FAILURE);
}
address.sin_family = AF_INET;
address.sin_addr.s_addr = INADDR_ANY;
address.sin_port = htons(PORT);
// Forcefully attaching socket to the port 8080
if (bind(server_fd, (struct sockaddr*)&address,
sizeof(address))
< 0) {
perror("bind failed");
exit(EXIT_FAILURE);
}
if (listen(server_fd, 3) < 0) {
perror("listen");
exit(EXIT_FAILURE);
}
if ((new_socket
= accept(server_fd, (struct sockaddr*)&address,
&addrlen))
< 0) {
perror("accept");
exit(EXIT_FAILURE);
}
// subtract 1 for the null
// terminator at the end
valread = read(new_socket, buffer,
1024 - 1);
printf("%s\n", buffer);
send(new_socket, hello, strlen(hello), 0);
printf("Hello message sent\n");
// closing the connected socket
close(new_socket);
// closing the listening socket
close(server_fd);
return 0;
}
client.c C
#include <arpa/inet.h>
#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>
#define PORT 8080
int main(int argc, char const* argv[])
{
int status, valread, client_fd;
struct sockaddr_in serv_addr;
char* hello = "Hello from client";
char buffer[1024] = { 0 };
if ((client_fd = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
printf("\n Socket creation error \n");
return -1;
}
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(PORT);
// Convert IPv4 and IPv6 addresses from text to binary
// form
if (inet_pton(AF_INET, "127.0.0.1", &serv_addr.sin_addr)
<= 0) {
printf(
"\nInvalid address/ Address not supported \n");
return -1;
}
if ((status
= connect(client_fd, (struct sockaddr*)&serv_addr,
sizeof(serv_addr)))
< 0) {
printf("\nConnection Failed \n");
return -1;
}
// subtract 1 for the null
// terminator at the end
send(client_fd, hello, strlen(hello), 0);
printf("Hello message sent\n");
valread = read(client_fd, buffer,
1024 - 1);
printf("%s\n", buffer);
// closing the connected socket
close(client_fd);
return 0;
}
Compiling
gcc client.c -o clientgcc server.c -o server
Output
Client:Hello message sentHello from serverServer:Hello from clientHello message sentComponents of Socket Programming 1. Sockets
Sockets are one of the core components used by the program to access the network to communicate with other processes/nodes over the network. It is simply a combination of an IP address and a port number that acts as an endpoint for Communication.
Example: 192.168.1.1:8080, where the two parts separated by the colon represent the IP address( 192.168.1.1 ) and the port number( 8080 ).
Socket Types:
The client-server model refers to the architecture used in socket programming, where a client and a server to interact with each other to exchange information or services. This architecture allows client to send service requests and the server to process and send response to those service requests.
State Diagram for Server and Client Model
State diagram for server and client model of SocketSocket programming in C is a powerful way to handle network communication.
Creating a Server-Side ProcessThe server is created using the following steps:
1. Socket CreationThis step involves the creation of the socket using the socket() function.
Parameters:
sockfd = socket(domain, type, protocol)
2. Setsockopt
This helps in manipulating options for the socket referred by the file descriptor sockfd. This is completely optional, but it helps in reuse of address and port. Prevents error such as: “address already in use”.
C
setsockopt(sockfd, level, optname, optval, socklen_t optlen);
3. Bind
After the creation of the socket, the bind() function binds the socket to the address and port number specified in addr(custom data structure). In the example code, we bind the server to the localhost, hence we use INADDR_ANY to specify the IP address.
C++
bind(sockfd, sockaddr *addr, socklen_t addrlen);
Parameters:
In this step the server uses the listen() function that puts the server socket in a passive mode, where it waits for the client to approach the server to make a connection. The backlog, defines the maximum length to which the queue of pending connections for sockfd may grow. If a connection request arrives when the queue is full, the client may receive an error with an indication of ECONNREFUSED.
CParameters:
In this step the server extracts the first connection request from the queue of pending connections for the listening socket, sockfd, creates a new connected socket using the accept() function, and returns a new file descriptor referring to that socket. At this point, the connection is established between client and server, and they are ready to transfer data.
C
new_socket= accept(sockfd, sockaddr *addr, socklen_t *addrlen);
Parameters:
In this step the server can send or receive data from the client .
Send(): to send data to the client
C
send(sockfd, *buf, len, flags);
Parameters:
Receive() : to recieve the data from the client.
C
recv( sockfd, *buf, len, flags);
Parameters:
After the exchange of information is complete, the server closes the socket using the close() function and releases the system resources.
CParameters:
Follow the below steps for creating a client-side process:
1. Socket connectionThis step involves the creation of the socket which is done in the same way as that of server’s socket creation
2. ConnectThe connect() system call connects the socket referred to by the file descriptor sockfd to the address specified by addr. Server’s address and port is specified in addr.
C++
connect(sockfd, sockaddr *addr, socklen_t addrlen);
Parameters
In this step the client can send or receive data from the server which is done using the send() and recieve() functions similar to how the server sends/recieves data from the client.
4. CloseOnce the exchange of information is complete, the client also needs to close the created socket and releases the system resources using the close() function in the same way as the server does.
Common Issues and their Fixes in Socket ProgrammingRetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4