Size of a library and the executable

Question

I have a static library *.lib created using MSVC on windows. The size of library is say 70KB. Then I have an application which links this library. But now the size of the final executable (*.exe) is 29KB, less than the library. What i want to know is :

1. Since the library is statically linked, I was thinking it should add directly to the executable size and the final exe size should be more than that? Does windows exe format also do some compression of the binary data?

2. How is it for linux systems, that is how do sizes of library on linux (*.a/*.la file) relate with size of linux executable (*.out) ?

-AD

Answer 1

A static library on both Windows and Unix is a collection of .obj/.o files. The linker looks at each of these object files and determines if it is needed for the program to link. If it isn't needed, then the object file won't get included in the final executable. This can lead to executables that are smaller then the library.

EDIT: As MSalters points out, on Windows the VC++ compiler now supports generating object files that enable function-level linking, e.g., see here. In fact, edit-and-continue requires this, since the edit-and-continue needs to be able to replace the smallest possible part of the executable.

Answer 2

There is additional bookkeeping information in the .lib file that is not needed for the final executable. This information helps the linker find the code to actually link. Also, debug information may be stored in the .lib file but not in the .exe file (I don't recall where debug info is stored for objs in a lib file, it might be somewhere else).