sync_file_range(2)

NAME

sync_file_range - sync a file segment with disk

SYNOPSIS

#define _GNU_SOURCE
#include <fcntl.h>
int sync_file_range(int fd, off64_t offset, off64_t nbytes,
                    unsigned int flags);

DESCRIPTION

sync_file_range() permits fine control when synchronizing the open file referred to by the file descriptor fd with disk.

offset is the starting byte of the file range to be synchronized. nbytes specifies the length of the range to be synchronized, in bytes;
if nbytes is zero, then all bytes from offset through to the end of file are synchronized. Synchronization is in units of the system page size: offset is rounded down to a page boundary; (offset+nbytes-1) is rounded up to a page boundary.

The flags bit-mask argument can include any of the following values:

SYNC_FILE_RANGE_WAIT_BEFORE

Wait upon write-out of all pages in the specified range that have already been submitted to the device driver for write-out before performing any write.

SYNC_FILE_RANGE_WRITE

Initiate write-out of all dirty pages in the specified range which are not presently submitted write-out. Note that even this may block if you attempt to write more than request queue size.

SYNC_FILE_RANGE_WAIT_AFTER

Wait upon write-out of all pages in the range after performing any write.

Specifying flags as 0 is permitted, as a no-op.

Warning

This system call is extremely dangerous and should not be used in portable programs. None of these operations writes out the file's metadata. Therefore, unless the application is strictly performing overwrites of already-instantiated disk blocks, there are no guarantees that the data will be available after a crash. There is no user interface to know if a write is purely an overwrite. On file systems using copy-on-write semantics (e.g., btrfs) an overwrite of existing allocated blocks is impossible. When writing into preallocated space, many file systems also require calls into the block allocator, which this system call does not sync out to disk. This system call does not flush disk write caches and thus does not provide any data integrity on systems with volatile disk write caches.

Some details

SYNC_FILE_RANGE_WAIT_BEFORE and SYNC_FILE_RANGE_WAIT_AFTER will detect any I/O errors or ENOSPC conditions and will return these to the caller.

Useful combinations of the flags bits are:

SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE

Ensures that all pages in the specified range which were dirty when sync_file_range() was called are placed under write-out. This is a start-write-for-data-integrity operation.

SYNC_FILE_RANGE_WRITE

Start write-out of all dirty pages in the specified range which are not presently under write-out. This is an asynchronous flush-to-disk operation. This is not suitable for data integrity operations.

SYNC_FILE_RANGE_WAIT_BEFORE (or SYNC_FILE_RANGE_WAIT_AFTER)

Wait for completion of write-out of all pages in the specified range. This can be used after an earlier SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE operation to wait for completion of that operation, and obtain its result.

SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE | SYNC_FILE_RANGE_WAIT_AFTER

This is a write-for-data-integrity operation that will ensure that all pages in the specified range which were dirty when sync_file_range() was called are committed to disk.

RETURN VALUE

On success, sync_file_range() returns 0; on failure -1 is returned and errno is set to indicate the error.

ERRORS

EBADF fd is not a valid file descriptor.

EINVAL flags specifies an invalid bit; or offset or nbytes is invalid.

EIO I/O error.

ENOMEM Out of memory.

ENOSPC Out of disk space.

ESPIPE fd refers to something other than a regular file, a block

device, a directory, or a symbolic link.

VERSIONS

sync_file_range() appeared on Linux in kernel 2.6.17.

CONFORMING TO

This system call is Linux-specific, and should be avoided in portable programs.

SEE ALSO

fdatasync(2), fsync(2), msync(2), sync(2), feature_test_macros(7)

COLOPHON

This page is part of release 3.25 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 2010-01-17 SYNC_FILE_RANGE(2)