And now rewrite half of it bc the previous approach didn't actually work...

ethercat.c

@@ -2,6 +2,7 @@
 #include <fcntl.h>
 #include <linux/if.h>
 #include <linux/if_tun.h>
+#include <poll.h>
 #include <signal.h>
 #include <stdbool.h>
 #include <stdio.h>
@@ -10,20 +11,25 @@
 #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;
-bool just_created_dup;
-int stdin_dup;
+// 0 file_descriptor
+// 1 stdin
+struct pollfd polls_pls[NUMBER_OF_POLLED_FILES];
 
 void stop() {
   stop_now = true;
@@ -35,12 +41,6 @@ void yeet(int 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) {
     fprintf(stderr, "Usage: %s TAP\n", argv[0]);
@@ -55,18 +55,15 @@ int main(int argc, char** argv) {
     return EXIT_RUNTIME;
   }
 
-  stdin_dup = dup(fileno(stdin));
-  if (stdin_dup == -1) {
-    fprintf(stderr, "Failed to dup() stdin: %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 | O_NONBLOCK);
+  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;
@@ -82,51 +79,64 @@ int main(int argc, char** argv) {
     return EXIT_RUNTIME;
   }
 
+  polls_pls[0].fd = file_descriptor;
+  polls_pls[0].events = POLLIN;
+
   signal(SIGINT, stop);
   signal(SIGTERM, stop);
-  signal(SIGALRM, recreate_dup);
-
-  ualarm(500000, 500000);
 
   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: %s\n", strerror(errno));
+    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;
     }
-    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));
+    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;
+        }
+      }
     }
-    just_created_dup = false;
-    read_size = read(stdin_dup, 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 && !just_created_dup) {
-      // probably EOF
-      break;
-    }
-    for (
-      write_size = 0;
-      read_size > 0 && !stop_now;
-      read_size = read_size - write_size
-    ) {
+
+    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 stdout: %s\n", strerror(errno));
+        fprintf(stderr, "Failed to write tap: %s\n", strerror(errno));
         yeet(file_descriptor);
         return EXIT_RUNTIME;
       }