How to apply a patch as part of a pipe? In other words, how to patch stdin?

I want to apply a unified diff from mypatch.diff to stdin and output the result to stdout. So far, I have tried: patch -i mypatch.diff -o - -u originalfile Which successfully applies mypatch.diff and prints the result to stdout. However, I still have to provide the original file as originalfile, not via stdin. And if I try something like: patch -i mypatch.diff -o - -u - Then the patch gets rejected:

patching file -
Hunk #1 FAILED at 1.
1 out of 1 hunk FAILED -- saving rejects to file -.rej