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### 4.31 `fwide`—set and determine the orientation of a FILE stream

Synopsis

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

    int _fwide_r(struct _reent *ptr, FILE *fp, int mode);

Description
When mode is zero, the `fwide` function determines the current orientation of fp. It returns a value > 0 if fp 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 fp is byte oriented, i.e. if char I/O is permitted but wide character I/O is disallowed. It returns zero if fp 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, unless the stream is re-opened with freopen, which removes the orientation of the stream.

When mode is non-zero, the `fwide` function first attempts to set fp’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.

Returns
The `fwide` function returns fp’s orientation, after possibly changing it. A return value > 0 means wide-character oriented. A return value < 0 means byte oriented. A return value of zero means undecided.

Portability
C99, POSIX.1-2001.