When you call fflush() after updating a text stream, fflush() calculates your current shift state. If you are not in initial shift state, z/OS® XL C looks forward in the record to see whether a shift-in character occurs before the end of the record or any shift-out. If not, z/OS XL C adds a shift-in to the data if it will not overwrite a shift-out character. The shift-in is placed such that there are complete multibyte characters between it and the shift-out that took the data out of initial state. z/OS XL C may accomplish this by skipping over the next byte in order to leave an even number of bytes between the shift-out and the added shift-in.
Updating a wide-oriented or byte-oriented multibyte stream is strongly discouraged. In a byte-oriented stream, you may have written only half of a multibyte character when you call fflush(). In such a case, z/OS XL C adds a padding byte before the shift-out. For both wide-oriented and byte-oriented streams, the addition of any shift or padding character does not move the current file position.
Calling fflush() has no effect on the current record when you are writing new data to a wide-oriented or byte-oriented multibyte stream, because the record is incomplete.