gcc/g++ option to place all object files into separate directory

Question

I am wondering why gcc/g++ doesn\'t have an option to place the generated object files into a specified directory.

For example:

mkdir builddir
mkdir          


        

Answer 1

I believe you got the concept backwards...?!

The idea behind Makefiles is that they only process the files that have been updated since the last build, to cut down on (re-)compilation times. If you bunch multiple files together in one compiler run, you basically defeat that purpose.

Your example:

gcc -c file1.c file2.c file3.c **--outdir=**../builddir/objdir

You didn't give the 'make' rule that goes with this command line; but if any of the three files has been updated, you have to run this line, and recompile all three files, which might not be necessary at all. It also keeps 'make' from spawning a seperate compilation process for each source file, as it would do for seperate compilation (when using the '-j' option, as I would strongly suggest).

I wrote a Makefile tutorial elsewhere, which goes into some extra detail (such as auto-detecting your source files instead of having them hard-coded in the Makefile, auto-determining include dependencies, and inline testing).

All you would have to do to get your seperate object directory would be to add the appropriate directory information to the OBJFILES := line and the %.o: %.c Makefile rule from that tutorial. Neil Butterworth's answer has a nice example of how to add the directory information.

(If you want to use DEPFILES or TESTFILES as described in the tutorial, you'd have to adapt the DEPFILES := and TSTFILES := lines plus the %.t: %.c Makefile pdclib.a rule, too.)