How to Use Your Own Static Libraries with g++ like the Standard Libraries

[Jonathan Wakely]

On 29 June 2011 22:11, GM wrote:
> I have my own libraries I've written in C++.  So for instance, I have
> written "cartman.h" and "cartman.cpp" and I have managed to end up
> with "cartman.h" and "libcartman.a" for use in my own programs.
>
> The issue I'm having is that I would like to be able to use my own
> libraries in the same manner as I can use the standard C++ libraries.
>
> If I do something like...
>
> #include <iostream>
> #include <vector>
>
>  ....then when I compile my program, g++ auto-magically finds the
> correct libraries and links them up with my code.
>
> On the other hand, if I try to use my libraries in this way....
>
> #include "cartman.h"
>
> ...then I must go the extra step of telling g++ to find and use my
> library by name...
>
> g++ -o myprogram.exe -lcartman object1.o object2.o [etc]
>
> This extra step seems not nearly as elegant as using the standard
> libraries, and also seems like it would quickly get unwieldy and
> tedious in a large program.
>
>
>
> So I considered doing something like this in my makefile...
>
> mylibs = -lcartman -lkenny -lstan -letc
> CC = g++ -L/path/to/mylibs/  $(mylibs)
>
> program.exe: program.o object1.o object2.o [etc]
>            $(CC) -o program.exe program.o object1.o object2.o [etc]
>
> However, this results in warning messages from ld whenever any of the
> libraries specified aren't used to link with anything.  For instance,
> if I #included "cartman.h" and "kenny.h" but did not use "stan.h",
> then when I compile my program, ld warns me that "libstan.a" was not
> used during the compilation of my program.  Also very irritating and
> inelegant.  I would rather not get into the habit of just ignoring
> tons of error messages from my linker.
>
> I would like to be able to just #include "cartman.h" and have g++
> magically Do The Right Thing (TM) and link with libcartman.a if
> necessary, and just not use libcartman.a if it's not necessary.  Just
> like the way things Just Work if you include a standard library like
> <vector> or <iostream>.
>
> How can I do that?

You can't with GCC. Some compilers support an auto-linking feature via
a special #pragma, but GCC doesn't.

One solution is to fix your makefile, as the problem comes because you
have unneeded libs in the $(CC) command. Instead of always listing all
the libraries in $(CC) consider:

CC=gcc
LDFLAGS = -L/path/to/mylibs/

program.exe: program.o object1.o object2.o [etc]
           $(CC) -o $@ $^ $(LDFLAGS) -lcartman -lkenny

i.e. program.exe only links to the required libraries.

You can also do that like this:

program.exe: LIBS += -lcartman -lkenny
program.exe: program.o object1.o object2.o [etc]
           $(CC) -o $@ $^ $(LDFLAGS) $(LIBS)

$ make
gcc    -c -o program.o program.c
gcc    -c -o object1.o object1.c
gcc    -c -o object2.o object2.c
gcc -o program.exe program.o object1.o object2.o -L. -lcartman -lkenny

The advantage is you use the same compilation rule for other programs,
just changing $(LIBS)

prog2.exe: LIBS += -lstan -lkenny
prog2.exe: prog2.o object1.o object3.o [etc]
           $(CC) -o $@ $^ $(LDFLAGS) $(LIBS)

$ make prog2.exe
gcc    -c -o prog2.o prog2.c
gcc    -c -o object3.o object3.c
gcc -o prog2.exe prog2.o object1.o object3.o -L. -lstan -lkenny