Saturday, 10 January 2015

Visual Studio - Getting to your debugging symbols

I've recently been reformulating my lib environment. Basically, it consists of:
  • A loading zone, where I install the library sources, and run the build process. It has a folder for each lib, with each version in its own sub-folder.
  • A library root folder, with sub-roots for mingw and MSVC, where I store the files resulting from the builds. Again, each lib folder has a sub-folder for each installed version.

There's a bit more to it, but it's not important for today's post.

This reformulation has gone through several iterations, and it's still a work in progress. This time, I had the following goals:
  • G1 Getting some further progress on automating the process of building libraries from source, both release and debug versions. Actually, it was this requirement for debug versions of all the libs that led to my patch to OpenSSL with a debug configuration for mingw32.
  • G2 Correcting some bad decisions regarding the lib folders' names, especially when it comes to version numbers. The goal is to use the same number format each lib uses for its own files, where applicable.
  • G3 Clearing the landing zone after building the libs. Now, when I finish building, I run make (actually, mingw32-make or nmake) clean.

Point G3 led me on another learning experience.

The most common debug option when using MSVC seems to be /Zi, which stores the debug symbols in PDB files. As such, I haven't looked into the other options, which store debugging information in the object files themselves; I won't discuss them in this post, but if I had to hazard a guess, I'd say the behaviour is the same as the one described below.

During a debug build, the compiler/linker stores the absolute path to the PDB file on the executable files (EXE or DLL). When you fire up the debugger, as it loads these executable files, it looks up the PDB using this path, to load the symbols. If it can't find the PDB, it then goes through other steps.

On the open-source projects I've seen, I've come across two default behaviours - either the PDB files are not copied to the library folder at all (e.g., Boost); or they're copied to the folder containing the static libraries/import libraries, but not the DLLs themselves (e.g., ICU). I call these default behaviours, because there may be options for gettting a different behaviour; I've looked for these, but found none.

This, combined with point G3 above, is not-so-good news, because make clean deletes the PDB files. So, when we fire up the debugger, it cannot find the symbols for these DLLs. It never happened before because my procedure has always been:
  • build release.
  • clean up.
  • build debug.
  • don't clean up.

So, the PDBs were always available at the landing zone, i.e., the path stored on the executables. And even though some libs copied them to the lib folders, these weren't actually used when the DLLs were loaded by the debugger.

There are several options for dealing with this, including these three:
  • O1 Setting up a local symbol server, which, as I understand it, is more of a local cache to store symbol files.
  • O2 Adding each individual folder where PDB files are installed to the _NT_SYMBOL_PATH env variable, which MS debuggers use to locate symbol files.
  • O3 Manually copying the PDB files to the DLLs folders.

I believe option O1 is the best, but I'll leave it for another iteration. For now, I'll go with option O3. Like I said, this is a work in progress, and I don't feel a particular pressure to go for the optimal solution (which I'll have to test before I adopt it), I prefer getting to a working solution faster, in order to keep my self-imposed deadline (which will end this weekend).

Of course, once you have all this worked out, you still need your debugger to load the correct DLLs. You may have other versions of those DLLs on your path; with popular libraries, like OpenSSL, this is more common than you may think.

On Qt Creator, assuring that you load the correct DLLs is very simple. On Projects mode (Ctrl + 5), you switch to the Run configuration and edit the PATH on the Run Environment. Since I don't have these libraries on the PATH, I always need to do this; if you usually have your debug libraries on the PATH, you don't need to edit anything.

Visual Studio is a different sort of creature. A larger sort of creature, where everything usually takes a bit more work to find.

I knew it was on Project Properties (Alt + F7). At first, I thought it was on Configuration Properties -> VC++ Directories -> Executable Directories. When I hit help, it took me here, where we can read: "Directories in which to search for executable files. Corresponds to the PATH environment variable". Fine, that's just what we need. Somewhat later, and after a moderate amount of gnashing of the dental (not mental, mind you) persuasion, I noticed that the description on the project Property Pages was a wee-bit more complete, namely "Directories in which to search for executable files while building a VC++ project". Ah... right... building the project... as in, "not running the executable".

Then, I turned to the next obvious choice, Configuration Properties -> Debugging -> Environment. This time, I read the description before going for the help button. It is quite helpful, it says "Specifies the environment for the debugee, or variables to merge with existing environment". After reading this, I knew exactly what to do. Which was hitting the help button, and hoping the help page for this option was more useful.

Fortunately, it was. We control the PATH here, using something like this: PATH=E:\Dev\lib\msvc\openssl\openssl1_0_1j\debug\bin;E:\Dev\lib\msvc\icu\icu54_1\debug\bin;%PATH%.

And, finally, after some gnashing of the mental persuasion, I can say that Visual Studio's debugger and me are finally getting along just fine.

As they say, until next time... when I turn this into reusable project configurations, instead of having to specify these manually on every project.

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