C++ programmer's guide to undefined behavior: part 11 of 11

by Dmitry Sviridkin

Developing multithreaded applications is always challenging. The problem of synchronizing access to shared data is a perennial headache. It'd be ideal if we had a well-tested, reliable library of containers, high-level primitives, and parallel algorithms that managed all invariants. It'd be ideal if static compiler checks prevented us from misusing all these things. How nice it would be... Before C++11 and the standardized memory model, we could use threads but only at our risk. Starting with C++11, there are some pretty low-level primitives in the standard library. Since C++17, there are still various parallel versions of algorithms, but we can't even fine-tune the number of threads or their priorities.

C++ programmer's guide to undefined behavior: part 10 of 11

by Dmitry Sviridkin

As you can see from the backtrace, the problematic object whose destructor caused the crash was a vector of strings in libgtest.so. In GTest sources, I found that this vector is a global variable where InitGoogleTest() stored recognized command line arguments. It's just a global variable declared in the compiled file and not presented in the header file. All seemed fine, except for one detail: it wasn't marked as static and was not wrapped in an anonymous namespace. So what? It worked, didn't it? Yeah, it worked. The trick was how the gMock library is built. Let's reproduce it step by step.

C++ programmer's guide to undefined behavior: part 9 of 11

by Dmitry Sviridkin

And you could write it only in C++. In C, however, this mess has been forbidden (see 6.5.3.2, note 104). Also, you can't use the dereference operator on invalid and null pointers anywhere in C. Meanwhile, C++ has its own, special way of doing things. These weird examples were built in constexpr context (let me remind you that UB is forbidden there, and the compiler checks for it).

C++ programmer's guide to undefined behavior: part 8 of 11

by Dmitry Sviridkin

To avoid issues, use conditional noexcept always and everywhere and carefully check every function you use. Or don't use noexcept at all. In the second case, however, it's worth remembering that both move operations and swap should be marked as noexcept (and really be noexcept!) to effectively work with standard containers. Don't forget to write negative tests. You may miss a false noexcept and get std::terminate in the release build without them.

C++ programmer's guide to undefined behavior: part 7 of 11

by Dmitry Sviridkin

In the early '70s, Ken Thompson, Dennis Ritchie, and Brian Kernighan worked on the first versions of C and Unix. They made a decision that resonates with pain, suffering, bugs, and inefficiency today, 50 years later. They decided that developers were to write strings—variable-length data—in a sequence that terminated with a null character. Assembly has it, and C should have it too, if people call it "high-level assembly"! After all, the poor old PDP has limited memory: it's better to have one extra byte per string than 2, 4, or even all 8 bytes (depending on the platform) to store the size... Nah, it's better to have a byte at the end! But even other languages store a size, a reference, or a pointer to the data...

C++ programmer's guide to undefined behavior: part 6 of 11

by Dmitry Sviridkin

I/O streams have other flags that represent the state of the stream: whether there were errors, whether we reached the end. Many people know that you can check whether an operation was successful by putting a stream object into a conditional statement (or any context where it is converted to bool). Those unfamiliar with it might use the while (!iss.eof()) check that will one day lead to the infinite loop issue. This happens when the file isn't finished, but can no longer be read—say, if the file is on a network drive, and the network has gone down. Well, that's a story for another time. Let's focus on the correct way to check readability.

C++ programmer's guide to undefined behavior: part 5 of 11

by Dmitry Sviridkin

However, all this fuss with removing and adding const anywhere in the code eliminates this set of optimizations. So, a repeated access by a constant reference to the same data member or member function doesn't need to be cached at all. Note. It's worth mentioning that programmers have unrealistic expectations about the compiler optimizing code when they add more const. Here's a good note on the topic: "Why const Doesn't Make C Code Faster".

C++ programmer's guide to undefined behavior: part 4 of 11

by Dmitry Sviridkin

In the C++98, the committee made a terrible decision that seemed reasonable at the time. They created a specialization for std::vector<bool>. Normally, sizeof(bool) == sizeof(char), but one bit is enough for bool. However, 99.99% of all possible platforms can't address memory one bit at a time. Let's pack bits in vector<bool> and store CHAR_BIT (usually 8) boolean values in one byte (char) for more efficient memory utilization. As a result, one needs to work with std::vector<bool> in a very special way...

C++ programmer's guide to undefined behavior: part 3 of 11

by Dmitry Sviridkin

This program, built by GCC 10.1, -std=c++20 -O3, doesn't crash, but it doesn't output anything either. If we take GCC 14.1 and the same keys, we suddenly get "helloworld" in the output. It's old but gold undefined behavior.

C++ programmer's guide to undefined behavior: part 2 of 11

by Dmitry Sviridkin

The compiler can be guided by the following logic: If the h value is positive—regardless of the c character—the h*27752 + c value will be positive: the c value is small, and there is no overflow. At the first iteration, h is positive, we sum up positive numbers. There are no overflows in a correct program, so at each iteration, the value will be positive. The result will be positive; we no need any check.