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DESCRIPTION top

sync_file_range() permits fine control when synchronizing the open file referred to by the file descriptorfd 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; ifnbytes is zero, then all bytes fromoffset 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. Theflags 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. Specifyingflags 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 detailsSYNC_FILE_RANGE_WAIT_BEFOREandSYNC_FILE_RANGE_WAIT_AFTERwill detect any I/O errors orENOSPCconditions and will return these to the caller. Useful combinations of theflags bits are:SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE Ensures that all pages in the specified range which were dirty whensync_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(orSYNC_FILE_RANGE_WAIT_AFTER) Wait for completion of write-out of all pages in the specified range. This can be used after an earlierSYNC_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 whensync_file_range() was called are committed to disk.

EBADFfd is not a valid file descriptor.EINVALflags specifies an invalid bit; oroffset ornbytes is invalid.EIOI/O error.ENOMEMOut of memory.ENOSPCOut of disk space.ESPIPEfd refers to something other than a regular file, a block device, or a directory.

VERSIONS top

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 ofsync_file_range() shown in the SYNOPSIS would force a register to be wasted as padding between thefd andoffset arguments. (Seesyscall(2) for details.) Therefore, these architectures define a different system call that orders the arguments suitably:int sync_file_range2(intfd, unsigned intflags,off_toffset, off_tnbytes); The behavior of this system call is otherwise exactly the same assync_file_range().

HISTORY top

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 namearm_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 wrapssync_file_range2() under the namesync_file_range().

_FILE_OFFSET_BITSshould be defined to be 64 in code that takes the address ofsync_file_range, if the code is intended to be portable to traditional 32-bit x86 and ARM platforms whereoff_t's width defaults to 32 bits.

COLOPHON top

This page is part of theman-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 isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17sync_file_range(2)