return nonzero exit code on parse failure

src/main.cc

@@ -100,6 +100,7 @@ int main(int argc, char * argv[])
 {
     int opt;
     int option_index;
+    int rv = 0;
     const char * output_fname = nullptr;
 
     static const struct option long_options[] = {
@@ -118,7 +119,7 @@ int main(int argc, char * argv[])
 
     if (optind >= argc)
     {
-        return -2;
+        return 2;
     }
 
     const char * input_fname = argv[optind];
@@ -136,6 +137,10 @@ int main(int argc, char * argv[])
         {
             emit_c(node, output_fname);
         }
+        else
+        {
+            rv = 1;
+        }
     }
     /* Clean up temporary files. */
     if (preprocessed_fname_created)
@@ -143,5 +148,5 @@ int main(int argc, char * argv[])
         unlink(preprocessed_fname);
     }
 
-    return 0;
+    return rv;
 }