sync_file_range - sync a file segment with disk


#define _GNU_SOURCE
#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:

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.
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.
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.
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:
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.
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.
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.
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.


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


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
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