Dtrace on OSX 10.7 uses a negative number for the 3rd parameter.

tools/isolate/trace_inputs.py

 #!/usr/bin/env python
 # coding=utf-8
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Traces an executable and its child processes and extract the files accessed
 by them.
 
 The implementation uses OS-specific API. The native Kernel logger and the ETL
@@ skipping 696 matching lines @@
     """Processes a dtrace log line and keeps the list of existent and non
     existent files accessed.
 
     Ignores directories.
     """
     # This is the most common format. index pid function(args) = result
     RE_HEADER = re.compile(r'^\d+ (\d+):(\d+) ([a-zA-Z_\-]+)\((.*?)\) = (.+)$')
 
     # Arguments parsing.
     RE_CHDIR = re.compile(r'^\"(.+?)\"$')
-    RE_OPEN = re.compile(r'^\"(.+?)\", (\d+), (\d+)$')
+    RE_OPEN = re.compile(r'^\"(.+?)\", (\d+), (-?\d+)$')
     RE_RENAME = re.compile(r'^\"(.+?)\", \"(.+?)\"$')
 
     O_DIRECTORY = 0x100000
 
     def __init__(self, blacklist):
       # TODO(maruel): Handling chdir() and cwd in general on OSX is tricky
       # because OSX only keeps relative directory names. In addition, cwd is a
       # process local variable so forks need to be properly traced and cwd
       # saved.
       self._cwd = {}
@@ skipping 41 matching lines @@
           self._cwd[pid] = cwd2
         else:
           logging.debug('handle_chdir(%d, %s)' % (pid, cwd))
           self._cwd[pid] = cwd
       else:
         assert False, 'Unexecpected fail: %s' % result
 
     def handle_open_nocancel(self, ppid, pid, function, args, result):
       return self.handle_open(ppid, pid, function, args, result)
 
-    def handle_open(self, _ppid, pid, _function, args, result):
-      args = self.RE_OPEN.match(args).groups()
+    def handle_open(self, _ppid, pid, function, args, result):
+      match = self.RE_OPEN.match(args)
+      assert match, (pid, function, args, result)
+      args = match.groups()
       flag = int(args[1])
       if self.O_DIRECTORY & flag == self.O_DIRECTORY:
         # Ignore directories.
         return
       self._handle_file(pid, args[0], result)
 
     def handle_rename(self, _ppid, pid, _function, args, result):
       args = self.RE_RENAME.match(args).groups()
       self._handle_file(pid, args[0], result)
       self._handle_file(pid, args[1], result)
@@ skipping 836 matching lines @@
       os.path.abspath(options.log),
       args,
       options.root_dir,
       options.cwd,
       options.product_dir,
       options.force)
 
 
 if __name__ == '__main__':
   sys.exit(main())