Compilation / Linking 101

Static Libraries: File Extensions

1. Linux/Unix/macOS:
• Static libraries usually have the extension .a (archive).
• Example:
• libprotoc.a
• libprotobuf.a

2. Windows:
• Static libraries typically use the extension .lib.
• Example:
• libprotoc.lib
• protobuf.lib

How Static Libraries Are Used

• Static Libraries Are Linked at Compile Time:
• Static libraries are not standalone files at runtime.
• During the build process, the compiler/linker embeds the code from the static library directly into the final executable (e.g., protoc).
• Once this happens, there is no need to ship the .a or .lib file alongside the executable.

• No Runtime Dependency:
• Unlike dynamically linked libraries (e.g., .so, .dll), static libraries are effectively "compiled into" the executable. This means the executable contains everything it needs and does not rely on external libraries at runtime.

How to Identify Static Linking

1. Inspect the Build Process:
• If the linker is instructed to use a static library, you'll see flags like static or l<library> pointing to a .a(on Linux/Unix) or .lib (on Windows) during the build process.

2. Check for External Dependencies:
• Use tools like ldd (Linux/macOS) or dumpbin (Windows) on the final executable.
• If no .so (Linux/macOS) or .dll (Windows) dependencies are listed for a library (like libprotoc), it’s likely statically linked.

Key Differences Between Static and Dynamic Libraries

Summary

• Static libraries do have specific file extensions (.a on Linux/macOS, .lib on Windows) during development and linking.

• After compilation, the static library is no longer needed, as its contents are embedded into the executable.

• The resulting program is fully self-contained, unlike dynamically linked executables that rely on .so, .dll, or .dylib files at runtime.