fwide - set and determine the orientation of a FILE stream

#include <wchar.h>
int fwide(FILE *stream, int mode);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

    _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE
        || _POSIX_C_SOURCE >= 200112L

When mode is zero, the fwide() function determines the current orientation of stream. It returns a positive value if stream is wide-character oriented, that is, if wide-character I/O is permitted but char I/O is disallowed. It returns a negative value if stream is byte oriented—that is, 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 nonzero, the fwide() function first attempts to set stream's orientation (to wide-character oriented if mode is greater than 0, or to byte oriented if mode is less than 0). It then returns a value denoting the current orientation, as above.

The fwide() function returns the stream's orientation, after possibly changing it. A positive return value means wide-character oriented. A negative return value means byte oriented. A return value of zero means undecided.

fprintf(3), fwprintf(3)