Node:Patching Time Stamps, Next:Multiple Patches, Up:Merging with patch

### Updating Time Stamps on Patched Files

When `patch` updates a file, it normally sets the file's last-modified time stamp to the current time of day. If you are using `patch` to track a software distribution, this can cause `make` to incorrectly conclude that a patched file is out of date. For example, if `syntax.c` depends on `syntax.y`, and `patch` updates `syntax.c` and then `syntax.y`, then `syntax.c` will normally appear to be out of date with respect to `syntax.y` even though its contents are actually up to date.

`patch` normally refrains from setting a file's time stamps if the file's original last-modified time stamp does not match the time given in the diff header, of if the file's contents do not exactly match the patch. However, if the `-f` or `--force` option is given, the file's time stamps are set regardless.

Due to the limitations of the current `diff` format, `patch` cannot update the times of files whose contents have not changed. Also, if you set file time stamps to values other than the current time of day, you should also remove (e.g., with `make clean`) all files that depend on the patched files, so that later invocations of `make` do not get confused by the patched files' times.