NAME
fsync, fdatasync - synchronize a file's in-core state with storage device SYNOPSIS
#include
int fsync(int fd); int fdatasync(int fd); Feature Test Macro Requirements for glibc (see featuretestmacros(7)): fsync(): BSDSOURCE || XOPENSOURCE || /* since glibc 2.8: */ POSIXCSOURCE >= 200112L fdatasync(): POSIXCSOURCE >= 199309L || XOPENSOURCE >= 500 DESCRIPTION fsync() transfers ("flushes") all modified in-core data of (i.e., modi‐ fied buffer cache pages for) the file referred to by the file descrip‐ tor 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 associ‐ ated 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 statime or stmtime (respectively, time of last access and time of last modifica‐ tion; see stat(2)) do not require flushing because they are not neces‐ sary for a subsequent data read to be handled correctly. On the other hand, a change to the file size (stsize, 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 synchro‐ nization. CONFORMING TO
4.3BSD, POSIX.1-2001. AVAILABILITY On POSIX systems on which fdatasync() is available, POSIXSYNCHRO‐ NIZEDIO is defined in
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 filesys‐ tems 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), syncfilerange(2), hdparm(8), mount(8), sync(8), update(8) COLOPHON This page is part of release 3.53 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)