fsync, fdatasync - synchronize a file's in-core state with storage device
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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
(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,
errno is set appropriately.
fd is not a valid open file descriptor.
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.
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
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.
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