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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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fsync(2) System Calls Manual fsync(2)
fsync, fdatasync - synchronize a file's in-core state with storage
device
Standard C library (libc, -lc)
#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
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.
On success, these system calls return zero. On error, -1 is
returned, and errno is set to indicate the error.
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 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.
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).)
POSIX.1-2008.
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 forbidden by POSIX, but nevertheless survives in HP-
UX and AIX.
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 fsync(2)
Pages that refer to this page: pv(1), sync(1), bdflush(2), close(2), io_uring_enter2(2), io_uring_enter(2), mount(2), open(2), posix_fadvise(2), statx(2), sync(2), sync_file_range(2), syscalls(2), write(2), aio_error(3), aio_fsync(3), aio_return(3), dbopen(3), fclose(3), fflush(3), io_uring_prep_fsync(3), cups-files.conf(5), systemd.exec(5), aio(7), signal-safety(7), mount(8), sfdisk(8), xfs_io(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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