FSYNC(2) Linux FSYNC(2)
fsync, fdatasync - synchronize a file's in-core state with storage
int fsync(int fd);
int fdatasync(int fd);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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
_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
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
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 appropriately.
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 where open on the file when the
error was recorded.
ENOSPC Disk space was exhausted while synchronizing.
fd is bound to a special file (e.g., a pipe, FIFO, or socket)
which does not support synchronization.
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.
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).)
On some UNIX systems (but not Linux), fd must be a writable file
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 does 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.
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)
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