Creating a library file in makefile and compiling

Question

My problem is fairly easy but I just don't know how to solve it. I know how to compile and library and link against it if I'm not using a make file because then I can just call ar separately and everything goes right.

Anyway I'm using a petsc library and I'm using a makefile what they provided:

CFLAGS          = 
FFLAGS          = 
CPPFLAGS        = 
FPPFLAGS        =
LOCDIR          = /home/user/.../.../   # Working folder
EXAMPLESC       = main.cpp class.cpp        #.cpp file names here
EXAMPLESF       =
#MANSEC          = Mat I don't know what this is but it seems to work without it.

include ${PETSC_DIR}/conf/variables
include ${PETSC_DIR}/conf/rules

myProgram: main.o class.o  chkopts
    -${CLINKER}  -o myProgram main.o class.o ${PETSC_MAT_LIB}
    ${RM} main.o class.o

include ${PETSC_DIR}/conf/test

ARFLAGS will be -rv as a default so where should I provide such a information as

ar -rv libclassdll.a class.o

and where should I add -L./-lclassdll ?

I'm quite a rookie with makefiles so that's why I'm a bit lost here :<

I tried to change the line to

myProgram: main.o class.o  chkopts
    -${CLINKER}  -o myProgram main.o class.o ${AR} libclassdll.a class.o ${PETSC_MAT_LIB}
    ${RM} main.o class.o

and then my compiling command seems to be mpicxx -o myProgram main.o class.o /usr/bin/ar/ libclassdll.a class.o -L ( a lot of linking here ) and at least it says: g++ classdll.a no such a file or dir.

So it doesn't generate even a lib file for me. So any ideas will be really appreciated.

A new problem when I uploaded makefile on the different machine, my current makefile looks like this

LibMyClass.so: MyClass.o  chkopts
    -${CLINKER}  -shared -Wl,-soname,${SONAME} -o ${VERS}   *.o  ${PETSC_MAT_LIB}

    mv ${VERS} ${LIBADD}
    ln -sf ${LIBADD}${VERS} ${LIBADD}${SOWOV}
    ln -sf ${LIBADD}${VERS} ${LIBADD}${SONAME}

That works on one machine but other machine gives following error

/usr/bin/ld: MyClass.o: relocation R_X86_64_32S against `.rodata' can not be used when making a shared object; recompile with -fPIC
MyClass.o: could not read symbols: Bad value

I did change the paths of course but I guess that indicates other kind of problem because even if I type "g++ -shared -Wl,-soname,libmyclass.so.1 -o libmyclass.so.1.0 MyClass.o" or "g++ -fPIC -share... " I'll get the same error.

Answer 1

Make myProgram depend on main.o and on the libclass.a (don't call it libclassdll.a; it is not a DLL, but a static library).

General gist of the solution:

# $@ means "the target of the rule"
# $^ means "the prerequisites: main.o and libclass.a"

myProgram: main.o libclass.a
        $(CC) -o $@ $^ # additional arguments for other libraries, not built in this make

libclass.a: class.o
        # your $(AR) command goes here to make the library

Answer 2

Ideally you should construct the library first, then use it, just as you would "by hand".

To construct (or update) the library, you need a rule something like this:

libclassdll.a: class.o
    ar -rv libclassdll.a class.o

Or more concisely, like this:

libclassdll.a: class.o
    ar $(ARFLAGS) $@ $^

Then the rule for myProgram becomes:

# Assuming CLINKER is something civilized, like gcc
myProgram: main.o libclassdll.a  chkopts
    -${CLINKER} -o myProgram main.o -L. -lclassdll ${PETSC_MAT_LIB}

or better:

myProgram: main.o libclassdll.a  chkopts
    -${CLINKER} -o $@ $< -L. -lclassdll ${PETSC_MAT_LIB}

So in your makefile, you would replace

myProgram: main.o class.o  chkopts
    -${CLINKER}  -o myProgram main.o class.o ${PETSC_MAT_LIB}
    ${RM} main.o class.o

with

myProgram: main.o libclassdll.a  chkopts
    -${CLINKER} -o $@ $< -L. -lclassdll ${PETSC_MAT_LIB}

libclassdll.a: class.o
    ar $(ARFLAGS) $@ $^

There are other refinements you can make, but that should be enough for now.