Networking, Part 6: Creating a UDP server - angrave/SystemProgramming GitHub Wiki
There are a variety of function calls available to send UDP sockets. We will use the newer getaddrinfo to help set up a socket structure.
Remember that UDP is a simple packet-based ('data-gram') protocol ; there is no connection to set up between the two hosts.
First, initialize the hints addrinfo struct to request an IPv6, passive datagram socket.
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_INET6; // use AF_INET instead for IPv4
hints.ai_socktype = SOCK_DGRAM;
hints.ai_flags = AI_PASSIVE;Next, use getaddrinfo to specify the port number (we don't need to specify a host as we are creating a server socket, not sending a packet to a remote host).
getaddrinfo(NULL, "300", &hints, &res);
sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
bind(sockfd, res->ai_addr, res->ai_addrlen);The port number is <1024, so the program will need root privileges. We could have also specified a service name instead of a numeric port value.
So far the calls have been similar to a TCP server. For a stream-based service we would call listen and accept. For our UDP-serve we can just start waiting for the arrival of a packet on the socket-
struct sockaddr_storage addr;
int addrlen = sizeof(addr);
// ssize_t recvfrom(int socket, void* buffer, size_t buflen, int flags, struct sockaddr *addr, socklen_t * address_len);
byte_count = recvfrom(sockfd, buf, sizeof(buf), 0, &addr, &addrlen);The addr struct will hold sender (source) information about the arriving packet.
Note the sockaddr_storage type is a sufficiently large enough to hold all possible types of socket addresses (e.g. IPv4, IPv6 and other socket types).
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <unistd.h>
#include <arpa/inet.h>
int main(int argc, char **argv)
{
int s;
struct addrinfo hints, *res;
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_INET6; // INET for IPv4
hints.ai_socktype = SOCK_DGRAM;
hints.ai_flags = AI_PASSIVE;
getaddrinfo(NULL, "300", &hints, &res);
int sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
if (bind(sockfd, res->ai_addr, res->ai_addrlen) != 0) {
perror("bind()");
exit(1);
}
struct sockaddr_storage addr;
int addrlen = sizeof(addr);
while(1){
char buf[1024];
ssize_t byte_count = recvfrom(sockfd, buf, sizeof(buf), 0, &addr, &addrlen);
buf[byte_count] = '\0';
printf("Read %d chars\n", byte_count);
printf("===\n");
printf("%s\n", buf);
}
return 0;
}