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fsync(2) [redhat man page]

FSYNC(2)						     Linux Programmer's Manual							  FSYNC(2)

NAME
fsync, fdatasync - synchronize a file's complete in-core state with that on disk SYNOPSIS
#include <unistd.h> int fsync(int fd); int fdatasync(int fd); DESCRIPTION
fsync copies all in-core parts of a file to disk, and waits until the device reports that all parts are on stable storage. It also updates metadata stat information. It does not necessarily ensure that the entry in the directory containing the file has also reached disk. For that an explicit fsync on the file descriptor of the directory is also needed. fdatasync does the same as fsync but only flushes user data, not the meta data like the mtime or atime. RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately. ERRORS
EBADF fd is not a valid file descriptor open for writing. EROFS, EINVAL fd is bound to a special file which does not support synchronization. EIO An error occurred during synchronization. NOTES
In case the hard disk has write cache enabled, the data may not really be on permanent storage when fsync/fdatasync return. When an ext2 file system is mounted with the sync option, directory entries are also implicitly synced by fsync. On kernels before 2.4, fsync on big files can be inefficient. An alternative might be to use the O_SYNC flag to open(2). CONFORMING TO
POSIX.1b (formerly POSIX.4) SEE ALSO
bdflush(2), open(2), sync(2), mount(8), update(8), sync(8) Linux 1.3.85 2001-04-18 FSYNC(2)

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FSYNC(2)						     Linux Programmer's Manual							  FSYNC(2)

NAME
fsync, fdatasync - synchronize a file's in-core state with storage device SYNOPSIS
#include <unistd.h> 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 DESCRIPTION
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 modi- fication; 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. RETURN VALUE
On success, these system calls return zero. On error, -1 is returned, and errno is set appropriately. ERRORS
EBADF fd is not a valid open file descriptor. EIO An error occurred during synchronization. EROFS, EINVAL fd is bound to a special file which does not support synchronization. CONFORMING TO
4.3BSD, POSIX.1-2001. AVAILABILITY
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).) NOTES
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. SEE ALSO
bdflush(2), open(2), sync(2), sync_file_range(2), hdparm(8), mount(8), sync(8), update(8) COLOPHON
This page is part of release 3.44 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2012-02-27 FSYNC(2)
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