fsync(2) - NetBSD Manual Pages

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FSYNC(2)                  NetBSD 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 neces-
     sarily 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() (with-
     out 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 on the
                      specified range.

           FDISKSYNC  Request the destination device to ensure that the rele-
                      vant 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 writ-
     ing, 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 is less than zero, or start + length is less
                        than start or triggers an integer overflow; or how
                        contains an invalid value.


NOTES

     For optimal efficiency, the fsync_range() call requires that the file
     system containing the file referenced by fd support partial synchroniza-
     tion of file data.  For file systems which do not support partial syn-
     chronization, 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 mod-
     eled after the function available in AIX.

NetBSD 10.1                    February 17, 2021                   NetBSD 10.1