COFF

COFF

Filename extension	none, .o, .obj
Developed by	AT&T Corporation
Type of format	Binary, executable, object, shared libraries
Extended to	XCOFF, ECOFF, Portable Executable

The Common Object File Format (COFF) is a format for executable, object code, and shared library computer files used on Unix systems. It was introduced in Unix System V, replaced the previously used a.out format, and formed the basis for extended specifications such as XCOFF and ECOFF, before being largely replaced by ELF, introduced with SVR4. COFF and its variants continue to be used on some Unix-like systems, on Microsoft Windows, in EFI environments and in some embedded development systems.

The original Unix object file format a.out is unable to adequately support shared libraries, foreign format identification, or explicit address linkage. As development of Unix-like systems continued both inside and outside AT&T, different solutions to these and other issues emerged.

COFF was introduced in AT&T's UNIX System V for non-VAX 32-bit platforms such as the 3B20. Improvements over the existing AT&T a.out format included arbitrary sections, explicit processor declarations, and explicit address linkage.

However, the COFF design was both too limited and incompletely specified: there was a limit on the maximum number of sections, a limit on the length of section names, included source files, and the symbolic debugging information was incapable of supporting real world languages such as C, much less newer languages like C++, or new processors. All real world implementations of COFF were necessarily violations of the standard as a result. This led to numerous COFF extensions. IBM used the XCOFF format in AIX. DEC, SGI and others used ECOFF; And numerous SysV ports and tool chains targeting embedded development each created their own, incompatible, variations.

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