FSYNC(P) FSYNC(P)
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 in an implementation-defined manner. 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 configu-
ration 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 config-
ured 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) dur-
ing 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 IEEE Std 1003.1-2001, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
POSIX 2003 FSYNC(P)
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