.O file signature
A Intel 80386 COFF file begins with the bytes 64 86 at the start of the file. The signature is the same whatever the file is named, which is why a renamed or extensionless COFF file is still recognisable.
Signature
| Offset | Hex | ASCII |
|---|---|---|
| 0 | 64 86x86-64 machine type; 4C 01 for i386. COFF has no universal magic. |
MIME type: application/x-object · usual extensions: .o, .obj
How to check a file's signature
Without installing anything: drop the file on our detector. It reads the bytes in your browser and shows the hex dump alongside the verdict; nothing is uploaded.
macOS and Linux (Terminal):
xxd -l 32 file.o
file file.oWindows (PowerShell):
Format-Hex -Path file.o -Count 32If the bytes do not match
A file named .o whose first bytes are something else is not a COFF file. The common explanations are a wrong or deliberately misleading extension, a download that saved an HTML error page instead of the file, or a container such as ZIP wrapping the real content. The detector reports what the bytes say; the COFF guide explains what a genuine file should contain.