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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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FSYNC(3P) POSIX Programmer's Manual FSYNC(3P)
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.
fsync — synchronize changes to a file
#include <unistd.h>
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 transfer is implementation-defined. The fsync() function
shall not return until the system has completed that action or
until an error is detected.
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 been completed.
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
file system.
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.
None.
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.
None.
sync(3p)
The Base Definitions volume of POSIX.1‐2017, unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. 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.opengroup.org/unix/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
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 FSYNC(3P)
Pages that refer to this page: aio.h(0p), unistd.h(0p), aio_fsync(3p), fdatasync(3p), sync(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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