Comment by kevin_thibedeau

> The linker won’t complain if your code calls functions that don’t exist – it will just mark them as external, expecting them to be provided by some other code. But they won’t be, and the program will just crash when it calls one of these missing functions.

You can verify no external symbols remain after linkage with: nm -D --undefined-only. This can be incorporated into a build script to verify the binary is valid.