I feel like Sherlock, if he were a developer

It’s a normal day at work. My boss assigned me an issue that I know nothing about. And I’m supposed to solve it as fast as possible.

Somewhere in a massive project is a piece of code that keeps crashing. To me, it feels as electrifying as a murder mystery.

Luckily for me, debugging and investigation go hand in hand.

Welcome to the crime scene!

There are clues. Some obvious suspicions. Some finger prints.

But nothing definite.

I track down the usual suspects, but they lead me nowhere.

“There is nothing more deceptive than an obvious fact.”
― Arthur Conan Doyle in The Boscombe Valley Mystery

I call for the help of my equivalent of a Dr. Watson: my IDE.

I place a few breakpoints. I add a few watches.

I brainstorm a little more.

I play back the crime scene again and again, investigating the facts.

Skimming the stack trace, I get a flash of insight that helps me narrow my search.

I feel a shot of elation as I go into a function, and add a specific breakpoint.

And a few moments later, I emerge from my state of concentration, having solved the bug.

You want to know what the problem was?

If you know some basic C, take a look at this block of code, and see if you can find a clue as to what went wrong:

FILE *fd;
char *filename="models/";
strcat(filename,"bullet");
strcat(filename,".h3d");
if( (fd = fopen(filename,"r"))==NULL )
{
    printf("\nFile or Directory not found");
    return;
}

All right, drum roll… here’s the cause of the problem:

It was a segmentation fault. Plain and simple.

char *filename="models/"; // This is a string literal stored in read-only memory
When we use strcat to append to "filename", it is undefined behavior because we aren't allowed to write to that read-only memory.

So how did I solve it?

I allocated a memory buffer big enough to store the full path of the file.

char filename[256]; // Alternatively you can allocate dynamically 
strcpy(filename, "models/");
strcat(filename,"bullet");
strcat(filename,".h3d");

Now you see how I can’t help being Sherlock-ed now.

Stay tuned for the next mystery!

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