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 builddir/objdir
cd srcdir

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

I know that it’s possible to achive this with separate -o options given to the compiler, e.g.:

gcc -c file1.c -o ../builddir/objdir/file1.o
gcc -c file2.c -o ../builddir/objdir/file2.o
gcc -c file3.c -o ../builddir/objdir/file3.o

… and I know that I can write Makefiles via VPATH and vpath directives to simplify this.

But that’s a lot of work in a complex build environment.

I could also use

gcc -c file1.c file2.c file3.c

But when I use this approach my srcdir is full of .o garbage afterwards.

So I think that an option with the semantics of –outdir would be very useful.

What is your opinion?

EDIT: our Makefiles are written in such a way that .o files actually placed into builddir/obj. But I am simply wondering if there might be a better approach.

EDIT: There are several approaches which place the burden to achieve the desired behavior to the build system (aka Make, CMake etc.). But I consider them all as being workarounds for a weakness of gcc (and other compilers too).

Answer 1

This is the chopped down makefile for one of my projects, which compiles the sources in ‘src’ and places the .o files in the directory “obj”. The key bit is the the use of the patsubst() function – see the GNU make manual (which is actually a pretty good read) for details:

OUT = lib/alib.a
CC = g++
ODIR = obj
SDIR = src
INC = -Iinc

_OBJS = a_chsrc.o a_csv.o a_enc.o a_env.o a_except.o \
        a_date.o a_range.o a_opsys.o
OBJS = $(patsubst %,$(ODIR)/%,$(_OBJS))


$(ODIR)/%.o: $(SDIR)/%.cpp 
    $(CC) -c $(INC) -o $@ $< $(CFLAGS) 

$(OUT): $(OBJS) 
    ar rvs $(OUT) $^

.PHONY: clean

clean:
    rm -f $(ODIR)/*.o $(OUT)