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.Dd March 30, 2020 .Dt FSYNC 2 .Os .Sh NAME .Nm fdatasync , .Nm fsync .Nd "synchronise changes to a file" .Sh LIBRARY .Lb libc .Sh SYNOPSIS n unistd.h .Ft int .Fn fdatasync "int fd" .Ft int .Fn fsync "int fd" .Sh DESCRIPTION The .Fn fsync system call causes all modified data and attributes of the file referenced by the file descriptor .Fa 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.

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

p The .Fn 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 .Fn fdatasync can be more efficient than .Fn fsync .

p Both .Fn fdatasync and .Fn fsync calls are cancellation points. .Sh RETURN VALUES .Rv -std fsync .Sh ERRORS The .Fn fsync and .Fn fdatasync calls fail if: l -tag -width Er t Bq Er EBADF The .Fa fd argument is not a valid descriptor. t Bq Er EINVAL The .Fa fd argument refers to a socket, not to a file. t Bq Er EIO An I/O error occurred while reading from or writing to the file system. t Bq Er EINTEGRITY Corrupted data was detected while reading from the file system. .El .Sh SEE ALSO .Xr fsync 1 , .Xr sync 2 , .Xr syncer 4 , .Xr sync 8 .Sh HISTORY The .Fn fsync system call appeared in x 4.2 . The .Fn fdatasync system call appeared in .Fx 11.1 . .Sh BUGS The .Fn fdatasync system call currently does not guarantee that enqueued .Xr aio 4 requests for the file referenced by .Fa fd are completed before the syscall returns.