How does format-patch determine the filename of the patch?

Hi.  I'm not familiar with the git source code, so forgive me if this
is a dumb question.  I'm trying to determine the algorithm that
git-format-patch uses to determine the name of the patch file it
creates (e.g. "0001-this-is-my-patch-without-spaces-and-trunca.patch")
 I'm looking at the function cmd_format_patch(), and I just don't see
where it creates any files.  Can someone show me where this code is?

The reason I ask is that I'm writing a script which calls
git-format-patch to create some patches for post-processing.  So I
need the name of the file that git-format-patch creates so that I can
open it and examine it.  I'd liked to see if there's a way to get the
name of the patch without actually creating the file.

-- 
Timur Tabi
Linux kernel developer at Freescale