The fwide function determines the orientation of the stream pointed at by stream.
If the orientation of stream has already been determined, fwide leaves it unchanged. Otherwise, fwide sets the orientation of stream according to mode.
If mode is less than zero, stream is set to byte-oriented. If it is greater than zero, stream is set to wide-oriented. Otherwise, mode is zero, and stream is unchanged.
The fwide function returns a value according to orientation after the call of fwide; a value less than zero if byte-oriented, a value greater than zero if wide-oriented, and zero if the stream has no orientation.
ferror(3), fgetc(3), fgetwc(3), fopen(3), fputc(3), fputwc(3), freopen(3), stdio(3)