SYNC_FILE_RANGE(2)

NAME
sync_file_range - sync a file segment with disk

SYNOPSIS
#define _GNU_SOURCE
#include <fcntl.h>

void sync_file_range(int fd, off64_t offset, off64_t nbytes,
                     unsigned int flags);

DESCRIPTION
sync_file_range() permits fine control when synchronising the open file referred to by the file descriptor fd with disk.

offset is the starting byte of the file range to be synchronised. nbytes specifies the length of the range to be synchronised, in bytes; if nbytes is zero, then all bytes from offset through to the end of file are synchronised. Synchronisation 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.

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.

NOTES
None of these operations write 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.

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 traditional fdatasync(2) operation. It 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.

ERRORS

EBADF
    fd is not a valid file descriptor.

EIO I/O error.

EINVAL
    flags specifies an invalid bit; or offset or nbytes is invalid.

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.

CONFORMING TO
This system call is Linux specific, and should be avoided in portable programs.

VERSIONS
sync_file_range() appeared on Linux in kernel 2.6.17.

SEE ALSO
fdatasync(2), fsync(2), msync(2), sync(2)