.OBJ

Introduction

On Windows, the compiler turns each source file into a .obj object file in the COFF format (Common Object File Format), which the linker then joins into a PE program or library. Microsoft’s compiler writes them, and so do Clang and other compilers that target Windows. A static library (.lib) is an archive of these object files.

The .obj extension has other meanings. A 3D model in the Wavefront OBJ format is a text file with the same extension, and older DOS and Windows compilers wrote object files in a different format, OMF.

Software

• Visual Studio, whose link joins them and dumpbin shows them (Windows)
• LLVM, with lld-link and llvm-objdump (Windows, macOS, Linux)
• GNU Binutils, with objdump (Windows, macOS, Linux)
• Ghidra (Windows, macOS, Linux)
• IDA Pro (Windows, macOS, Linux)

Structure

A COFF object file has no signature. It starts directly with the 20-byte file header, which is the same header a PE file has after its PE\0\0 signature: the target machine (0x14C for 32-bit x86, 0x8664 for x64, 0xAA64 for 64-bit ARM), the number of sections, a timestamp, the position and size of the symbol table, and an optional header size that is 0 for object files.

The section table follows, then each section’s raw data and its relocations: the places the linker must patch once it knows where everything ends up. The symbol table at the end lists the functions and variables the file defines and the ones it needs from elsewhere, in 18-byte records, followed by a string table for names longer than eight characters.

See also

Other formats that use the same extension:

• Wavefront OBJ .obj