fflush(3) - NetBSD Manual Pages

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FFLUSH(3)               NetBSD Library Functions Manual              FFLUSH(3)


NAME

     fflush, fpurge -- flush a stream


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <stdio.h>

     int
     fflush(FILE *stream);

     int
     fpurge(FILE *stream);


DESCRIPTION

     The function fflush() forces a write of all buffered data for the given
     output or update stream via the stream's underlying write function.  The
     open status of the stream is unaffected.

     If the file is opened only for reading and not for writing, nothing is
     done.

     If the stream argument is NULL, fflush() flushes all open output streams.

     The function fpurge() erases any input or output buffered in the given
     stream.  For output streams this discards any unwritten output.  For
     input streams this discards any input read from the underlying object but
     not yet obtained via getc(3); this includes any text pushed back via
     ungetc(3).


RETURN VALUES

     Upon successful completion 0 is returned.  Otherwise, EOF is returned and
     the global variable errno is set to indicate the error.


ERRORS

     [EBADF]            stream is not an open stream.

     The function fflush() may also fail and set errno for any of the errors
     specified for the routine write(2).


SEE ALSO

     write(2), fclose(3), fopen(3), setbuf(3)


STANDARDS

     The fflush() function conforms to ANSI X3.159-1989 (``ANSI C89'').


HISTORY

     The fflush() function first appeared in Version 4 AT&T UNIX.  The
     fpurge() function first appeared in 4.4BSD.


BUGS

     According to IEEE Std 1003.1-2024 (``POSIX.1''), calling fflush() on a
     readonly stream that is not at EOF and is seekable, the file offset of
     the FILE pointer should be set to the file position of the underlying
     file descriptor and any characters that have been pushed back via
     ungetc(3) or ungetwc(3) should be discarded.  This is currently not
     implemented.

NetBSD 11.99                   December 18, 2025                  NetBSD 11.99