fix compiler warning

[bmcdonnell-ionx]

Resolves the compiler warnings first mentioned here:

../mbed-os/features/filesystem/fat/FATFileSystem.cpp: In member function 'virtual void FATFileSystem::dir_seek(mbed::fs_dir_t, off_t)':
../mbed-os/features/filesystem/fat/FATFileSystem.cpp:661:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     if (offset < dh->dptr) {
         ~~~~~~~^~~~~~~~~~
../mbed-os/features/filesystem/fat/FATFileSystem.cpp:664:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     while (dh->dptr < offset) {
            ~~~~~~~~~^~~~~~~~

I don't actually understand how dir_seek() is doing its job, though. Can someone explain?

[mbed-ci]

User not whitelisted, CI not run.

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1084
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6018/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 761
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6018/

[mbed-ci]

Test : SUCCESS

Build number : 891
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6018/891

[geky]

Thanks for the pr!

The conditions in dir_seek are handling two different cases:

1. offset > dptr:

             dptr
             v
   blah blah blah blah blah
                  ^
                  off
   

   Here we can just iterate forward until dptr == off (the while loop).

2. offset < dptr:

             dptr
             v
   blah blah blah blah blah
        ^
        off
   

   But here we would need to iterate backwards. In theory we could iterate backwards, but it gets complicated because of LFNs. Since ChanFS targets embedded systems and we care more about code footprint than speed (especially in the weird corner case of iterating backwards in a directory), we just rewind completely (the rewind call) and then iterate forwards until dptr == off (the while loop).

It's an interesting case of reusing as much code as possible to keep the code footprint small.

Does that help clarify things?

[bmcdonnell-ionx]

Does that help clarify things?

Yes, thanks!

I think my confusion stemmed from figuring that offset would be relative to the current position, not an "absolute" position within the directory.

...This PR should stay open until someone merges it, right? (Just checking, since I have the option to close it.)

[0xc0170]

...This PR should stay open until someone merges it, right? (Just checking, since I have the option to close it.)

@geky Approved?

[geky]

Ah my bad, I forgot to click the approve button.

Only open prs are candidates for being merged. Closing a pr is the same as saying you don't want the pr merged.

Although it's easy to reopen closed prs and issues.

---

Thinking about it, offset is kinda a funny name, but the name comes from the fact that it's an offset from the start of the file/directory.