Segment fault when ifstream declared in a function, but not main, only when optimizations are turned on

Question

As the title says, I'm having an issue with ifstream. I was trying to read a text file and that would work fine when debugging but not in the release build. I was able to narrow it down to the example below.

The following code segfaults when it reaches the declaration of file in foo() if optimizations are turned on.

#include <iostream>
#include <fstream>

void foo (){
    std::cout << "I'm here 3" << std::endl;

    std::ifstream file;

    std::cout << "I'm here 4" << std::endl;
}

int main()
{
    std::cout << "I'm here 1" << std::endl;

    std::ifstream file;

    std::cout << "I'm here 2" << std::endl;

    foo();

    return 0;
}

Output:

I'm here 1
I'm here 2
I'm here 3

I'm compiling and linking with the following commands:

g++ -Wall -std=c++17 -O1 main.cpp -o main.exe 

If I change the -O1 to O0 it doesn't segfault and runs to the end. I'm not really sure what the issue is.

My system is Windows 10 and I'm running g++.exe (x86_64-win32-seh-rev0, Built by MinGW-W64 project) 8.1.0. Is this a compiler bug or am I just doing something stupid?

Answer 1

Since --static fixes it, this means your program picks up incorrect DLL versions.

Open your compiler's bin directory, and make a list of all DLLs in there.

Then:

• Search for those DLLs in C:\Windows (recursively). Delete any matching DLLs, they shouldn't be there.

• Make sure your compiler is the first thing in the PATH, or at least that any directories before it don't contain those DLLs (or better, all those directories except the compiler one).

  This includes not having two different MinGW versions in the PATH (or you can just uninstall all but one).