Ubuntu Manpage: sync_file_range - sync a file segment with disk

NAME

       sync_file_range - sync a file segment with disk

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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);

DESCRIPTION

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.

RETURN VALUE

       On  success,  sync_file_range()  returns  0; on failure -1 is returned and errno is set to
       indicate the error.

ERRORS

       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.

VERSIONS

   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().

STANDARDS

       Linux.

HISTORY

       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().

NOTES

       _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.