complete initial implementation

Never tested. Let's see how many bugs we find.

ethercat.c

@@ -2,12 +2,14 @@
 #include <fcntl.h>
 #include <linux/if.h>
 #include <linux/if_tun.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
+#define DEVICE_NODE_TUN "/dev/net/tun"
 #define BUFFER_SIZE 0x1000
 
 #define EXIT_SUCCESS 0
@@ -17,6 +19,27 @@
 int file_descriptor;
 char buffer[BUFFER_SIZE];
 struct ifreq ifr;
+bool stop_now = false;
+ssize_t read_size;
+ssize_t write_size;
+bool just_created_dup;
+int stdin_dup;
+
+void stop() {
+  stop_now = true;
+}
+
+void yeet(int fd) {
+  if (fcntl(fd, F_GETFD) != -1) {
+    close(fd);
+  }
+}
+
+void recreate_dup() {
+  just_created_dup = true;
+  yeet(stdin_dup);
+  stdin_dup = dup(fileno(stdin));
+}
 
 int main(int argc, char** argv) {
   if (argc != 2) {
@@ -27,32 +50,88 @@ int main(int argc, char** argv) {
     return EXIT_SUCCESS;
   }
 
+  if (freopen(NULL, "rb", stdin) == NULL) {
+    fprintf(stderr, "Failed to set stdin to binary mode: %i\n", errno);
+    return EXIT_RUNTIME;
+  }
+
+  stdin_dup = dup(fileno(stdin));
+  if (stdin_dup == -1) {
+    fprintf(stderr, "Failed to dup() stdin: %i\n", errno);
+  }
+
+  if (freopen(NULL, "wb", stdout) == NULL) {
+    fprintf(stderr, "Failed to set stdout to binary mode: %i\n", errno);
+    return EXIT_RUNTIME;
+  }
+
+  file_descriptor = open(DEVICE_NODE_TUN, O_RDWR | O_NONBLOCK);
+  if (file_descriptor == -1) {
+    fprintf(stderr, "Failed to open %s: %i\n", DEVICE_NODE_TUN, errno);
+    return EXIT_RUNTIME;
+  }
+
   memset(&ifr, 0, sizeof(ifr));
   ifr.ifr_flags = IFF_TAP | IFF_NO_PI;
   strncpy(ifr.ifr_name, argv[1], IFNAMSIZ);
 
-  file_descriptor = open("/dev/net/tun", O_RDWR);
-  if (file_descriptor == -1) {
-    fprintf(stderr, "An error occurred while trying to open DEVICE_NODE_TUN: %i\n", errno);
-    return EXIT_RUNTIME;
-  }
-
   if (ioctl(file_descriptor, TUNSETIFF, &ifr) == -1) {
     fprintf(stderr, "An error occurred while trying to ioctl: %i\n", errno);
     close(file_descriptor);
     return EXIT_RUNTIME;
   }
 
-  //TODO:
-  // while input not EOF
-  // and the tap is still a valid device (it might go away)
-  // read from tap to stdout
-  // read from stdin to tap
+  signal(SIGINT, stop);
+  signal(SIGTERM, stop);
+  signal(SIGALRM, recreate_dup);
 
-  //TODO: signal handler for SIGINT (and SIGHUP? maybe others?)
+  ualarm(500000, 500000);
 
-  if (fcntl(fd, F_GETFD) != -1 || errno != EBADF) {
-    close(file_descriptor);
+  while (fcntl(file_descriptor, F_GETFD) != -1 && !stop_now) {
+    read_size = read(file_descriptor, buffer, BUFFER_SIZE);
+    if (read_size == -1) {
+      fprintf(stderr, "Failed to read from tap: %i\n", 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: %i\n", errno);
+        yeet(file_descriptor);
+        return EXIT_RUNTIME;
+      }
+    }
+    just_created_dup = false;
+    read_size = read(stdin_dup, buffer, BUFFER_SIZE);
+    if (read_size == -1) {
+      fprintf(stderr, "Failed to read from stdin: %i\n", errno);
+      yeet(file_descriptor);
+      return EXIT_RUNTIME;
+    }
+    if (read_size == 0 && !just_created_dup) {
+      // probably EOF
+      break;
+    }
+    for (
+      write_size = 0;
+      read_size > 0 && !stop_now;
+      read_size = read_size - write_size
+    ) {
+      write_size = write(file_descriptor, buffer, read_size);
+      if (write_size == -1) {
+        fprintf(stderr, "Failed to write stdout: %i\n", errno);
+        yeet(file_descriptor);
+        return EXIT_RUNTIME;
+      }
+    }
   }
+
+  yeet(file_descriptor);
   return EXIT_SUCCESS;
 }