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// Initially based on https://stackoverflow.com/a/35570418
#include <netinet/in.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>
static unsigned char udp_buffer[UINT16_MAX];
static inline bool IsDigit(char c) { return c >= '0' && c <= '9'; }
static uint32_t ParseAddress(char const *str, FILE *log_stream)
{
uint32_t addr = 0;
unsigned byte_counter = 0, byte = 0, digits_counter = 0;
for (size_t i = 0;; i++) {
char const c = str[i];
if (c == '\0') {
if (byte_counter != 3) {
fprintf(log_stream, "Invalid address specified: %u bytes of 4 expected are specified\n", byte_counter + 1);
return INADDR_NONE;
}
if (digits_counter == 0) {
fprintf(log_stream, "Invalid address specified: no digits on byte %u\n", byte_counter + 1);
return INADDR_NONE;
}
addr |= byte << (8 * (3 - byte_counter));
break;
} else if (IsDigit(c)) {
digits_counter++;
if (digits_counter > 3) {
fprintf(log_stream, "Invalid address specified: too many digits for byte %u\n", byte_counter + 1);
return INADDR_NONE;
}
byte = byte * 10 + (c - '0');
if (byte > UINT8_MAX) {
fprintf(log_stream, "Invalid address specified: too big number for byte %u\n", byte_counter + 1);
return INADDR_NONE;
}
} else if (c == '.') {
if (digits_counter == 0) {
fprintf(log_stream, "Invalid address specified: no digits on byte %u\n", byte_counter + 1);
return INADDR_NONE;
}
digits_counter = 0;
addr |= byte << (8 * (3 - byte_counter));
byte = 0;
byte_counter++;
if (byte_counter > 3) {
fprintf(log_stream, "Invalid address specified: unexpected dot after 4th byte\n");
return INADDR_NONE;
}
} else {
fprintf(log_stream, "Invalid address specified: char '%c' (0x%02x) is not allowed\n", c, c);
return INADDR_NONE;
}
}
if (addr == INADDR_NONE) {
fprintf(log_stream, "Invalid address specified\n");
}
return addr;
}
int main(int const argc, char *const argv[])
{
if (argc < 3) {
fprintf(stderr, "Usage: %s <address> <port>\n", argv[0]);
return 1;
}
unsigned const addr = ParseAddress(argv[1], stderr);
if (addr == INADDR_NONE) {
return 1;
}
int const port = atoi(argv[2]);
if (port <= 0 || port > UINT16_MAX) {
fprintf(stderr, "Invalid port specified\n");
return 1;
}
int const fd = socket(AF_INET, SOCK_DGRAM, 0);
if (fd < 0) {
perror("socket failed");
return 1;
}
struct sockaddr_in const serveraddr = {
.sin_family = AF_INET,
.sin_port = htons(port),
.sin_addr.s_addr = htonl(addr),
};
ssize_t const ret = sendto(
fd, "\x2a", 1, 0, (struct sockaddr const *)&serveraddr, sizeof(serveraddr));
if (ret < 0) {
perror("sendto failed");
return 1;
}
printf("message sent\n");
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);
(void) length;
uint32_t const addr2 = ntohl(source_address.sin_addr.s_addr);
uint8_t const a0 = addr2 >> 24;
uint8_t const a1 = addr2 >> 16;
uint8_t const a2 = addr2 >> 8;
uint8_t const a3 = addr2;
uint16_t const port2 = ntohs(source_address.sin_port);
printf("Received: %u from %u.%u.%u.%u:%u\n", udp_buffer[0], a0, a1, a2, a3, port2);
close(fd);
}
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