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FSYNC(2)                      System Calls Manual                     FSYNC(2)

NAME
     fsync, fsync_range -- synchronize a file's in-core state with that on
     disk

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

     int
     fsync(int fd);

     int
     fsync_range(int fd, int how, off_t start, off_t length);

DESCRIPTION
     fsync() causes all modified data and attributes of fd to be written to a
     permanent storage device.  This normally results in all in-core modified
     copies of buffers for the associated file to be written to a disk.

     fsync_range() is similar, but provides control over the region of the
     file to be synchronized, and the method of synchronization.

     These functions should be used by programs that require a file to be in a
     known state, for example, in building a simple transaction facility.

     Note that writing the data to a permanent storage device does not
     necessarily write the data to permanent storage media within that device;
     for example, after writing data to a disk device, the data might reside
     in a cache within the device, but not yet on more permanent storage
     within the device.  Neither fsync() nor the default behavior of
     fsync_range() (without the FDISKSYNC flag) will flush disk caches,
     because they assume that storage devices are able to ensure that
     completed writes are transferred to media some time between the write and
     a power failure or system crash.

     fsync_range() causes all modified data starting at start for length
     length of fd to be written to a permanent storage device.  If the length
     parameter is zero, fsync_range() will synchronize all of the file data.

     fsync_range() takes a how parameter which contains one or more of the
     following flags:

           FDATASYNC  Synchronize the file data and sufficient meta-data to
                      retrieve the data for the specified range.  This is
                      equivalent to fdatasync(2) on the specified range.

           FFILESYNC  Synchronize all modified file data and meta-data for the
                      specified range.  This is equivalent to fsync(2) on the
                      specified range.

           FDISKSYNC  Request the destination device to ensure that the
                      relevant data and meta-data is flushed from any cache to
                      permanent storage media.  In the present implementation,
                      the entire cache on the affected device will be flushed,
                      and this may have a significant impact on performance.

     The FDATASYNC and FFILESYNC flags are mutually exclusive.  Either of
     those flags may be combined with the FDISKSYNC flag.

     Note that fsync_range() requires that the file fd must be open for
     writing, whereas fsync() does not.

RETURN VALUES
     A 0 value is returned on success.  A -1 value indicates an error.

ERRORS
     fsync() or fsync_range() fail if:

     [EBADF]            fd is not a valid descriptor.

     [EINVAL]           fd refers to a socket, not to a file.

     [EIO]              An I/O error occurred while reading from or writing to
                        the file system.

     Additionally, fsync_range() fails if:

     [EBADF]            fd is not open for writing.

     [EINVAL]           start + length is less than start.

NOTES
     For optimal efficiency, the fsync_range() call requires that the file
     system containing the file referenced by fd support partial
     synchronization of file data.  For file systems which do not support
     partial synchronization, the entire file will be synchronized and the
     call will be the equivalent of calling fsync().

SEE ALSO
     fdatasync(2), sync(2), sync(8)

HISTORY
     The fsync() function call appeared in 4.2BSD.

     The fsync_range() function call first appeared in NetBSD 2.0 and is
     modeled after the function available in AIX.

NetBSD 7.1.2                  September 22, 2013                  NetBSD 7.1.2