Manpage of FSYNC
FSYNCSection: Linux Programmer's Manual (2)
NAMEfsync, fdatasync - 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)):
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
DESCRIPTIONfsync() transfers ("flushes") all modified in-core data of (i.e., modified buffer cache pages for) the file referred to by the file descriptor fdto 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_atimeor 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.
RETURN VALUEOn success, these system calls return zero. On error, -1 is returned, and errnois set appropriately.
- fdis not a valid open file descriptor.
- An error occurred during synchronization.
- EROFS, EINVAL
- fdis bound to a special file (e.g., a pipe, FIFO, or socket) which does not support synchronization.
CONFORMING TOPOSIX.1-2001, POSIX.1-2008, 4.3BSD.
AVAILABILITYOn POSIX systems on which fdatasync() is available, _POSIX_SYNCHRONIZED_IOis defined in <unistd.h>to a value greater than 0. (See also sysconf(3).)
NOTESOn some UNIX systems (but not Linux), fdmust be a writablefile 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.
SEE ALSOsync(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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