The --directory=directory (-d directory) option to patch makes directory directory the current directory for interpreting both file names in the patch file, and file names given as arguments to other options (such as -B and -o). For example, while in a mail reading program, you can patch a file in the /usr/src/emacs directory directly from a message containing the patch like this:
| patch -d /usr/src/emacs
Sometimes the file names given in a patch contain leading directories, but you keep your files in a directory different from the one given in the patch. In those cases, you can use the --strip=number (-pnumber) option to set the file name strip count to number. The strip count tells patch how many slashes, along with the directory names between them, to strip from the front of file names. A sequence of one or more adjacent slashes is counted as a single slash. By default, patch strips off all leading directories, leaving just the base file names.
For example, suppose the file name in the patch file is /gnu/src/emacs/etc/NEWS. Using -p0 gives the entire file name unmodified, -p1 gives gnu/src/emacs/etc/NEWS (no leading slash), -p4 gives etc/NEWS, and not specifying -p at all gives NEWS.
patch looks for each file (after any slashes have been stripped) in the current directory, or if you used the -d directory option, in that directory.