.OBJ File Format

Introduction

A classic COFF object file counts its sections in 16 bits and numbers them in 16-bit symbol fields, so it can have at most 65,279. Heavily templated C++ can need more. Microsoft’s compiler in Visual Studio writes the /bigobj format for those: a longer header with 32-bit counts, and symbols with 32-bit section numbers. Clang writes the same format with -Wa,-mbig-obj.

Structure

The file starts with a 56-byte header that begins 00 00 FF FF, the same bytes as other anonymous headers; a class identifier inside it tells a bigobj from the others. The section table follows, with 40-byte entries as in COFF, and then each section’s data and relocations. The symbol table has 20-byte records instead of 18, and a string table follows it.

Also used for

• Wavefront OBJ – A plain text mesh of vertices, normals and faces, read by almost every 3D program.

Variants

• OMF object file – Intel’s object format for 16-bit DOS and Windows compilers, in typed records
• COFF object file – compiled code from Windows compilers, waiting for the linker
• COFF object file (bigobj) – the extended form of COFF, for objects with very many sections
• Anonymous object header – the wrapper Microsoft’s compiler writes for link-time code generation

Links