How I solved a bug by disabling C++ extensions

I was recently investigating a bug and would like to share an unexpected, yet interesting discovery regarding the cause of the issue. In order to provide an idea of what I was working on, consider the following code as a simplified representation:

void read_to_buffer(std::size_t size) {
    char buffer[size];
    std::memset(buffer, 0, size);
    // call some function to fill the buffer
    // process the content of the buffer

    // to make sure that the function is not compiled out
    std::cout << buffer << std::endl;
}

int main(int argc, char *argv[]) {
    auto size = std::stoul(argv[1]);
    read_to_buffer(size);
    return 0;
}

In my case, the code was working perfectly, until the size was changed somewhere deep in the configuration. After that, it started to crash due to a segmentation fault, as the code was trying to allocate too much memory on stack. The solution is to simply allocate the memory on heap. Fortunately, C++ provides plenty of options, such as std::vector v(size), std::string s(size, 0), std::unique_ptr(new char[size]), and so forth.

My only question was, “How was the code successfully compiled in the first place?”

According to the C++ standard, the size of objects allocated on stack must be known at compile time. In the sample I mentioned above, char buffer[size] is a variable-length array, which is actually a feature from the C99 standard, and not related to C++. The interesting catch is that while variable-length array is not supported by the C++ standard, GCC and clang still attempt to compile it because they innately comply with both standards. I can offer the following recommendation in order to avoid such a subtle nuisance: the variable-length arrays need to be explicitly disabled by adding -Werror=vla to the CXX_FLAGS.

Although the optimal and safest solution would be to use the -pedantic flag, which instructs the compiler to adhere to the C++ standard and forbidding all extensions.

And now, ladies and gentlemen,

Exhibit A:

$make
/home/user/example/main.cpp:7:16: error: variable length arrays are a C99 feature [-Werror,-Wvla-extension]
    char buffer[size];
               ^
1 error generated.

Exhibit B:

$make
/home/user/example/main.cpp:7:21: error: ISO C++ forbids variable length array ‘buffer’ [-Werror=vla]
     char buffer[size];
                     ^
cc1plus: all warnings being treated as errors

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See also