I wanted to write about a really weird problem I recently had while debugging in C++ (technically, it’s all C). Unfortunately, I was doing this in kernel debugging mode, which made life a bit harder, but it would have happened the same in userland.

I had an .hpp file (we’ll call it process_internal.hpp) that was originally an internal file just to be included from a .cpp file (we’ll call it process.cpp), so it contained global variables as symbols. I ended up needing to include this process_internal.hpp file elsewhere (for testing, we’ll call it test.cpp). Because of this, the same symbol was included in multiple files, so the separate .o builds were not properly interacting. I ended up using “#ifdef”s to only include the parts I needed in the test.cpp file, and doing “extern” defines of the global variables for it. It looked something like the following:

enum { FT_Inbound, FT_Outbound };
typedef struct FilteringLayer {
	int FilterTypeNum, OriginalID;
	const char *Name;
} FilteringLayer;
const int FT_NumTypes=2;

#ifdef _PROCESS_INTERNAL
	FilteringLayer FilterTypes[FT_NumTypes]={
		{FT_Inbound,  5, "Inbound"),
		{FT_Outbound, 8, "Outbound"),
	};
#else
	extern "C" FilteringLayer *FilterTypes;
#endif

So I was accessing this variable in test.cpp and getting a really weird problem. The code looked something like this:

struct foo { int a, b; };
foo Stuff[]={...};
void FunctionBar()
{
	for(int i=0;i<FT_NumTypes;i++)
		Stuff[FilterTypes[i].OriginalID].b=1;
}

This was causing an access exception, which blue screened my debug VM. I tried running the exact same statements in the visual studio debugger, and things were working just as they were supposed to! So I decided to go to the assembly level. It looked something like this: (I included descriptions)

I noticed that line #9 was putting 0x0000000C`00000000 into RCX instead of &FilterTypes. I knew the instruction should have been an “lea” instead of a “mov” to fix this. My first thought was compiler bug, but as many programming mantras say, that is very very rarely the case. If you want to guess now what the problem is, now is the time. I’ve given you all the information (and more) to make the guess.

The answer: extern "C" FilteringLayer *FilterTypes; should have been extern "C" FilteringLayer FilterTypes[];. Oops! The debugger was getting it right because it had the extra information of the real definition of the FilterTypes variable.

It has always really bugged me that in Chrome, when you want to view the response and form data for an AJAX request listed in the console, you have to go through multiple annoying clicks to view these two pieces of data, which are also on separate tabs. There is a great Chrome extension though called AJAX-Debugger that gets you all the info you need on the console. However, it also suffered from the having-to-click-through problem for the request data (5th object deep in an object nest), and it did not support JSONP. I’ve gone ahead and fixed these 2 problems :-).

Now to get around to making the other Chrome plugin I’ve been needing for a while ... (Automatic devtool window focus when focusing its parent window)

[Edit on 2015-08-20 @ 8:10am] I added another patch to the Chrome extension to atomically run the group calls (otherwise, they sometimes showed out of order).

Also, the auto focusing thing is not possible as a pure extension due to chrome API inadequacies. While it would be simple to implement using an interval poll via something like Auto Hot Key, I really hate [the hack of] making things constantly poll to watch for something. I’m thinking of a hybrid chrome extension+AHK script as a solution.