Man

Command Section
FSYNC(2)                  FreeBSD System Calls Manual                 FSYNC(2)

NAME
     fdatasync, fsync - synchronise changes to a file

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <unistd.h>

     int
     fdatasync(int fd);

     int
     fsync(int fd);

DESCRIPTION
     The fsync() system call causes all modified data and attributes of the
     file referenced by the file descriptor fd to be moved 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.

     The fdatasync() system call causes all modified data of fd to be moved to
     a permanent storage device.  Unlike fsync(), the system call does not
     guarantee that file attributes or metadata necessary to access the file
     are committed to the permanent storage.

     The fsync() system call should be used by programs that require a file to
     be in a known state, for example, in building a simple transaction
     facility.  If the file metadata has already been committed, using
     fdatasync() can be more efficient than fsync().

     Both fdatasync() and fsync() calls are cancellation points.

RETURN VALUES
     The fsync() function returns the value 0 if successful; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

ERRORS
     The fsync() and fdatasync() calls fail if:

     [EBADF]            The fd argument is not a valid descriptor.

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

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

SEE ALSO
     sync(2), syncer(4), sync(8)

HISTORY
     The fsync() system call appeared in 4.2BSD.  The fdatasync() system call
     appeared in FreeBSD 11.1.

BUGS
     The fdatasync() system call currently does not guarantee that enqueued
     aio(4) requests for the file referenced by fd are completed before the
     syscall returns.

FreeBSD 11.1-RELEASE-p4          May 24, 2017          FreeBSD 11.1-RELEASE-p4
Command Section