Synchronize a file's in-core state with storage device
int fsync(int fd);
int fdatasync(int fd);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fsync(): _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 after the system crashed or was rebooted. This includes writing through or flushing a disk cache if present. The call blocks until the device reports that the transfer has completed. It also flushes metadata information associated with the file (see stat(2)).
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 stat(2)) 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 appropriately.
fd is not a valid open file descriptor.
An error occurred during synchronization.
fd is bound to a special file which does not support synchronization.
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 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 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.
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