SDFS File::flush() appears to do nothing

[everslick]

I have a long running process that regularly appends data to a file on SD card and calls flush() on it afterwards. Still when I open the very same file and read its content it has no data appended nor its file size updated. When I close the file between writes, the data is there though. The exact same code works as expected when the file lifes on SPIFFS. So I assume that SD flush() is either not implemented or is buggy.

[everslick]

Or do I just see this behaviour mentioned here: #514 (comment) ?

why should a file handle be closed if another one is opened for the same file? this is completely not to be expected, is it?

[everslick]

I see there have been reports about this issue, putting them here for reference: #1173 and #1169

[psykokwak-com]

I had the same issue.
I fixed it by adding fsync(fileno(_f)); after fflush(_f); in the void VFSFileImpl::flush() in file vfs_api.cpp

[k8C]

I confirm that File.flush() does not work as expected. For example instead of file.print(text); file.flush(); I have to open and close everytime: file = SD.open(fileName, FILE_APPEND); file.print(text); file.close();. Please fix this soon

[ericlangel]

Same Problem with Flash (FFAT) File write.

This works...but its to slow
loop(){
File Data = FFat.open("/Data.txt", FILE_APPEND);
Data.write(Buffer,18);
Data.close();
}

And this doesn't work... nothing is written
loop(){
File Data = FFat.open("/Data.txt", FILE_APPEND);
while(1){
Data.write(Buffer,18);
Data.flush
}
}

I have to write 18 Bytes every 1ms.
How can i get it working?
FFat.open() needs 8ms.

[psykokwak-com]

Just use the POSIX implementation : fopen, fwrite, fsync, fclose.

[rrroonn]

fsync(fileno(myFileDesc)); does indeed work around the problem.

[Bascy]

I'm getting a 'fileno' was not declared in this scope when compiling
Where should this method be living?

[everslick]

@Bascy : the workaround has already been merged 5 days ago. I think it will land in the stable release sooner or later. fileno() is an internal function, you cannot use it in a sketch.

[Bascy]

I'm getting the compile error when compiling FS library through my script, not using fileno() myself
Definition of fileno() is disabled in esp32\tools\sdk\include\newlib\stdio.h

#if 0 /*ndef STRICT_ANSI - FIXME: must initialize stdio first, use fn */
#define fileno(p) __sfileno(p)
#endif

[Bascy]

Solved, was caused by a stray compiler option

[ArosPrince]

For me. the error is caused by -std=c++17. But I wouldn't call that a stray compiler option. There are libraries that require this option.

[davefiddes]

@ArosPrince If you are looking to use a newer standard from the one defined in the platform.txt you need to use the GNU extension variant. So the ESP32 Arduino project currently uses -std=gnu++11 if you wanted to use C++17 you change this to -std=gnu++17. Using a newer C++ standard should probably be raised as a separate issue.

[ArosPrince]

@davefiddes OK, thanks. Good to know.