Building a Static Library

Organizing code in function libraries is a good practice that can help to avoid code duplication by increasing code reuse.

Suppose to have a header file my_stuff.h with all declarations to be exposed as the library API, for example:

#ifndef MY_STUFF_H_
#define MY_STUFF_H_

int my_stuff_foo(int, int);

#endif  // MY_STUFF_H_

then we have the source code for my_stuff_foo function in my_stuff.cc:

int my_stuff_foo(int a, int b) {
    int result;
    
    result = a + b;
    
    return result;
}

To build the library we need the following Makefile, which:

1. Compiles my_stuff.cc into my_stuff.o

$(CXX) $(CXXFLAGS) -c my_stuff.cc

1. Invokes the archiver ar to produce a static library (named libmystuff.a) out of the object file my_stuff.o.

ar rcs libmystuff.a my_stuff.o

Note that the library must start with the three letters lib and have the suffix .a.

Here’s the complete source of the Makefile:

CXX=g++
CXXFLAGS=-g -std=c++11 -Wall -pedantic
INCLUDES=-I./includes
CXXFLAGS+=$(INCLUDES)

all: my_stuff_lib

my_stuff.o:
	$(CXX) $(CXXFLAGS) -c my_stuff.cc

my_stuff_lib: my_stuff.o
	ar rcs libmystuff.a my_stuff.o

clean:
	rm -f *.o
	rm -f *.a

Linking The Library

We now have our libmystuff.a file, which is basically a collection of all object files compiled into a single file.

To use the functions we packed into libmystuff.a in our next project we have to link it. To link our library we need to tell the compiler its name and where it is located.

To do this we use two Makefile variables:

1. LDLIBS, which contains all libraries names we need to link.
2. LIBS, which contains all non standard search paths to look for to find the libraries we need.

Note we also need to tell the compiler where it’s located the header file which contains all libmystuff.a APIs.

Here’s the source of the Makefile:

CXX=g++
CXXFLAGS=-g -std=c++11 -Wall -pedantic
INCLUDES=-I. -I<path/to/my_stuff.h>
CXXFLAGS+=$(INCLUDES)
LDLIBS=-lmystuff
LIBS=-L<path/to/libmystuff.a>

all: my_cool_project

my_cool_project: my_cool_project.o
	$(CXX)							\
	$(CXXFLAGS)						\
	my_cool_project.cc					\
	-o							\
	my_cool_project						\
	$(LDLIBS)						\
	$(LIBS)

clean:
	rm -f *.o
	rm -f my_cool_project

This Makefile can be used to build my_cool_project.cc which can use all API functions exposed by libmystuff.a, for example:

#include <iostream>

#include "my_stuff.h"

int main(int argc, char* argv[]) {

    int result = my_stuff_foo(42, 23);

    std::cout << "my_stuff_foo() result is:" << result << std::endl;

    return 0;
}