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.