9.4 Avoiding Recompilation of Some Files
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:
Use the command `make' to recompile the source files that really
need recompilation, ensuring that the object files are up-to-date
before you begin.
Make the changes in the header files.
Use the command `make -t' to mark all the object files as
up to date. The next time you run
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 section 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:
Recompile the source files that need compilation for reasons independent
of the particular header file, with `make -o headerfile'.
If several header files are involved, use a separate `-o' option
for each header file.
Touch all the object files with `make -t'.