fsync(2) System Calls Manual fsync(2)
NAME
fsync, fdatasync - synchronize a file's in-core state with storage de-
vice
LIBRARY
Standard C library (libc, -lc)
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
DESCRIPTION
fsync() transfers ("flushes") all modified in-core data of (i.e., modi-
fied 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 in-
formation associated with the file (see inode(7)).
Calling fsync() does not necessarily ensure that the entry in the direc-
tory 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 re-
trieval to be correctly handled. For example, changes to st_atime or
st_mtime (respectively, time of last access and time of last modifica-
tion; see inode(7)) do not require flushing because they are not neces-
sary 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.
RETURN VALUE
On success, these system calls return zero. On error, -1 is returned,
and errno is set to indicate the error.
ERRORS
EBADF fd is not a valid open file descriptor.
EINTR The function was interrupted by a signal; see signal(7).
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 pre-
cise 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.
VERSIONS
On POSIX systems on which fdatasync() is available, _POSIX_SYNCHRO-
NIZED_IO is defined in <unistd.h> to a value greater than 0. (See also
sysconf(3).)
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001, 4.2BSD.
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.
Under AT&T UNIX System V Release 4 fd needs to be opened for writing.
This is by itself incompatible with the original BSD interface and for-
bidden by POSIX, but nevertheless survives in HP-UX and AIX.
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)
Linux man-pages 6.9.1 2024-05-02 fsync(2)
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