синхронизировать изменения в файле (synchronize changes to a file)
Пролог (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.
Имя (Name)
fsync — synchronize changes to a file
Синопсис (Synopsis)
#include <unistd.h>
int fsync(int fildes);
Описание (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.
Возвращаемое значение (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.
Ошибки (Error)
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.
Примеры (Examples)
None.
Использование в приложениях (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.
Обоснование (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.
Будущие направления (Future directions)
None.
Смотри также (See also)
sync(3p)
The Base Definitions volume of POSIX.1‐2017, unistd.h(0p)
