Question

Suppose I have

• an object file (source.o) without function main.

• a shared object (libmain.so) with function main.

How will the linker take care of the entry point when both are linked dynamically to create binary source.bin?

Answer 1

Whenever you have a dynamic linked program, there are several “entry points” involved. The first is in entry point of the dynamic linker itself, e.g. /lib/ld-linux.so.2 (on Linux/x86) or similar. The dynamic linker runs first, resolving all symbol names to their definitions (regardless of whether the definitions are in the main program or a library), then passes execution to the second entry point in the main program binary. This is not main but part of the “C runtime” (thus crt.o and similar names) that takes care of some pre-main stuff (like C++ ctors, setting up the pointer to the environment variables, and constructing the right arguments for main). This code ends with (the equivalent of) exit(main(argc, argv)); which will use the relocated (by the dynamic linker) addresses of both exit and main.