Lib Files 101
Vineel Kovvuri - http://vineelkovvuri.com


    1 - Introduction
    2 - Static Library
    3 - Dynamic Linked Library
    4 - Static Library with __declspec(dllexport)
    5 - References

1. Introduction

    During the compilation one of the crucial step after assembling is
    creating the Object files. The collection of these object files is
    called a lib file. We can create these .lib files through following
    visual studio project types
    1. Static Library
    2. Dynamic Linked Library

    The format of these .lib files is specified in 'Archive (Library) File Format.'
    section of PE Format|https://docs.microsoft.com/en-us/windows/desktop/debug/pe-format#archive-library-file-format
    specification. As per the spec, .lib is an archive of individual .obj
    files with some metadata. Multiple tools can be used to
    extract lib files. Visual Studio installation contains Lib.exe tool.
    Since .lib and .obj files follow Unix COFF format Unix binutil's
    'ar' tool can be used to extract it.

    Lib StaticLib1.lib /list
    Lib StaticLib1.lib /EXTRACT:Debug\sub.obj /out:sub.obj <-- Extracts one file at a time!
    ar -x StaticLib1.lib
    use 7-Zip to extract it

    Visual Studio New Project dialog for 'Static Library' and 'Dynamic Linked Library'

2. Static Library

    Static Library is created when you want to provide the complete code to
    link into another dll or exe. For example, If a static library
    project contains 4 files add.c,sub.c,mul.c,div.c containing functions
    for their operations like below respectively.

    int add(int a)
        return a + 1;

    When you build the project what we get is a .lib file containing obj files
    of each of the above .c file. So a static library contains all
    the code that gets compiled from your project, and this .lib will
    be consumed by any other project types(dll or exe).

    NOTE: The functions inside a Static Library is not declared with
    __declspec(dllexport) because all functions declared in a static
    library are meant to be consumed/included by others directly.

    dumpbin /exports StaticLib.lib  <-- shows nothing because .lib itself does not
    export anything
    dumpbin /symbols StaticLib.lib  <-- shows all the symbols present

    Workflow for Static Library creation and consumption

    Concluded based on HxD view of the file

3. Dynamic Linked Library

    Dynamic Linked Library(DLL) is in many ways similar to Static Library because
    it also provides the code to be used by other projects like dll or exe,
    but the difference is in the way the code gets re-used by consumers.
    In DLL, the outcome of the project is not only a .lib file but also a
    .dll file. In fact, in case of DLL project, the .lib file does not contain
    any .obj file instead it contains only pointers of exported functions to
    the dll containing the actual code. In a Dll, all of the code is indeed
    present inside the .dll file.

    Since .lib does not contain any real .obj unlike static lib, we can
    only see the exported symbols but not their code in case of dll's .lib file.

    __declspec(dllexport) int add(int a)
        return a + 1;

    dumpbin /exports Dll1.lib <-- shows all the exported functions

    Workflow for Dynamic Linked Library creation and consumption

    Concluded based on HxD view of the file

    NOTE: The functions inside a Dynamic Linked Library need to be declared
    with __declspec(dllexport) if they have to be visible and consumed
    by others(indirectly).

4. Static Library with __declspec(dllexport)

    This is interesting. When a function inside a static library is declared
    with __declspec(dllexport), like any other function, it gets included
    by the consumer in his binary(dll) but because it is declared as
    __declspec(dllexport) that function gets exported as well from the consumer
    binary! Below is a screenshot of Consumer.exe which is exporting sub2 function
    because sub2 is actually declared with __declspec(dllexport) in the
    original static library.

    sub2 function got exported from the final consumer binary

    The takeaway here is Static Libraries are just a convenient archive format
    to hold multiple .obj files nothing more or nothing less! So we should be
    cautious of how the functions are declared.

5. References

    - PE Format|https://docs.microsoft.com/en-us/windows/desktop/debug/pe-format