TCP Connections in C: A Socket Programming Guide

Updated on August 10, 2023

We are all aware that programming languages and frameworks are growing at a breakneck pace. It may appear irrelevant to learn C programs to build network connections. But wait, why don't we delve into the golden age of programming and build some simple programs to pique our interest in the under hood workings of our well-built tech setup?

My idea is simple; I just want to present a short code snippet that shows how to establish up a TCP or UDP connection and transport data over it in C.

Setup server
Setup client
Run server and client executable
Additional setups

1. Setup server

Declare client and server structures variables and other necessary variables:

struct sockaddr_in client, server;
int lfd, n, confd;
char r_buff[100] = "", s_buff[100] = "";

Create a tcp socket using socket() and assign values to server structure:
```
lfd = socket(AF_INET, SOCK_STREAM, 0);
server.sin_family = AF_INET;
server.sin_port = 2000;
server.sin_addr.s_addr = inet_addr("127.0.0.1");
```
Here AF_INET is the address family, SOCK_STREAM is the type and 0 is for the default protocol.
Bind the socket with address and server structure using bind() and listen for connection using listen():
```
bind(lfd, (struct sockaddr *)&server, sizeof server);
listen(lfd, 1);
```

Accept the connection from client using accept():

n = sizeof client;
confd = accept(lfd, (struct sockaddr *)&client, &n);

Receive message from client using recv():

recv(confd, r_buff, sizeof r_buff, 0);
printf("\n[client] %s", r_buff);

send message back to client using send()

printf("\nserver: ");
gets(s_buff);
send(confd, s_buff, sizeof s_buff, 0);

Stop socket and end connection using close()
```
close(confd);
close(lfd);
```

Final code for server.c

#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/types.h>

int main() {
    struct sockaddr_in client, server;
    int lfd, n, confd;
    char r_buff[100] = "", s_buff[100] = "";

    lfd = socket(AF_INET, SOCK_STREAM, 0);
    server.sin_family = AF_INET;
    server.sin_port = 2000;
    server.sin_addr.s_addr = inet_addr("127.0.0.1");

    bind(lfd, (struct sockaddr *)&server, sizeof server);
    listen(lfd, 1);

    n = sizeof client;
    confd = accept(lfd, (struct sockaddr *)&client, &n);

    while (1) {
        recv(confd, r_buff, sizeof r_buff, 0);
        printf("\n[client] %s", r_buff);
        if (strcmp(r_buff, "exit") == 0)
            break;

        printf("\nserver: ");
        gets(s_buff);
        send(confd, s_buff, sizeof s_buff, 0);
        if (strcmp(s_buff, "exit") == 0)
            break;
        printf("\n");
    }

    close(confd);
    close(lfd);

    return 0;
}

2. Setup client

Declare server structure variable and neccessary variables:

struct sockaddr_in server;
int lfd;
char r_buff[100] = "", s_buff[100] = "";

Create a tcp socket using socket() and assign values to server structure:

lfd = socket(AF_INET, SOCK_STREAM, 0);
server.sin_family = AF_INET;
server.sin_port = 2000;
server.sin_addr.s_addr = inet_addr("127.0.0.1");

Connect to server using connect():

connect(lfd, (struct sockaddr *)&server, sizeof server);

Send message to server using send():

printf("\nclient: ");
gets(s_buff);
send(lfd, s_buff, sizeof s_buff, 0);

Receive back message from server using recv():

recv(lfd, r_buff, sizeof r_buff, 0);
printf("[server] %s", r_buff);

Stop socket and end connection using close()
```
close(lfd);
```

Final code for client.c

#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/types.h>

int main() {
struct sockaddr_in server;
int lfd;
char r_buff[100] = "", s_buff[100] = "";

lfd = socket(AF_INET, SOCK_STREAM, 0);
server.sin_family = AF_INET;
server.sin_port = 2000;
server.sin_addr.s_addr = inet_addr("127.0.0.1");

connect(lfd, (struct sockaddr *)&server, sizeof server);

while (1) {
    printf("\nclient: ");
    gets(s_buff);
    send(lfd, s_buff, sizeof s_buff, 0);
    if (strcmp(s_buff, "end") == 0)
    break;

    recv(lfd, r_buff, sizeof r_buff, 0);
    printf("[server] %s", r_buff);
    if (strcmp(r_buff, "end") == 0)
    break;

    printf("\n");
}

close(lfd);

return 0;
}

Note: The system calls socket(), bind(), listen(), connect(), accept(), send(), and recv() in the above programs are imported from libraries that may only operate in linux distributions (os); obviously, these programs cannot be executed in other operating systems such as Windows.

3. Run server and client executables

Compile server and client code

gcc server.c -o server.out
gcc client.c -o client.out

Execute server and client (in the respective order)

# in terminal-1 or system-1
./server.out

# in terminal-2 or system-2
./client.out

Make sure to execute them in different terminals (or different systems if you are assigned a public IP address)

If everything is fine, you can expect something like this.
- client
- server

4. Additional setups

logging requests into a file
- Edit server.c and add,
```
FILE *fp;
fp = fopen("log.txt", "a");

fputs(r_buff, fp);
fputs("\t", fp);
fputs(s_buff, fp);
fputs("\n", fp);
fclose(fp);
```
  Make sure not to use only fputs() inside loops; in that case, you may need to use fopen() and fclose() within the loop body at the beginning and end, respectively.
You can try adding functions like prime number checker, palindrome checker, etc to make this program more useful

TCP Connections in C: A Socket Programming Guide

contents

1. Setup server

2. Setup client

3. Run server and client executables

4. Additional setups