21.c

#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netinet/udp.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>

#define Fail(msg)\
	do {perror(msg); return 1; } while(0)

int main() {


	//Allocate socket, datagram
	int sock = socket(AF_INET, SOCK_DGRAM, 0);
	if (sock == -1) 
	  Fail("Failed to create socket");
	
	struct sockaddr_in local_addr = {
		.sin_family = AF_INET,
		.sin_port = htons(7542),
		.sin_addr.s_addr = htonl(INADDR_ANY)
	};

        struct sockaddr_in remote_addr;
        socklen_t remote_addr_size = sizeof(remote_addr);
        char remote_addr_str[INET_ADDRSTRLEN];
	socklen_t local_addr_size= sizeof(local_addr);


	if (bind(sock, (const struct sockaddr *)&local_addr, sizeof(local_addr)))
		Fail("Failed to bind");
	char buf[1]="H";
	if ((sendto(sock, buf, 1,MSG_NOSIGNAL,(struct sockaddr*)&local_addr, local_addr_size))<0)
		Fail("Failed in send to");

		char data[2048];
		ssize_t data_len = recvfrom(sock, data, sizeof(data), 0, (const struct sockaddr *)&remote_addr, &remote_addr_size);
		if (data_len < 0) {
			perror("Failed to receive");
			return 0;
		}
       if (strcmp(inet_ntop(remote_addr.sin_family, &remote_addr.sin_addr, remote_addr_str, sizeof(remote_addr_str)),"NULL")==0)
	       Fail("Failed while inet_ntop");
	printf("received packet from %s:%d, content: '%.*s'\n", remote_addr_str, ntohs(remote_addr.sin_port), (int) data_len, data);

	close(sock);
	return 0;
}