Is it allowed to open a file for both reading and writing?  What I'd like to do is use the same file pointer for read and write access.

I'm doing this:

fin = esxdos_f_open(argv[1],ESXDOS_MODE_R|ESXDOS_MODE_W|ESXDOS_MODE_OE);

which amounts to a call to f_open with mode flag 0x03.  The emulator (ZEsarUX) is coming up with error 6: I/O ERROR.  I have no real hardware to test this on so I'm not sure if this is an emulator problem or not.