crt0.o and crt1.o -- What's the difference?

Question

Recently I've been trying to debug some low-level work and I could not find the crt0.S for the compiler (avr-gcc) but I did find a crt1.S (and the same with the corresponding .o files).

What is the difference between these two files? Is crt1 something completely different or what? They both seem to have to do with something for 'bootstrapping' (setting up stack frame and such), but why the distinction?

Answer 1

Both crt0/crt1 do the same thing, basically do what is needed before calling main() (like initializing stack, setting irqs, etc.). You should link with one or the other but not both. They are not really libraries but really inline assembly code.

As far as I understand, crt comes in two "flavors"

• crt1 is used on systems that support constructors and destructors (functions called before and after main and exit). In this case main is treated like a normal function call.
• crt0 is used on systems that do not support constructors/destructors.