TIL – Linking

I was unaware of the difference between static linking and dynamic linking in linux. Thankfully Ben Kelly explained to me in some details these concepts on a Slack chat. I wanted to document them here for future references.

Static linking: when you link the program (the step after compilation that combines all the compiler outputs into a single runnable program), the linker tracks down the libraries the program needs and copies them into the final program file.

Dynamic linking: at link time, the linker merely records which libraries are needed, and when you run the program, the “dynamic linker” reads that information and runs around loading those libraries into memory and making them accessible to the program.

Advantage of the latter is smaller (potentially *much* smaller programs) and you can upgrade the libraries without rebuilding everything that uses them. Disadvantage is that those upgrades can break compatibility, and it’s another external dependency for the program (and thus another point of failure).

The dynamic linker has a bunch of ways it figures out where the libraries are stored; `man ld.so` for all the gory details.

But the tl;dr is that it has a few system paths it looks in (typically /lib and /usr/lib), plus whatever is listed in the environment variable LD_LIBRARY_PATH, plus whatever is recorded as the “rpath” in the executable itself.