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What it takes to pass a file path to a Windows API in C++

Ashley Gullen (@AshleyGullen@mastodon.gamedev.place)

https://mastodon.gamedev.place/@AshleyGullen/111109299141510319

If you thought JavaScript was a mess, here's what it takes to pass a file path to a Windows API in C++: 🧵 - Windows uses UTF-16, but most modern software uses UTF-8 - Converting UTF-8 to UTF-16 requires calling MultiByteToWideChar twice (once for size, second to convert) - Alternatively you can set the process code page to UTF-8 and call the 'A' variant API directly, but only sometimes, and only with Windows 10 v1903+, and you might still have to change the system locale setting and reboot (1/3)

4 comments

Kissaki

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How are we still not past three-splitting content…

It's not as shit as twitter UI/UX/Accessibility - but this mastodon split-post doesn't make me feel much better.

/edit: Actually, it's a 5-split.

I'll do what lemmy-post-OP didn't and copy the content for accessibility:

Ashley Gullen:

If you thought JavaScript was a mess, here's what it takes to pass a file path to a Windows API in C++: 🧵

Windows uses UTF-16, but most modern software uses UTF-8

Converting UTF-8 to UTF-16 requires calling MultiByteToWideChar twice (once for size, second to convert)

Alternatively you can set the process code page to UTF-8 and call the 'A' variant API directly, but only sometimes, and only with Windows 10 v1903+, and you might still have to change the system locale setting and reboot

Now you can pass a file path! But...

The path is limited to MAX_PATH (260 chars)

Unless you prefix the path with \\?\

However using \\?\ disables automatic conversion of / to \ in the path (and some other normalization)

Alternatively set a reg key LongPathsEnabled, but only with Windows 10 v1607+ and the app needs to opt in to longPathAware, and changing the registry might need a reboot

Even then not all Windows APIs support longer paths, e.g. PathCanonicalize. Swap for PathCchCanonicalizeEx with PATHCCH_ALLOW_LONG_PATHS, etc. Check docs per API.

Congratulations, your app can pass a long file path to Windows! But the shell still doesn't support long paths. So your app can write to long paths, but you can't access them with Windows Explorer. Even in Windows 11.

If the API returns a path, do all that in reverse.

I probably missed a few other details and quirks.

References:

https://learn.microsoft.com/en-us/windows/apps/design/globalizing/use-utf8-code-page

https://learn.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation

See also individual APIs e.g. https://learn.microsoft.com/en-us/windows/win32/api/shlwapi/nf-shlwapi-pathcanonicalizea

Unrelated promotional text and link

ReakDuck

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I assume this is not thr case for Linux

AProfessional

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Linux APIs are 8bit, instead of 16bit, however the filesystem encoding can be anything if the user wants.

In practice we all use UTF-8 but correct software has to encode to the correct one just in case.

There is also still a max path length, but it’s longer like 4096.

Max_P

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It's better and worse at the same time: it just doesn't bother with it for the most part. If you have files named with UTF-8 characters, and run it with a locale that uses an ISO-whatever charset, it just displays them wrong. As long as the byte is not a zero or an ASCII forward slash, it'll take it.

There's still a path length limit but it's bigger: 255 bytes for filenames and 4096 bytes for a whole path. That's bytes, not characters. So if you use UTF-16 like on Windows, those numbers are halved.

That said, it's assumed to be UTF-8 these days and should be interpreted as UTF-8, nobody uses non-UTF-8 locales anymore. But you technically can.