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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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sync_file_range(2) System Calls Manual sync_file_range(2)
sync_file_range - sync a file segment with disk
Standard C library (libc, -lc)
#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);
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 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.
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, or a directory.
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().
Linux.
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().
_FILE_OFFSET_BITS should be defined to be 64 in code that takes
the address of sync_file_range, if the code is intended to be
portable to traditional 32-bit x86 and ARM platforms where off_t's
width defaults to 32 bits.
fdatasync(2), fsync(2), msync(2), sync(2)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 sync_file_range(2)
Pages that refer to this page: fsync(2), io_uring_enter2(2), io_uring_enter(2), posix_fadvise(2), syscall(2), syscalls(2), io_uring_prep_sync_file_range(3), off_t(3type)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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