I/O fixed and tested

src/file.c

@@ -37,6 +37,10 @@ int closeFile(int fd) {
 }
 
 int readByte(const int fd, uint8_t *data) {
+  if (files[fd] == NULL) {
+    return STATUS_READY | STATUS_ERROR;
+  }
+
   int status = 0;
   int count = fread(data, 1, 1, files[fd]);
   // Error or EOF
@@ -54,17 +58,18 @@ int readByte(const int fd, uint8_t *data) {
 }
 
 int writeByte(const int fd, const uint8_t data) {
+  if (files[fd] == NULL) {
+    return STATUS_READY | STATUS_ERROR;
+  }
+
   int status = 0;
   int count = fwrite(&data, 1, 1, files[fd]);
   // Error or EOF
-  if (count != 1) {
-    int error = ferror(files[fd]);
-    int eof = feof(files[fd]);
-    if (error != 0) {
-      status = STATUS_ERROR;
-    } else if (eof != 0) {
-      status = STATUS_EOF;
-    }
+  if (count != 1 && ferror(files[fd]) != 0) {
+    return STATUS_READY | STATUS_ERROR;
+  }
+  if (fflush(files[fd]) != 0) {
+    return STATUS_READY | STATUS_ERROR;
   }
 
   return STATUS_READY | status;