What is the C++ iostream to creating a file with O_EXCL?

I want to create an output file in a threadsafe manner, and only if it does not exist. I want to use the file system for synchronization. With open() I would use the flags O_RWRONLY|O_CREAT|O_EXCL. Is there a way to do this in C++17 using the iostream or fstream ?

Prior to C++23 there is no way of opening an ofstream in exclusive mode.

Workaround: Use std::fopen which has this capability since C++17.

Example:

#include <cstdio>

// Mode "x" to make it fail if it already exists
std::FILE* fp = std::fopen("filename", "wx");

if(fp) {
    // created exclusively

    // work with fp ...

    std::fclose(fp);
}

If you really want an ofstream you could create a helper function:

template<class Stream>
Stream open_exclusively(const std::string& filename) {
    Stream rv;
    
    if(std::FILE* fp = std::fopen(filename.c_str(), "wx"); fp) {
        std::fclose(fp);

        // overwrite the file that was created exclusively:
        rv.open(filename);
    } else {
        // could not create file exclusivly, set the failbit in the stream:
        rv.setstate(Stream::failbit);
    }

    return rv;
}

int main() {
    auto os = open_exclusively<std::ofstream>("filename");

    if(os) {
        std::cout << "file created exclusively\n";
    }
}

Demo

Edit:

Even though the above demo is compliant and works on all platforms I've tested it - wine (v6.16) can't handle it, so I opened a bug report at bugs.winehq.org. You can follow the progress here:

Standard library call fopen(..., "wx") not recognized - causes destruction of data

Edit 2:

The Wine bugfix ucrtbase: Add support for x mode in fopen is now included in Wine 6.20 so after upgrading to 6.20 (or later), this will be working as it should in Wine too.

From C++23 you can use the std::ios::noreplace openmode:

std::ofstream os("filename", std::ios::noreplace);

if(os) {
    std::cout << "file created exclusively\n";
}