This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
The fsync() function shall request that all data for the open file
descriptor named by fildes is to be transferred to the storage device
associated with the file described by fildes. The nature of the
transfer is implementation-defined. The fsync() function shall not
return until the system has completed that action or until an error
If _POSIX_SYNCHRONIZED_IO is defined, the fsync() function shall
force all currently queued I/O operations associated with the file
indicated by file descriptor fildes to the synchronized I/O
completion state. All I/O operations shall be completed as defined
for synchronized I/O file integrity completion.
Upon successful completion, fsync() shall return 0. Otherwise, −1
shall be returned and errno set to indicate the error. If the fsync()
function fails, outstanding I/O operations are not guaranteed to have
The fsync() function shall fail if:
EBADF The fildes argument is not a valid descriptor.
EINTR The fsync() function was interrupted by a signal.
EINVAL The fildes argument does not refer to a file on which this
operation is possible.
EIO An I/O error occurred while reading from or writing to the
In the event that any of the queued I/O operations fail, fsync()
shall return the error conditions defined for read() and write().
The following sections are informative.
The fsync() function should be used by programs which require
modifications to a file to be completed before continuing; for
example, a program which contains a simple transaction facility might
use it to ensure that all modifications to a file or files caused by
a transaction are recorded.
The fsync() function is intended to force a physical write of data
from the buffer cache, and to assure that after a system crash or
other failure that all data up to the time of the fsync() call is
recorded on the disk. Since the concepts of ``buffer cache'',
``system crash'', ``physical write'', and ``non-volatile storage''
are not defined here, the wording has to be more abstract.
If _POSIX_SYNCHRONIZED_IO is not defined, the wording relies heavily
on the conformance document to tell the user what can be expected
from the system. It is explicitly intended that a null implementation
is permitted. This could be valid in the case where the system cannot
assure non-volatile storage under any circumstances or when the
system is highly fault-tolerant and the functionality is not
required. In the middle ground between these extremes, fsync() might
or might not actually cause data to be written where it is safe from
a power failure. The conformance document should identify at least
that one configuration exists (and how to obtain that configuration)
where this can be assured for at least some files that the user can
select to use for critical data. It is not intended that an
exhaustive list is required, but rather sufficient information is
provided so that if critical data needs to be saved, the user can
determine how the system is to be configured to allow the data to be
written to non-volatile storage.
It is reasonable to assert that the key aspects of fsync() are
unreasonable to test in a test suite. That does not make the function
any less valuable, just more difficult to test. A formal conformance
test should probably force a system crash (power shutdown) during the
test for this condition, but it needs to be done in such a way that
automated testing does not require this to be done except when a
formal record of the results is being made. It would also not be
unreasonable to omit testing for fsync(), allowing it to be treated
as a quality-of-implementation issue.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 FSYNC(3P)