Question

If I have a.o, b.o, and c.o, how do I make ld link them into d.o, which is then linked into my main object file? All that I want to have happen is that all the symbols in the input files get combined into one big output file.

Answer 1

A concatenation of .o files is called a library. You create one
with the ar library utility:

ar rvs mylib.a a.o b.o c.o

You can then link against the library:

cc main.c mylib.a