This example shows how to separate multiple functions into a separate library. Libraries can be statically or dynamically linked.
Functions referenced by a program to a statically linked library are resolved at compile time. In Linux, static libraries contain the name lib<name>.a. In Windows, static libraries are named <name>.lib.
To build both the main executable and static library, run the following commands:
mkdir build
cd build
cmake -DCMAKE_BUILD_TYPE=Release ..
make
The files will be build in build and build/lib. The ex06p2 program can then be installed on the system path by running make install. At compile time, the machine code from the static library is essentially copied into the binary executable ex06p2.
Note that per our instructions in lib/CMakeLists.txt this will also copy the header file fibonacci.hpp to ~/inc and the shared library libfibonacci.a to ~/lib.
We can also build the library as a standalone unit, without creating any executable program (generally, you would want to build a comprehensive suite of unit and functional tests for your library). To build this static library, run the following commands from the main directory:
mkdir build
cd build
cmake -DCMAKE_BUILD_TYPE=Release ../lib
make
If we wish to deploy our library to other programmers, we can provide these two library files (libfibonacci.a and fibonacci.hpp) and perhaps the source code example main.cpp showing how to use it.
Another option for building an application that uses a library is to use a dynamically linked library. In Linux, dynamic libraries are named lib<name>.so where the .so extention stands for Shared Object. In Windows, dynamic libraries are named <name>.dll for Dynamically Linked Library.
Dynamic libraries are required to deploy along with the binary executable that references functions within that library. This is because a dynamically linked function is not copied inside the executable, like in the statically linked case. Instead, the executable is referenced to a specific location within that .so or .dll file where the function's machine code resides. In this manner, many different executables can be built upon a common dynamic library file without replicating the source code for each executable being built.
To create a dynamically linked library with CMake, simply change STATIC to SHARED in the function add_library(). You may then build the project as before in the static library case. In order to "see" the functions and objects provided within a shared object library, you can use the nm command as follows from the build directory.
nm -D --demangle lib/libfibonacci.so
If an executable is built with a dynamic library, the dependencies (i.e., required libraries) can be viewed by using the ldd command. This command will show that libfibonacci.so is required by the executable in order to run.
ldd ex06p2
For a more complete description of shared libraries in Linux, refer to tldp.org.
You can also look at any Linux machine and browse the folders /usr/lib to see a listing of numerous shared object (i.e., dynamically linked) libraries and /usr/include to see various header files and associated static libraries.