diff --git a/src/calibre/utils/ipc/launch.py b/src/calibre/utils/ipc/launch.py
index 9c50208935..e13c9e0cb6 100644
--- a/src/calibre/utils/ipc/launch.py
+++ b/src/calibre/utils/ipc/launch.py
@@ -183,6 +183,12 @@ class Worker(object):
             args['stdout'] = _windows_null_file
             args['stderr'] = subprocess.STDOUT
 
+        if not iswindows:
+            # Close inherited file descriptors in worker
+            # On windows, this is done in the worker process
+            # itself
+            args['close_fds'] = True
+
         self.child = subprocess.Popen(cmd, **args)
         if 'stdin' in args:
             self.child.stdin.close()
diff --git a/src/calibre/utils/ipc/worker.py b/src/calibre/utils/ipc/worker.py
index 42618f7996..4d23d5a928 100644
--- a/src/calibre/utils/ipc/worker.py
+++ b/src/calibre/utils/ipc/worker.py
@@ -47,6 +47,11 @@ PARALLEL_FUNCS = {
       ('calibre.ebooks.metadata.worker', 'save_book', 'notification'),
 }
 
+try:
+    MAXFD = os.sysconf("SC_OPEN_MAX")
+except:
+    MAXFD = 256
+
 class Progress(Thread):
 
     def __init__(self, conn):
@@ -80,7 +85,7 @@ def main():
     # Close open file descriptors inherited from parent
     # as windows locks open files
     if iswindows:
-        os.closerange(3, 1000)
+        os.closerange(3, MAXFD)
     from calibre.constants import isosx
     if isosx and 'CALIBRE_WORKER_ADDRESS' not in os.environ:
         # On some OS X computers launchd apparently tries to