TCP/UDP Server-Client implementation in C

TCP Server-Client implementation in C

Prerequisites – Socket Programming in C/C++, TCP and UDP server using select, UDP Server-Client implementation in C
If we are creating a connection between client and server using TCP then it has few functionality like, TCP is suited for applications that require high reliability, and transmission time is relatively less critical. It is used by other protocols like HTTP, HTTPs, FTP, SMTP, Telnet. TCP rearranges data packets in the order specified. There is absolute guarantee that the data transferred remains intact and arrives in the same order in which it was sent. TCP does Flow Control and requires three packets to set up a socket connection, before any user data can be sent. TCP handles reliability and congestion control. It also does error checking and error recovery. Erroneous packets are retransmitted from the source to the destination.

The entire process can be broken down into following steps:

The entire process can be broken down into following steps:

TCP Server –

1. using create(), Create TCP socket.
2. using bind(), Bind the socket to server address.
3. using listen(), put the server socket in a passive mode, where it waits for the client to approach the server to make a connection
4. using accept(), At this point, connection is established between client and server, and they are ready to transfer data.
5. Go back to Step 3.

TCP Client –

1. Create TCP socket.
2. connect newly created client socket to server.

TCP Server:

#include <stdio.h> 
#include <netdb.h> 
#include <netinet/in.h> 
#include <stdlib.h> 
#include <string.h> 
#include <sys/socket.h> 
#include <sys/types.h> 
#define MAX 80 
#define PORT 8080 
#define SA struct sockaddr 
  
// Function designed for chat between client and server. 
void func(int sockfd) 
{ 
    char buff[MAX]; 
    int n; 
    // infinite loop for chat 
    for (;;) { 
        bzero(buff, MAX); 
  
        // read the message from client and copy it in buffer 
        read(sockfd, buff, sizeof(buff)); 
        // print buffer which contains the client contents 
        printf("From client: %s	 To client : ", buff); 
        bzero(buff, MAX); 
        n = 0; 
        // copy server message in the buffer 
        while ((buff[n++] = getchar()) != '
') 
            ; 
  
        // and send that buffer to client 
        write(sockfd, buff, sizeof(buff)); 
  
        // if msg contains "Exit" then server exit and chat ended. 
        if (strncmp("exit", buff, 4) == 0) { 
            printf("Server Exit...
"); 
            break; 
        } 
    } 
} 
  
// Driver function 
int main() 
{ 
    int sockfd, connfd, len; 
    struct sockaddr_in servaddr, cli; 
  
    // socket create and verification 
    sockfd = socket(AF_INET, SOCK_STREAM, 0); 
    if (sockfd == -1) { 
        printf("socket creation failed...
"); 
        exit(0); 
    } 
    else
        printf("Socket successfully created..
"); 
    bzero(&servaddr, sizeof(servaddr)); 
  
    // assign IP, PORT 
    servaddr.sin_family = AF_INET; 
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY); 
    servaddr.sin_port = htons(PORT); 
  
    // Binding newly created socket to given IP and verification 
    if ((bind(sockfd, (SA*)&servaddr, sizeof(servaddr))) != 0) { 
        printf("socket bind failed...
"); 
        exit(0); 
    } 
    else
        printf("Socket successfully binded..
"); 
  
    // Now server is ready to listen and verification 
    if ((listen(sockfd, 5)) != 0) { 
        printf("Listen failed...
"); 
        exit(0); 
    } 
    else
        printf("Server listening..
"); 
    len = sizeof(cli); 
  
    // Accept the data packet from client and verification 
    connfd = accept(sockfd, (SA*)&cli, &len); 
    if (connfd < 0) { 
        printf("server acccept failed...
"); 
        exit(0); 
    } 
    else
        printf("server acccept the client...
"); 
  
    // Function for chatting between client and server 
    func(connfd); 
  
    // After chatting close the socket 
    close(sockfd); 
} 

TCP Client:

#include <netdb.h> 
#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 
#include <sys/socket.h> 
#define MAX 80 
#define PORT 8080 
#define SA struct sockaddr 
void func(int sockfd) 
{ 
    char buff[MAX]; 
    int n; 
    for (;;) { 
        bzero(buff, sizeof(buff)); 
        printf("Enter the string : "); 
        n = 0; 
        while ((buff[n++] = getchar()) != '
') 
            ; 
        write(sockfd, buff, sizeof(buff)); 
        bzero(buff, sizeof(buff)); 
        read(sockfd, buff, sizeof(buff)); 
        printf("From Server : %s", buff); 
        if ((strncmp(buff, "exit", 4)) == 0) { 
            printf("Client Exit...
"); 
            break; 
        } 
    } 
} 
  
int main() 
{ 
    int sockfd, connfd; 
    struct sockaddr_in servaddr, cli; 
  
    // socket create and varification 
    sockfd = socket(AF_INET, SOCK_STREAM, 0); 
    if (sockfd == -1) { 
        printf("socket creation failed...
"); 
        exit(0); 
    } 
    else
        printf("Socket successfully created..
"); 
    bzero(&servaddr, sizeof(servaddr)); 
  
    // assign IP, PORT 
    servaddr.sin_family = AF_INET; 
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1"); 
    servaddr.sin_port = htons(PORT); 
  
    // connect the client socket to server socket 
    if (connect(sockfd, (SA*)&servaddr, sizeof(servaddr)) != 0) { 
        printf("connection with the server failed...
"); 
        exit(0); 
    } 
    else
        printf("connected to the server..
"); 
  
    // function for chat 
    func(sockfd); 
  
    // close the socket 
    close(sockfd); 
} 

Compilation –
Server side:
gcc server.c -o server
./server

Client side:
gcc client.c -o client
./client

Output –
Server side:

Socket successfully created..
Socket successfully binded..
Server listening..
server acccept the client...
From client: hi
     To client : hello
From client: exit
     To client : exit
Server Exit... 

Client side:

Socket successfully created..
connected to the server..
Enter the string : hi
From Server : hello
Enter the string : exit
From Server : exit
Client Exit... 

UDP Server-Client implementation in C

There are two major transport layer protocols to communicate between hosts : TCP and UDP. Creating TCP Server/Client was discussed in a previous post.

Prerequisite : Creating TCP Server/Client

Theory
In UDP, the client does not form a connection with the server like in TCP and instead just sends a datagram. Similarly, the server need not accept a connection and just waits for datagrams to arrive. Datagrams upon arrival contain the address of sender which the server uses to send data to the correct client.

The entire process can be broken down into following steps :
UDP Server :

1. Create UDP socket.
2. Bind the socket to server address.
3. Wait until datagram packet arrives from client.
4. Process the datagram packet and send a reply to client.
5. Go back to Step 3.

UDP Client :

1. Create UDP socket.
2. Send message to server.
3. Wait until response from server is recieved.
4. Process reply and go back to step 2, if necessary.
5. Close socket descriptor and exit.

Necessary Functions :

int socket(int domain, int type, int protocol)
Creates an unbound socket in the specified domain.
Returns socket file descriptor.

Arguments :
domain – Specifies the communication
domain ( AF_INET for IPv4/ AF_INET6 for IPv6 )
type – Type of socket to be created
( SOCK_STREAM for TCP / SOCK_DGRAM for UDP )
protocol – Protocol to be used by socket.
0 means use default protocol for the address family.

int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen)
Assigns address to the unbound socket.

Arguments :
sockfd – File descriptor of socket to be binded
addr – Structure in which address to be binded to is specified
addrlen – Size of addr structure

ssize_t sendto(int sockfd, const void *buf, size_t len, int flags,
               const struct sockaddr *dest_addr, socklen_t addrlen)
Send a message on the socket

Arguments :
sockfd – File descriptor of socket
buf – Application buffer containing the data to be sent
len – Size of buf application buffer
flags – Bitwise OR of flags to modify socket behaviour
dest_addr – Structure containing address of destination
addrlen – Size of dest_addr structure

ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
                 struct sockaddr *src_addr, socklen_t *addrlen)
Receive a message from the socket.

Arguments :
sockfd – File descriptor of socket
buf – Application buffer in which to receive data
len – Size of buf application buffer
flags – Bitwise OR of flags to modify socket behaviour
src_addr – Structure containing source address is returned
addrlen – Variable in which size of src_addr structure is returned

int close(int fd)
Close a file descriptor

Arguments :
fd – File descriptor

In the below code, exchange of one hello message between server and client is shown to demonstrate the model.

UDPServer.c

// Server side implementation of UDP client-server model 
#include <stdio.h> 
#include <stdlib.h> 
#include <unistd.h> 
#include <string.h> 
#include <sys/types.h> 
#include <sys/socket.h> 
#include <arpa/inet.h> 
#include <netinet/in.h> 

#define PORT     8080 
#define MAXLINE 1024 

// Driver code 
int main() { 
    int sockfd; 
    char buffer[MAXLINE]; 
    char *hello = "Hello from server"; 
    struct sockaddr_in servaddr, cliaddr; 
    
    // Creating socket file descriptor 
    if ( (sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0 ) { 
        perror("socket creation failed"); 
        exit(EXIT_FAILURE); 
    } 
    
    memset(&servaddr, 0, sizeof(servaddr)); 
    memset(&cliaddr, 0, sizeof(cliaddr)); 
    
    // Filling server information 
    servaddr.sin_family = AF_INET; // IPv4 
    servaddr.sin_addr.s_addr = INADDR_ANY; 
    servaddr.sin_port = htons(PORT); 
    
    // Bind the socket with the server address 
    if ( bind(sockfd, (const struct sockaddr *)&servaddr, 
            sizeof(servaddr)) < 0 ) 
    { 
        perror("bind failed"); 
        exit(EXIT_FAILURE); 
    } 
    
    int len, n; 

    len = sizeof(cliaddr); //len is value/resuslt 

    n = recvfrom(sockfd, (char *)buffer, MAXLINE, 
                MSG_WAITALL, ( struct sockaddr *) &cliaddr, 
                &len); 
    buffer[n] = ''; 
    printf("Client : %s
", buffer); 
    sendto(sockfd, (const char *)hello, strlen(hello), 
        MSG_CONFIRM, (const struct sockaddr *) &cliaddr, 
            len); 
    printf("Hello message sent.
"); 
    
    return 0; 
} 

UDPClient.c

// Client side implementation of UDP client-server model 
#include <stdio.h> 
#include <stdlib.h> 
#include <unistd.h> 
#include <string.h> 
#include <sys/types.h> 
#include <sys/socket.h> 
#include <arpa/inet.h> 
#include <netinet/in.h> 

#define PORT     8080 
#define MAXLINE 1024 

// Driver code 
int main() { 
    int sockfd; 
    char buffer[MAXLINE]; 
    char *hello = "Hello from client"; 
    struct sockaddr_in     servaddr; 

    // Creating socket file descriptor 
    if ( (sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0 ) { 
        perror("socket creation failed"); 
        exit(EXIT_FAILURE); 
    } 

    memset(&servaddr, 0, sizeof(servaddr)); 
    
    // Filling server information 
    servaddr.sin_family = AF_INET; 
    servaddr.sin_port = htons(PORT); 
    servaddr.sin_addr.s_addr = INADDR_ANY; 
    
    int n, len; 
    
    sendto(sockfd, (const char *)hello, strlen(hello), 
        MSG_CONFIRM, (const struct sockaddr *) &servaddr, 
            sizeof(servaddr)); 
    printf("Hello message sent.
"); 
        
    n = recvfrom(sockfd, (char *)buffer, MAXLINE, 
                MSG_WAITALL, (struct sockaddr *) &servaddr, 
                &len); 
    buffer[n] = ''; 
    printf("Server : %s
", buffer); 

    close(sockfd); 
    return 0; 
} 

Output :

$ ./server
Client : Hello from client
Hello message sent.

$ ./client
Hello message sent.
Server : Hello from server

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