#220 Static Libraries

Basics of building and using static libraries with C++.

Notes

Distributing code as static libraries for C++:

library maker produces the binary library files with associated header files
library user:
- compiles the program with the library header files
- links the program with the library binary files

The “static” reference means the library references are resolved at compile/link time, so the resulting executable has includes the library built-in. i.e. no runtime dependency on the library binaries.

Making the Library

The animals folder defines a simple library. It provides a “Cat” class that knows how to speak().

The Makefile is setup to generate a library file. Basically, instead of targeting an executable like an actual program, we compile an link to a library archive file. Since I am on MacOS, I’m using the GCC archiver

$ cd animals
$ make
c++ -std=c++17 -Wall -O3   -c -o cat.o cat.cpp
ar -rsc libanimals.a cat.o

The library follows Static Libraries conventions:

"lib" + library-name + ".a"

So given our library is called “animals”, the library file is “libanimals.a”

NB: Shared Libraries conventions:

"lib" + library-name + ".so"

Distributing the library

For others to use the library they need to things - the compiled library, and header files. For this test, I’m simply going to “distribute” using cp(!) …

$ cp animals/libanimals.a demo/lcklib/
$ cp animals/*.h demo/lcklib/include

NB: I’ve arbitrarily decided to put the binaries in a lcklib folder, and headers in lcklib/include.

Using the Library

For client programs to use the library, they need two things:

compiler needs to be told where to find the header files (-I./lcklib/include)
the linker needs to be told where to find the binaries (-L./lcklib) and what libraries to link (-lanimals)

The Makefile is setup to do that for us:

$ cd demo
$ make
c++ -std=c++17 -Wall -O3 -I./lcklib/include   -c -o demo.o demo.cpp
c++ -L./lcklib -lanimals  demo.o   -o demo
./demo
Hello there!!!
Meeeouwww!!!

Credits and References