I have a working integration of FatFS in my C++ application running on a Cortex M4-based platform.
My application consists of logging data to a data format called MDF.
On the implementation side, I log data (to a given file) in batches of buffers; The number of buffers depends on how fast I acquire the data: log batch of one buffer . . . do other stuff . . . log batch of five buffer . . . do other stuff . . . etc.
There is also a header which is 24 bytes and contains the number of bytes of data. On a PC, I would just save the header at the end of the measurement but this is an embedded product which could be de-powered at any point in time. If I don't save the header periodically, the file becomes "corrupted".
Therefore, in order to maintain coherency I need to re-save the header after saving every batch of data and that's where my issue is.
This means that I have to call f_lseek before writing the header and then before I write the batch of data.
I am using f_cache_fptr so f_lseek is not painfully slow but I'd like to avoid needing to call f_lseek so frequently.
QUESTION
Is it possible to somehow have 2 seek locations so that I don't need to call f_seek to ping-pong between header-location and data-location?
I am open to modifying FatFS.
The problem, at the low-level, is simpler because the header only shares one 512 byte sector with the data: 24 bytes of header followed by 488 bytes of data.
Is it possible to somehow have 2 seek locations so that I don't need to call f_seek to ping-pong between header-location and data-location?
Not as far as I can tell, no, and it doesn't really seem to make sense. A FIL has only one current position, indicating where the next data written to it will go. What would it even mean for there to be two? How would the system know where to write? It certainly wouldn't be correct to write to both places.
Note in particular that with some operating systems and file systems, it is possible to open the same file more than once, but FatFS supports duplicate file opens only when all openings involved are for read-only mode.
I guess it would be possible to modify FatFS to give it the ability to store one file position when you seek to another, and then later to return to the first. So that would mean adding at least one member to the FIL structure, and adding at least one new function.
But why muck with the innards of FatFS? That's going to be at least a little risky. As long as you have to add a function anyway, how about just implementing a FRESULT my_f_write_at_beginning(FIL* fp, const void* buff, UINT btw, UINT* bw) on top of the existing functions? It can store the current position, seek to the beginning of the file, perform the write (maybe ensuring that the full number of bytes specified is written), and then seek back to the original position.
But fundamentally, no, there is no escaping ping-ponging back and forth, because doing so is part of the requirement you laid out.