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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.
NAME
fsync — synchronize changes to a file
SYNOPSIS
#include <unistd.h> int fsync(int fildes);
DESCRIPTION
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.
RETURN VALUE
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.
ERRORS
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.
EXAMPLES
None.
APPLICATION USAGE
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.
RATIONALE
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.
FUTURE DIRECTIONS
None.
SEE ALSO
sync()
The Base Definitions volume of POSIX.1‐2017, <unistd.h>
COPYRIGHT
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 .