Linking multiple languages

Question

If I compile c++ code, I am supposed to use g++. This automatically does some magic that calling just Gcc would not do. The same with Fortran code and gfortran. So my question is which frontend should I use to link if I have object files from both c++ as well as Fortran, c, and maybe another language still?

Answer 1

You can really do it in whatever way you want. If you have an application with mixed src code, you could compile it as FORTRAN and link to the c library, or compile as c and link to the FORTRAN library.
Whether you compile as c or c++ doesn't make any real difference.

You need separate compile rules for each compiler (c, c++, FORTRAN). These will compile the objects from your mixed src. Then you need a build rule the will link the objects, including one of the libraries.

Here is a sample, where your src is,

.src/main.cpp
.src/MY_FUNCTION1.FOR
.src/my_function2.cpp
.src/my_function2.c

SOURCELOC = ./src
BDIR = ./bld

# mixed app objects objects
MIXED_OBJS = 
 $(BDIR)/main.o 
 $(BDIR)/MY_FUNCTION1.o 
 $(BDIR)/my_function2.o 
 $(BDIR)/my_function3.o

# build my_app, build as gfortran and link to c library
$(BDIR)/my_mixed_app.exe: $(MIXED_OBJS)
 gfortran -o $@ $(MIXED_OBJS) -lstdc++

# compile src gfortran objects with FORTRAN preprocessor
$(BDIR)/%.o: $(SOURCELOC)/%.FOR
 gfortran -c -o $@ $<

# compile src c++ objects
$(BDIR)/%.o: $(SOURCELOC)/%.cpp
 g++ -c -o $@ $<

# compile src c objects
$(BDIR)/%.o: $(SOURCELOC)/%.c
 gcc -c -o $@ $<

You should also be able to compile your .c files as g++, but since you are using the file extension to indicate the compiler, you will need a rule that lets make know to use g++ for .c.

# compile src c objects
$(BDIR)/%.o: $(SOURCELOC)/%.c
 g++ -c -o $@ $<

You could do the same thing with a link rule to build as g++ and link to the fortran library.

# build my_app, build as g++ and link to fortran library
$(BDIR)/my_mixed_app.exe: $(MIXED_OBJS)
 g++ -o $@ $(MIXED_OBJS) -lgfortran

If I remember right, one of the reasons to build as FORTRAN and link to the c library is that many systems may not have a FORTRAN runtime library installed, but most will have a c library somewhere. I'm not 100% sure about that, so perhaps someone else will comment.

Answer 2

You can probably use gcc but explicitly link in each language's support library.