EtherCat/ethercat.c

/*
  Copyright 2025 Interzero Product Cycle GmbH

  This program is free software under the terms of the BSD-3-Clause license.
  A copy of this license should have been provided with the code.
  If not, see https://opensource.org/license/BSD-3-Clause or https://spdx.org/licenses/BSD-3-Clause.html
*/

#include <errno.h>
#include <fcntl.h>
#include <linux/if.h>
#include <linux/if_tun.h>
#include <poll.h>
#include <signal.h>
#include <stdbool.h>
#include <stdio.h>
#include <string.h>
#include <sys/ioctl.h>
#include <unistd.h>

#define DEVICE_NODE_TUN "/dev/net/tun"
// This number mus be larger than the MTU.
#define BUFFER_SIZE 0x1000
#define NUMBER_OF_POLLED_FILES 2
#define POLL_TIMEOUT 100

#define EXIT_SUCCESS 0
#define EXIT_USAGE 1
#define EXIT_RUNTIME 2

int file_descriptor;
int poll_result;
char buffer[BUFFER_SIZE];
struct ifreq ifr;
bool stop_now = false;
ssize_t read_size;
ssize_t write_size;
// 0 file_descriptor
// 1 stdin
struct pollfd polls_pls[NUMBER_OF_POLLED_FILES];

void stop() {
  stop_now = true;
}

void yeet(int fd) {
  if (fcntl(fd, F_GETFD) != -1) {
    close(fd);
  }
}

int main(int argc, char** argv) {
  if (argc != 2) {
    fprintf(stderr, "Usage: %s TAP\n", argv[0]);
    return EXIT_USAGE;
  } else if ((strlen(argv[1])==2 && strncmp(argv[1], "-h", 2)==0) || (strlen(argv[1])==6 && strncmp(argv[1], "--help", 6)==0)) {
    fprintf(stdout, "Usage: %s TAP\n", argv[0]);
    return EXIT_SUCCESS;
  }

  if (freopen(NULL, "rb", stdin) == NULL) {
    fprintf(stderr, "Failed to set stdin to binary mode: %s\n", strerror(errno));
    return EXIT_RUNTIME;
  }

  polls_pls[1].fd = fileno(stdin);
  polls_pls[1].events = POLLIN;

  if (freopen(NULL, "wb", stdout) == NULL) {
    fprintf(stderr, "Failed to set stdout to binary mode: %s\n", strerror(errno));
    return EXIT_RUNTIME;
  }

  file_descriptor = open(DEVICE_NODE_TUN, O_RDWR);
  if (file_descriptor == -1) {
    fprintf(stderr, "Failed to open %s: %s\n", DEVICE_NODE_TUN, strerror(errno));
    return EXIT_RUNTIME;
  }

  memset(&ifr, 0, sizeof(ifr));
  ifr.ifr_flags = IFF_TAP | IFF_NO_PI;
  strncpy(ifr.ifr_name, argv[1], IFNAMSIZ);

  if (ioctl(file_descriptor, TUNSETIFF, &ifr) == -1) {
    fprintf(stderr, "An error occurred while trying to ioctl: %s\n", strerror(errno));
    close(file_descriptor);
    return EXIT_RUNTIME;
  }

  polls_pls[0].fd = file_descriptor;
  polls_pls[0].events = POLLIN;

  signal(SIGINT, stop);
  signal(SIGTERM, stop);

  while (fcntl(file_descriptor, F_GETFD) != -1 && !stop_now) {
    poll_result = poll(polls_pls, NUMBER_OF_POLLED_FILES, POLL_TIMEOUT);
    if (poll_result == -1 && errno != EINTR) {
      fprintf(stderr, "An error occurred while polling: %s\n", strerror(errno));
      yeet(file_descriptor);
      return EXIT_RUNTIME;
    }
    if (poll_result == 0) {
      continue;
    }

    if (polls_pls[0].revents & POLLIN) {
      read_size = read(file_descriptor, buffer, BUFFER_SIZE);
      if (read_size == -1) {
        fprintf(stderr, "Failed to read from tap: %s\n", strerror(errno));
        yeet(file_descriptor);
        return EXIT_RUNTIME;
      }
      for (
        write_size = 0;
        read_size > 0 && !stop_now;
        read_size = read_size - write_size
      ) {
        // using read_size as the amount of remaining bytes
        write_size = write(fileno(stdout), buffer, read_size);
        if (write_size == -1) {
          fprintf(stderr, "Failed to write stdout: %s\n", strerror(errno));
          yeet(file_descriptor);
          return EXIT_RUNTIME;
        }
      }
    }

    if (polls_pls[1].revents & POLLIN) {
      read_size = read(fileno(stdin), buffer, BUFFER_SIZE);
      if (read_size == -1) {
        fprintf(stderr, "Failed to read from stdin: %s\n", strerror(errno));
        yeet(file_descriptor);
        return EXIT_RUNTIME;
      }
      if (read_size == 0) {
        //TODO: is this still how to check for eof with poll?
        // or is there another way?
        break;
      }

      //TODO: How to properly deal with partial reads?

      // accepts exactly one complete packet (Ethernet Frame?) at a time
      write_size = write(file_descriptor, buffer, read_size);
      if (write_size == -1) {
        fprintf(stderr, "Failed to write tap: %s\n", strerror(errno));
        yeet(file_descriptor);
        return EXIT_RUNTIME;
      }
    }
  }

  yeet(file_descriptor);
  return EXIT_SUCCESS;
}