Spooky C++

Scary Stories

You want to see something… spooky? Do you want to hear a story, so haunting, so bone-chilling, it keeps you up at night? Well, let’s look at this program that grants permissions:

#include <iostream>
#include <string>
#include <string_view>

std::string_view Sanitize(const std::string& input) {
  std::string_view sanitized = input;
  while (!sanitized.empty() && sanitized.front() == ' ') { 
    sanitized.remove_prefix(1);
  }
  return sanitized;
}

int main() {
  std::cout << std::boolalpha; 
  std::string_view username = Sanitize("foobarbazboo");
  std::string admin_id = "root_1231231";
  std::cout << "Grant foobarbazboo root permissions = " << (username == admin_id) << "\n";
  return 0;
}

When I compile and run this code (clang++-19 -std=c++17 -O0 -Wall -Wextra -Wpedantic cursed.cpp -o cursed_clang && ./cursed_clang), everything works fine and my user foobarbazboo does not get root permissions:

Grant foobarbazboo root permissions = false

However, if I compile this code in a forest, at midnight on halloween, on top of a Ouija board, after saying “Candyman” in front of a mirror 5 times, and changing the optimization flag to -O2, this program will mysteriously grant foobarbazboo root permissions…

clang++-19 -std=c++17 -O2 -Wall -Wextra -Wpedantic cursed.cpp -o cursed_clang_optimized && ./cursed_clang_optimized

Grant foobarbazboo root permissions = true

Is this the work of g-g-ghosts? Is my computer h-h-haunted?

• No - you can reproduce this on your computer as well.

Is C++17 c-c-cursed?

• Yes, but not by the supernatural. This can happen on C++20 and C++23 as well:

clang++-19 -std=c++20 -O2 -Wall -Wextra -Wpedantic cursed.cpp -o cursed_clang_c20 && ./cursed_clang_c23

Grant foobarbazboo root permissions = true

clang++-19 -std=c++23 -O2 -Wall -Wextra -Wpedantic cursed.cpp -o cursed_clang_c23 && ./cursed_clang_c17

Is my compiler c-c-cursed?

• No, because GCC will do the same thing:

g++ -std=c++17 -O0 -Wall -Wextra -Wpedantic cursed.cpp -o cursed_gcc && ./cursed_gcc
Grant foobarbazboo root permissions = false

g++ -std=c++17 -O2 -Wall -Wextra -Wpedantic cursed.cpp -o cursed_gcc_optimized && ./cursed_gcc_optimized
Grant foobarbazboo root permissions = true

What’s going on

As you can guess, this is an example of undefined behavior causing unintuitive results.

Even though this is a funny example, I doubt a bug like this will surface in the wild (nor be easily exploited) as this behavior is incredibly hard to reproduce.

For example, this only works because the string foobarbazboo is the same length as admin_id, and is passed as an inline parameter. If you change the input length or refactor the program to store foobarbazboo in a temporary variable, the behavior goes away.

Even if you didn’t know anything about UB, I think most people would think this code is ill-formed and weird. A handful of readability/refactoring principles will eliminate this particular behavior. For instance, refactoring admin_id to be a singleton/global variable will make the program behave correctly.

Anyway, Happy Halloween

Appendix

uname -r
6.18.54.1-microsoft-standard-WSL2

clang++ -v
Debian clang version 19.1.7 (3+b1)
Target: x86_64-pc-linux-gnu
Debian clang version 19.1.7 (3+b1)

g++ -v
Target: x86_64-linux-gnu
# ... 
gcc version 14.2.0 (Debian 14.2.0-19)