exe checksum different after each recompile

Question

So I'm trying to figure out how to get my exe to have the same hash code/checksum when it's recompiled. I'm using FastSum to generate the checksum. Currently, no code changes are made, I'm just rebuilding the project in VS and the checksum comes out different. The code is written in c++.

I'm not familiar with using hash codes and/or checksums in this manner, but I did some research and read something about needing a consistent GUID. But I have no idea how that would tie into the checksum generation program...

Well, I'll leave it at that, thanks in advance.

Answer 1

Is this a managed binary? Managed binaries have a GUID section that changes from build to build and there's not much you can do to stop that.

You can get a better look at the changes in your binary by running "link /dump /all [filename]" or "link /dump /disasm [filename]". The /all option will show you all the hex values as well as their ascii equivalent, while the /disasm option will disassemble the code and show it to you in assembly, which can be easier to read but might ignore some trivial differences which might have caused the hash to change.

Answer 2

Have you examined the differences between the exes? I suspect the compiler/linker is inserting the date or time into the binary and as a result each binary will be different from another. Or it could be worse, sometimes compilers/linkers build static tables in their own system memory then copy that into the binary, say you have 9 bytes of something and for alignment reasons the compiler chooses to use 12 bytes in the binary, I have seen compilers/linkers take whatever 3 bytes are in system memory of that computer and copy that into the file. Ideally you would want the tools to zero out memory they are using for such a thing so you get repeatable results.

Basically do a binary diff between the files you should then find out why they dont match.

Answer 3

From what I recall, the EXE format includes a build timestamp so a hash of the exe, including that timestamp, would change on each recompile.