Is it possible to find sizes of structures declared in a DLL?

Question

We have the situation where we have a large number of inter-related DLLs that form our product. This is a very "old" product (as in it has been developed over 20 years) and has suffered in the past from different defaults for structure packing over several versions of Visual Studio.

So, in the many cases where #pragma pack has not been used in the DLL header files, but the structure alignment has been set instead in the project properties, we can have the situation where a project that imports the DLL (via its lib and header) has a different structure alignment and potentially results in mismatched structure sizes.

This is complicated by the fact that structs can be sized correctly by "accident" - e.g. if all members of the struct are unsigned int then pack(4) in the DLL and pack(2) in the importing project can work ok. Until, of course, someone amends a struct to add a bool for example.

I would like to remove all of this potential confusion by adding #pragma pack statements to the header files of all exporting modules but would first like to assess whether we have any such exposures in our current code (thinking about hard-to-track runtime bugs here). Also, it may be useful to introduce some automated checking into our build process to make sure we never hit these situations, even with third-party DLLs or LIBs.

So, my question:

Is it possible, from a compiled DLL, or its associated LIB, to determine what structure alignment was in force at the time the DLL was compiled? Similarly, is it possible to discover this for an EXE?

What am I wondering is if there is anything in the PE format or LIB (is that COFF?) that can be used to find this information?

UPDATE

Well, no good came from examining libs and dlls with dumpbin, so I'm going to try to get some info from the PDB files we generate from our Release builds. I found this as a starting point...

Answer 1

I would say that is not possible. C++ doesn't have type-information applied to it (unless enabled with RTTI, but won't be of much help for this problem). Structure is nothing but a sequence of bytes, for the programmer. Compiler will replace the variable.member with appropriate byte-alignment to access that data.

I doubt you have correct debugging information (i.e. PDB file) for the DLL to lookup the symbols. Even with that, it is not possible to find "packing" of a structure.

I have faced problem with structure sizes in different EXE/DLLs (having full source code), where sizeof is only tool we can use to find the difference (and go nested to find the root of problem). Even with this technique, it it not possible which packing is enabled for a particular structure.