Programming

I recently stumbled upon a subtle bug in a benchmark code which again reminds me to never use C++ again, if I can.

Here’s a buggy snippet from this code (simplified):

// BUGGY
ostringstream os;
int i = 1;
os << "foo-" << i << ".dat";
const char *filename = os.str().c_str();
int fd = open(filename, O_RDONLY);

You may expect above code to try open a file named foo-1.dat but that’s not what is happening here.

In this snippet, os.str() create a temporary string object which is destroyed immediately after call to c_str() method. So, filename ends up pointing to freed memory which can of course contain arbitrary content (till you reach a NULL).

Very old but still as relevant… and very interesting too! Directly go to “anti-tip” section of this article.

“The moral of the story is: don’t get tricky. C programmers often try to minimize the number of lines of C in their program without consideration for what the compiler will generate. When in doubt, write clear code and give the optimizer a chance to maximize performance. Look at the compiler output. Your code will be easier to debug and probably faster too.”