FSYNC(3P) POSIX Programmer's Manual FSYNC(3P)PROLOG
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.
NAMEfsync — synchronize changes to a file
int fsync(int fildes);
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 trans‐
fer is implementation-defined. The fsync() function shall not return
until the system has completed that action or until an error is
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
Upon successful completion, fsync() shall return 0. Otherwise, −1 shall
be returned and errno set to indicate the error. If the fsync() func‐
tion fails, outstanding I/O operations are not guaranteed to have been
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 oper‐
ation is possible.
EIO An I/O error occurred while reading from or writing to the file
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 modifica‐
tions to a file to be completed before continuing; for example, a pro‐
gram which contains a simple transaction facility might use it to
ensure that all modifications to a file or files caused by a transac‐
tion 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 permit‐
ted. 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 actu‐
ally cause data to be written where it is safe from a power failure.
The conformance document should identify at least that one configura‐
tion 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 con‐
figured to allow the data to be written to non-volatile storage.
It is reasonable to assert that the key aspects of fsync() are unrea‐
sonable 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 auto‐
mated 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-
The Base Definitions volume of POSIX.1‐2008, <unistd.h>
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 Electri‐
cal 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 .
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IEEE/The Open Group 2013 FSYNC(3P)