How does GDB know the architecture of an executable?

[fei ding]

Hi guys:

I'm wondering how does GDB know the architecture of an executable, for
example, if I use -m32 options in gcc and I can get an i386
executable, if I use nothing I'll get a x86-64 executable while I am
compiling code on a 64-bit Linux.

I've see the source code for one day but I cannot get what I want, at
first I think '''struct gdbarch''' has something with it. Can anyone
help me? I want to know everything about this.

Thanks a lot.