синхронизировать состояние файла в ядре с устройством хранения (synchronize a file's in-core state with storage device)
Имя (Name)
fsync, fdatasync - synchronize a file's in-core state with
storage device
Синопсис (Synopsis)
#include <unistd.h>
int fsync(int fd);
int fdatasync(int fd);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
fsync():
Glibc 2.16 and later:
No feature test macros need be defined
Glibc up to and including 2.15:
_BSD_SOURCE || _XOPEN_SOURCE
|| /* Since glibc 2.8: */ _POSIX_C_SOURCE >= 200112L
fdatasync():
_POSIX_C_SOURCE >= 199309L || _XOPEN_SOURCE >= 500
Описание (Description)
fsync() transfers ("flushes") all modified in-core data of (i.e.,
modified buffer cache pages for) the file referred to by the file
descriptor fd to the disk device (or other permanent storage
device) so that all changed information can be retrieved even if
the system crashes or is rebooted. This includes writing through
or flushing a disk cache if present. The call blocks until the
device reports that the transfer has completed.
As well as flushing the file data, fsync() also flushes the
metadata information associated with the file (see inode(7)).
Calling fsync() does not necessarily ensure that the entry in the
directory containing the file has also reached disk. For that an
explicit fsync() on a file descriptor for the directory is also
needed.
fdatasync() is similar to fsync(), but does not flush modified
metadata unless that metadata is needed in order to allow a
subsequent data retrieval to be correctly handled. For example,
changes to st_atime or st_mtime (respectively, time of last
access and time of last modification; see inode(7)) do not
require flushing because they are not necessary for a subsequent
data read to be handled correctly. On the other hand, a change
to the file size (st_size, as made by say ftruncate(2)), would
require a metadata flush.
The aim of fdatasync() is to reduce disk activity for
applications that do not require all metadata to be synchronized
with the disk.
Возвращаемое значение (Return value)
On success, these system calls return zero. On error, -1 is
returned, and errno is set to indicate the error.
Ошибки (Error)
EBADF fd is not a valid open file descriptor.
EIO An error occurred during synchronization. This error may
relate to data written to some other file descriptor on
the same file. Since Linux 4.13, errors from write-back
will be reported to all file descriptors that might have
written the data which triggered the error. Some
filesystems (e.g., NFS) keep close track of which data
came through which file descriptor, and give more precise
reporting. Other filesystems (e.g., most local
filesystems) will report errors to all file descriptors
that were open on the file when the error was recorded.
ENOSPC Disk space was exhausted while synchronizing.
EROFS, EINVAL
fd is bound to a special file (e.g., a pipe, FIFO, or
socket) which does not support synchronization.
ENOSPC, EDQUOT
fd is bound to a file on NFS or another filesystem which
does not allocate space at the time of a write(2) system
call, and some previous write failed due to insufficient
storage space.
Стандарты (Conforming to)
POSIX.1-2001, POSIX.1-2008, 4.3BSD.
On POSIX systems on which fdatasync() is available,
_POSIX_SYNCHRONIZED_IO is defined in <unistd.h> to a value
greater than 0. (See also sysconf(3).)
Примечание (Note)
On some UNIX systems (but not Linux), fd must be a writable file
descriptor.
In Linux 2.2 and earlier, fdatasync() is equivalent to fsync(),
and so has no performance advantage.
The fsync() implementations in older kernels and lesser used
filesystems do not know how to flush disk caches. In these cases
disk caches need to be disabled using hdparm(8) or sdparm(8) to
guarantee safe operation.
Смотри также (See also)
sync(1), bdflush(2), open(2), posix_fadvise(2), pwritev(2),
sync(2), sync_file_range(2), fflush(3), fileno(3), hdparm(8),
mount(8)
