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// Initially based on https://stackoverflow.com/a/35570418
#include <errno.h>
#include <fcntl.h>
#include <netinet/in.h>
#include <poll.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>
enum frame_type {
FT_NONE = 0,
FT_HELLO = 42,
};
struct frame {
uint32_t addr;
uint16_t port;
enum frame_type type;
};
static unsigned char udp_buffer[UINT16_MAX];
static int ReceiveFrame(int const fd, struct frame *const frame)
{
frame->type = FT_NONE;
struct sockaddr_in source_address;
socklen_t source_address_len = sizeof(source_address);
int const length = recvfrom(
fd,
udp_buffer,
sizeof(udp_buffer) - 1,
0,
(struct sockaddr*)&source_address,
&source_address_len);
if (length == 0) {
return -EWOULDBLOCK;
}
if (length < 0) {
int const err = errno;
if (err != EWOULDBLOCK) {
perror("recvfrom failed");
}
return -err;
}
uint32_t const addr = ntohl(source_address.sin_addr.s_addr);
uint32_t const port = ntohs(source_address.sin_port);
if (length) {
unsigned char frame_type = udp_buffer[0];
if (frame_type == FT_HELLO) {
*frame = (struct frame){
.addr = addr,
.port = port,
.type = frame_type,
};
}
}
uint8_t const a0 = addr >> 24;
uint8_t const a1 = addr >> 16;
uint8_t const a2 = addr >> 8;
uint8_t const a3 = addr;
printf("Received %u from %u.%u.%u.%u:%u\n", udp_buffer[0], a0, a1, a2, a3, port);
return 0;
}
static int SendFrame(int const fd, struct frame const *const frame)
{
udp_buffer[0] = frame->type;
struct sockaddr_in const addr = {
.sin_family = AF_INET,
.sin_port = htons(frame->port),
.sin_addr.s_addr = htonl(frame->addr),
};
ssize_t const ret = sendto(
fd, udp_buffer, 1, 0, (struct sockaddr const *)&addr, sizeof(addr));
if (ret < 0) {
int const err = errno;
perror("sendto failed");
return -err;
}
printf("message sent\n");
return 0;
}
int main(void)
{
int const fd = socket(AF_INET, SOCK_DGRAM, 0);
if (fd < 0) {
perror("socket failed");
return 1;
}
fcntl(fd, F_SETFL, fcntl(fd, F_GETFL, 0) | O_NONBLOCK);
struct sockaddr_in const serveraddr = {
.sin_family = AF_INET,
.sin_port = htons(50037u),
.sin_addr.s_addr = htonl(INADDR_ANY),
};
if (bind(fd, (struct sockaddr *)&serveraddr, sizeof(serveraddr)) < 0) {
perror("bind failed");
return 1;
}
struct pollfd fds[1] = { { .fd = fd, .events = POLLIN, }, };
for (;;) {
int const timeout_msecs = 500;
int const pollret = poll(fds, sizeof(fds)/sizeof(*fds), timeout_msecs);
if (pollret > 0) {
if (fds[0].revents & POLLIN) {
while (1) {
struct frame frame = { .type = FT_NONE, };
int const ret = ReceiveFrame(fd, &frame);
if (ret) {
break;
}
if (frame.type == FT_HELLO) {
SendFrame(fd, &frame);
}
}
}
if (fds[0].revents & POLLRDNORM) {
printf("POLLRDNORM, ignoring...\n");
}
if (fds[0].revents & POLLRDBAND) {
printf("POLLRDBAND, ignoring...\n");
}
if (fds[0].revents & POLLPRI) {
printf("POLLPRI, ignoring...\n");
}
if (fds[0].revents & POLLPRI) {
printf("POLLPRI, ignoring...\n");
}
if (fds[0].revents & POLLHUP) {
printf("POLLHUP, ignoring...\n");
}
if (fds[0].revents & POLLERR) {
printf("POLLERR, ignoring...\n");
}
if (fds[0].revents & POLLNVAL) {
printf("POLLNVAL, exiting...\n");
return 1;
}
}
}
close(fd);
}
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