XBD 3.172 defines a File Offset:
3.172 File Offset
The byte position in the file where the next I/O operation begins. Each
open file description
associated with a regular file, block special file, or directory has a file
offset. A character special
file that does not refer to a terminal device may
POSIX FILE streams opened for update need either a fflush() or a file
positioning function
when switching from writing to reading, and a file positioning function when
switching from reading to writing.
The Newlib C library as used in Cygwin fails with perror() reporting "Illegal
seek" when
fsee
