BigObj COFF Object Files: Binary Structure Explained

I recently implemented BigObj COFF file parsing in cgo (golang/go#24341). In the process, I quickly discovered that Microsoft doesn’t document the binary format anywhere. Their official documentation is the only reference they have to BigObj as far as I can tell, and it doesn’t say anything about the binary format. I didn’t see any other blogs or resources covering this topic either. I figured it out by reading binutils and LLVM source code, so I’m documenting what I learned while the knowledge is still fresh in my memory. ...

September 28, 2025 · 4 min