Summary

I'm using the Mbed OS FATFileSystem on an SD card (via sd-driver). Is this thread-safe?

Details

I'm writing to /fs/dir1/file1 from thread 1, and writing to /fs/dir2/file2 from thread 2. The files are in different directories (obviously), but live on the same volume.

I noticed that ChaN's FatFs (which the Mbed OS FATFileSystem wraps) has an FF_FS_REENTRANT option, which Mbed OS doesn't support. Would that be necessary for thread safety?

Hi Brendan,

Yes, the FATFileSystem is thread safe. However, the synchronization is actually implemented in the C++ layer: https://github.com/ARMmbed/mbed-os/blob/master/features/filesystem/fat/FATFileSystem.cpp#L681-L690

ChanN's FF_FS_REENTRANT option would allow us to synchronize the filesystem through the ChaN C API, but synchronizing in the C++ layer allows us to directly use the C++ RTOS APIs and use the same locks for the C++ class's state.

Let us know if that helps clear things up.

Thanks for the info.

I think it's worth a mention in the documentation, or at least in code comments at the function and/or file level. Do you have an opinion on which is more appropriate? I'm wondering whether to report the issue on mbed-os or the handbook.

Also, I'll just add this link for future reference to be clear of what you were pointing at, since master is a moving target. https://github.com/ARMmbed/mbed-os/blob/mbed-os-5.9.3/features/filesystem/fat/FATFileSystem.cpp#L681-L690

Hmm, we could add a note on the functions about thread safety, for example: https://github.com/ARMmbed/mbed-os/blob/master/drivers/Serial.h#L37

More in depth documentation on why we don't turn on REENTRANT could go in a comment in the ffconf.h.

Both of these would go through mbed-os.

But an issue in either place would work.

I've created an issue for this in the documentation repo: https://github.com/ARMmbed/mbed-os-5-docs/issues/638