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Linux 2.6 - man page for fflush (linux section 3)

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FFLUSH(3)			    Linux Programmer's Manual				FFLUSH(3)

NAME
       fflush - flush a stream

SYNOPSIS
       #include <stdio.h>

       int fflush(FILE *stream);

DESCRIPTION
       For  output streams, fflush() forces a write of all user-space buffered data for the given
       output or update stream via the stream's underlying write function.   For  input  streams,
       fflush()  discards  any	buffered data that has been fetched from the underlying file, but
       has not been consumed by the application.  The open status of the stream is unaffected.

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

       For a nonlocking counterpart, see unlocked_stdio(3).

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

ERRORS
       EBADF  Stream is not an open stream, or is not open for writing.

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

ATTRIBUTES
   Multithreading (see pthreads(7))
       The fflush() function is thread-safe.

CONFORMING TO
       C89, C99, POSIX.1-2001, POSIX.1-2008.

       The standards do not specify the behavior for input streams.  Most  other  implementations
       behave the same as Linux.

NOTES
       Note  that  fflush()  only  flushes  the user-space buffers provided by the C library.  To
       ensure that the data is physically stored on disk the kernel buffers must be flushed  too,
       for example, with sync(2) or fsync(2).

SEE ALSO
       fsync(2), sync(2), write(2), fclose(3), fopen(3), setbuf(3), unlocked_stdio(3)

COLOPHON
       This  page  is  part of release 3.55 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at
       http://www.kernel.org/doc/man-pages/.

GNU					    2013-07-15					FFLUSH(3)
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