Sometimes you may have changed a source file but you do not want to
recompile all the files that depend on it. For example, suppose you add
a macro or a declaration to a header file that many other files depend
on. Being conservative,
make assumes that any change in the
header file requires recompilation of all dependent files, but you know
that they do not need to be recompiled and you would rather not waste
the time waiting for them to compile.
If you anticipate the problem before changing the header file, you can
use the ‘-t’ flag. This flag tells
make not to run the
commands in the rules, but rather to mark the target up to date by
changing its last-modification date. You would follow this procedure:
make, the changes in the header files will not cause any recompilation.
If you have already changed the header file at a time when some files do need recompilation, it is too late to do this. Instead, you can use the ‘-o file’ flag, which marks a specified file as “old” (see Summary of Options). This means that the file itself will not be remade, and nothing else will be remade on its account. Follow this procedure: