Reading commit objects

Hello,

I'm building a library to manipulate git repositories (interacting directly with the filesystem).

Currently, we're trying to parse commit objects. After decompressing the contents of a commit object file we got the following output:

commit 191 author Francisco Sokol <chico.sokol@gmail.com> 1369140112 -0300 committer Francisco Sokol <chico.sokol@gmail.com> 1369140112 -0300

first commit

We hoped to get the same output of a "git cat-file -p <sha1>", but that didn't happened. From a commit object, how can I find tree object hash of this commit?

Thanks,

-- Chico Sokol