UDP Server-Client implementation in C

04 May, 2022
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 the sender which the server uses to send data to the correct client. 
The entire process can be broken down into the following steps : 
UDP Server :  
Create a UDP socket.Bind the socket to the server address.Wait until the datagram packet arrives from the client.Process the datagram packet and send a reply to the client.Go back to Step 3.
Create a UDP socket.
Bind the socket to the server address.
Wait until the datagram packet arrives from the client.
Process the datagram packet and send a reply to the client.
Go back to Step 3.
UDP Client :  
Create a UDP socket.Send a message to the server.Wait until response from the server is received.Process reply and go back to step 2, if necessary.Close socket descriptor and exit.
Create a UDP socket.
Send a message to the server.
Wait until response from the server is received.
Process reply and go back to step 2, if necessary.
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 the 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 a socket to be bonded addr – Structure in which address to be bound 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 the socket buf – Application buffer containing the data to be sent len – Size of buf application buffer flags – Bitwise OR of flags to modify socket behavior dest_addr – Structure containing the address of the 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 the socket buf – Application buffer in which to receive data len – Size of buf application buffer flags – Bitwise OR of flags to modify socket behavior 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, the exchange of one hello message between server and client is shown to demonstrate the model.
Filename: UDPServer.c
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/result         n = recvfrom(sockfd, (char *)buffer, MAXLINE,                  MSG_WAITALL, ( struct sockaddr *) &cliaddr,                 &len);     buffer[n] = '\0';     printf("Client : %s\n", buffer);     sendto(sockfd, (const char *)hello, strlen(hello),          MSG_CONFIRM, (const struct sockaddr *) &cliaddr,             len);     printf("Hello message sent.\n");              return 0; }
Filename: UDPClient.c
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");                 n = recvfrom(sockfd, (char *)buffer, MAXLINE,                  MSG_WAITALL, (struct sockaddr *) &servaddr,                 &len);     buffer[n] = '\0';     printf("Server : %s\n", 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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