sync_file_range() - Unix, Linux System Call

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:

Tag	Description
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:

Tag	Description
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

Tag	Description
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.