Doing an 'strace -ff -o food ls' gives an output file 'food' instead of 'food.pid_of_ls'. My guess is that strace has to fork() and then execve(), so it should be trivial to derive the pid of the child.
Fixed in 10de62bb052b541af3d0566f05f3f870cce0e028.
Fixed in 10de62bb052b541af3d0566f05f3f870cce0e028.