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SYNC_FILE_RANGE(2) Linux Programmer's Manual SYNC_FILE_RANGE(2)
sync_file_range - sync a file segment with disk
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fcntl.h>
int sync_file_range(int fd, off64_t offset, off64_t nbytes,
unsigned int flags);
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.
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.
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.
On success, sync_file_range() returns 0; on failure -1 is returned and errno
is set to indicate the error.
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.
sync_file_range() appeared on Linux in kernel 2.6.17.
This system call is Linux-specific, and should be avoided in portable
programs.
Some architectures (e.g., PowerPC, ARM) need 64-bit arguments to be aligned in
a suitable pair of registers. On such architectures, the call signature of
sync_file_range() is flawed, since it forces a register to be wasted as
padding between the fd and offset arguments. Therefore, these architectures
define a different system call that orders the arguments suitably:
int sync_file_range2(int fd, unsigned int flags,
off64_t offset, off64_t nbytes);
The behavior of this system call is otherwise exactly the same as
sync_file_range().
A system call with this signature first appeared on the ARM architecture in
Linux 2.6.20, with the name arm_sync_file_range(). It was renamed in Linux
2.6.22, when the analogous system call was added for PowerPC. On
architectures where glibc support is provided, glibc transparently wraps
sync_file_range2() under the name sync_file_range().
fdatasync(2), fsync(2), msync(2), sync(2)
This page is part of release 3.32 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 2010-10-09 SYNC_FILE_RANGE(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface