Fix for issue 6768

runtime/bin/gen_snapshot.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Generate a snapshot file after loading all the scripts specified on the
 // command line.
 
 #include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
@@ skipping 117 matching lines @@
   }
 
   // Get the script name.
   if (i < argc) {
     *script_name = argv[i];
     i += 1;
   } else {
     *script_name = NULL;
   }
 
+  if (snapshot_filename == NULL) {
+    Log::PrintErr("No snapshot output file specified.\n\n");
+    return -1;
+  }
+
   return 0;
 }
 
 
 static void WriteSnapshotFile(const uint8_t* buffer, const intptr_t size) {
   File* file = File::Open(snapshot_filename, File::kWriteTruncate);
   ASSERT(file != NULL);
   for (intptr_t i = 0; i < size; i++) {
     file->WriteByte(buffer[i]);
   }
@@ skipping 203 matching lines @@
   Dart_Handle lib;
   // Load the builtin library to make it available in the snapshot
   // for importing.
   lib = Builtin::LoadAndCheckLibrary(id);
   ASSERT(!Dart_IsError(lib));
   return lib;
 }
 
 
 static void PrintUsage() {
-  Log::PrintErr("dart [<vm-flags>] [<dart-script-file>]\n");
+  Log::PrintErr(
+"Usage:\n"
+"\n"
+"  gen_snapshot [<vm-flags>] [<options>] \\\n"
+"               {--script_snapshot=<out-file> | --snapshot=<out-file>} \\\n"
+"               [<dart-script-file>]\n"
+"\n"
+"  Writes a snapshot of <dart-script-file> to <out-file>. If no\n"
+"  <dart-script-file> is passed, a generic snapshot of all the corelibs is\n"
+"  created. One of the following is required:\n"
+"\n"
+"    --script_snapshot=<file>   Generates a script snapshot.\n"
+"    --snapshot=<file>          Generates a complete snapshot. Uses the url\n"
+"                               mapping specified on the command line to load\n"
+"                               the libraries.\n"
+"Supported options:\n"
+"\n"
+"--package_root=<path>\n"
+"  Where to find packages, that is, \"package:...\" imports.\n"
+"\n"
+"--url_mapping=<mapping>\n"
+"  Uses the URL mapping(s) specified on the command line to load the\n"
+"  libraries. For use only with --snapshot=.\n");
 }
 
 
 static void VerifyLoaded(Dart_Handle library) {
   if (Dart_IsError(library)) {
     const char* err_msg = Dart_GetError(library);
     Log::PrintErr("Errors encountered while loading: %s\n", err_msg);
     Dart_ExitScope();
     Dart_ShutdownIsolate();
     exit(255);
@@ skipping 68 matching lines @@
 
   // Parse command line arguments.
   if (ParseArguments(argc,
                      argv,
                      &vm_options,
                      &app_script_name) < 0) {
     PrintUsage();
     return 255;
   }
 
-  if (snapshot_filename == NULL) {
-    Log::PrintErr("No snapshot output file specified\n");
-    return 255;
-  }
-
   DartUtils::SetOriginalWorkingDirectory();
 
   Dart_SetVMFlags(vm_options.count(), vm_options.arguments());
 
   // Initialize the Dart VM.
   // Note: We don't expect isolates to be created from dart code during
   // snapshot generation.
   if (!Dart_Initialize(NULL, NULL, NULL, NULL)) {
     Log::PrintErr("VM initialization failed\n");
     return 255;
@@ skipping 107 matching lines @@
       CreateAndWriteSnapshot(true);
 
       free(snapshot_buffer);
     }
   } else {
     SetupForGenericSnapshotCreation();
     CreateAndWriteSnapshot(false);
   }
   return 0;
 }