fwide(3) -- Linux man page
NAMEfwide - set and determine the orientation of a FILE stream
#include <wchar.h> int fwide(FILE *stream, int mode);
DESCRIPTIONWhen mode is zero, the fwide function determines the current orientation of stream. It returns a value > 0 if stream is wide-character oriented, i.e. if wide character I/O is permitted but char I/O is disallowed. It returns a value < 0 if stream is byte oriented, i.e. if char I/O is permitted but wide character I/O is disallowed. It returns zero if stream has no orientation yet; in this case the next I/O operation might change the orientation (to byte oriented if it is a char I/O operation, or to wide-character oriented if it is a wide character I/O operation).
Once a stream has an orientation, it cannot be changed and persists until the stream is closed.
When mode is non-zero, the fwide function first attempts to set stream's orientation (to wide-character oriented if mode > 0, or to byte oriented if mode < 0). It then returns a value denoting the current orientation, as above.
RETURN VALUEThe fwide function returns the stream's orientation, after possibly changing it. A return value > 0 means wide-character oriented. A return value < 0 means byte oriented. A return value zero means undecided.
CONFORMING TOISO/ANSI C, UNIX98
SEE ALSOfprintf(3), fwprintf(3)
NOTESWide-character output to a byte oriented stream can be performed through the fprintf function with the %lc and %ls directives.
Char oriented output to a wide-character oriented stream can be performed through the fwprintf function with the %c and %s directives.