sayhello.c

/* cc -o sayhello.c '-DHELLO_ADDRESS="foo.example.com"' -DHELLO_PORT=4567
   sends a UDP packet containing this machine's hostname (and nothing else)
   to HELLO_ADDRESS:HELLO_PORT every five seconds.
 */

#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>

#ifndef HELLO_ADDRESS
#define HELLO_ADDRESS "hello.example.com"
#endif
#ifndef HELLO_PORT
#define HELLO_PORT 1234
#endif

#define DIE_S2(s) #s
#define DIE_S1(s) DIE_S2(s)
#define DIE(...)                                                               \
	do {                                                                   \
		int _errno = errno;                                            \
		fprintf(stderr, __VA_ARGS__);                                  \
		errno = _errno;                                                \
		perror("File: " __FILE__ "\nLine: " DIE_S1(__LINE__));         \
		abort();                                                       \
	} while (0)
#define EH(LABEL, ...)                                                         \
	do {                                                                   \
		if (errno == EINTR) {                                          \
			goto LABEL;                                            \
		}                                                              \
		DIE(__VA_ARGS__);                                              \
	} while (0)

int main(int argc, char **argv)
{
	ssize_t hostnamelen = 1;
	char *hostname = malloc(hostnamelen);

	if (hostname == NULL) {
		DIE("malloc hates me.\n");
	}
	while (gethostname(hostname, hostnamelen)) {
		if (errno != ENAMETOOLONG) {
			DIE("gethostname fail.\n");
		}
		hostnamelen <<= 1;
		free(hostname);
		hostname = malloc(hostnamelen);
		if (hostname == NULL) {
			DIE("malloc hates me.\n");
		}
	}
	hostnamelen = strlen(hostname) + 1;

	socklen_t addrlen;
	struct sockaddr *addr;

	struct addrinfo ai_hints, *ai_lookup;
	memset(&ai_hints, 0, sizeof(ai_hints));
	ai_hints.ai_family = PF_INET;
	ai_hints.ai_socktype = SOCK_DGRAM;
	ai_hints.ai_flags = AI_NUMERICSERV;
	if (getaddrinfo("tigger", "8087", &ai_hints, &ai_lookup)) {
		DIE("Looking up 'tigger' failed.\n");
	}
	addrlen = ai_lookup->ai_addrlen;
	addr = malloc((size_t)addrlen);
	if (addr == NULL) {
		DIE("malloc hates me.\n");
	}
	memcpy(addr, ai_lookup->ai_addr, (size_t)addrlen);
	freeaddrinfo(ai_lookup);

	int s = socket(AF_INET, SOCK_DGRAM, 0);
	if (s == -1) {
		DIE("Can't get a udp4 socket.\n");
	}

	for (;;) {
		ssize_t r = sendto(s, hostname, hostnamelen, 0, addr, addrlen);
		if (r != hostnamelen) {
			EH(stopped, "sendto returned %zd.\n", r);
		}
		if (sleep(5)) {
			EH(stopped, "nightmare\n");
		}
	}
stopped:
	free(addr);
	return 0;
}