sync_file_range(2) System Calls Manual sync_file_range(2)
NAME
sync_file_range - sync a file segment with disk
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#define _GNU_SOURCE /* See feature_test_macros(7) */
#define _FILE_OFFSET_BITS 64
#include <fcntl.h>
int sync_file_range(int fd, off_t offset, off_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 meta-
data. Therefore, unless the application is strictly performing over-
writes 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 filesystems using copy-on-
write semantics (e.g., btrfs) an overwrite of existing allocated blocks
is impossible. When writing into preallocated space, many filesystems
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,
or a directory.
VERSIONS
sync_file_range2()
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() shown in the SYNOPSIS would force a
register to be wasted as padding between the fd and offset arguments.
(See syscall(2) for details.) Therefore, these architectures define a
different system call that orders the arguments suitably:
int sync_file_range2(int fd, unsigned int flags,
off_t offset, off_t nbytes);
The behavior of this system call is otherwise exactly the same as
sync_file_range().
STANDARDS
Linux.
HISTORY
Linux 2.6.17.
sync_file_range2()
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().
NOTES
_FILE_OFFSET_BITS should be defined to be 64 in code that takes the ad-
dress of sync_file_range, if the code is intended to be portable to tra-
ditional 32-bit x86 and ARM platforms where off_t's width defaults to 32
bits.
SEE ALSO
fdatasync(2), fsync(2), msync(2), sync(2)
Linux man-pages 6.9.1 2024-05-02 sync_file_range(2)
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