( Avoiding Compilation

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 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:
   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" (
 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'.
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