Blank Variable Watch, or OMF Errors

During a debugging session I faced a weird situation where the code compiled and ran just fine, yet some class members appeared blank at the watch window, and others showed –

CXX0033: error in OMF type information

Skip to the bottom line: there’s a compiler switch, /Yl, that’s specifically tailored to address this symptom. In my project, the issue was solved by adding  /YlSomeFunctionIUse to the stdafx.cpp compiler command line (in the project that defined, not consumed, the blank symbols).

The root cause seems to be a clash of two intended behaviors: (1) Debug info of defined symbols is embedded in the PCH object module (~= .obj file) itself, (2) when a source file refers to the PCH but does not directly use any of the functions defined in it, the PCH object module is dropped from that compilation unit altogether, and thus relevant debug info is lost.

Quoting the /Yl msdn page:

An error can occur when you store the precompiled header in a library, use the library to build an object module, and the source code does not refer to any of the functions the precompiled header file defines.

– so it might be that the symptom is revealed only when the PCH is used in a build of a static library.

This KB article says this behaviour is by design, which seems weird to me. I see no justification for dropping the debug info along with unused function definitions. I’d gladly pay the price of some PDB bloat, to avoid having to use arcane, hidden compiler switches just to be able to debug properly.

BTW, WTF is OMF?

It really stands for [Relocatable] Object Module Format, a relic of ~20 years (which goes to show how old this debugger code is, really). It’s an object-file format designed by Intel in the 70s, and used by MS prior to adoption of their own COFF flavour sometime in the early 90s. The spec is still around, but seems untouched since 1993. The industry probably gave up on the idea of using different vendors for different parts of the compiler-linker-loader tool chain, and so standardization efforts have halted since.

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