File: make.info,  Node: Avoiding Compilation,  Next: Overriding,  Prev: Instead of Execution,  Up: Running

9.4 Avoiding Recompilation of Some Files
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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 recipes in
the rules, but rather to mark the target up to date by changing its
last-modification date.  You would follow this procedure:

  1. 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.

  2. Make the changes in the header files.

  3. 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" (*note Summary
of Options: Options Summary.).  This means that the file itself will not
be remade, and nothing else will be remade on its account.  Follow this
procedure:

  1. 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.

  2. Touch all the object files with 'make -t'.