FWIDE(3) Linux Programmer's Manual FWIDE(3)
fwide - set and determine the orientation of a FILE stream
int fwide(FILE *stream, int mode);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fwide(): _XOPEN_SOURCE >= 500 || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
When mode is zero, the fwide() function determines the current orienta- tion of stream. It returns a positive value if stream is wide-charac- ter 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 ori- ented--that is, if char I/O is permitted but wide-character I/O is dis- allowed. It returns zero if stream has no orientation yet; in this case the next I/O operation might change the orientation (to byte ori- ented 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.
POSIX.1-2001, POSIX.1-2008, C99.
Wide-character output to a byte oriented stream can be performed through the fprintf(3) function with the %lc and %ls directives.
Char oriented output to a wide-character oriented stream can be per- formed through the fwprintf(3) function with the %c and %s directives.
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GNU 2016-03-15 FWIDE(3)