Correction: when input is followed by output, the call needed to avoid undefined behaviour has to be to a "file positioning function" (fseek, fsetpos, or rewind, but not fflush). Since fileobject.c does not use wide I/O operations, it should be sufficient to use _portable_fseek(fp, 0, SEEK_SET).

(_portable_fseek may call some function that is not strictly defined to be a "file positioning function", e.g. fseeko() or fseek64(). However, it would be insane for a stdio implementation not to treat those as being file positioning functions as far as the intent of the C or POSIX standards is concerned.)