man2/sync_file_range.2

.\" Copyright (c) 2006 Andrew Morton <akpm@osdl.org>
.\" and Copyright 2006 Michael Kerrisk <mtk.manpages@gmail.com>
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.\" 2006-07-05 Initial creation, Michael Kerrisk based on
.\"     Andrew Morton's comments in fs/sync.c
.\" 2010-10-09, mtk, Document sync_file_range2()
.\"
.TH SYNC_FILE_RANGE 2 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
sync_file_range \- sync a file segment with disk
.SH SYNOPSIS
.nf
.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
.B #include <fcntl.h>
.PP
.BI "int sync_file_range(int " fd ", off64_t " offset ", off64_t " nbytes ,
.BI "                    unsigned int " flags );
.fi
.SH DESCRIPTION
.BR sync_file_range ()
permits fine control when synchronizing the open file referred to by the
file descriptor
.I fd
with disk.
.PP
.I offset
is the starting byte of the file range to be synchronized.
.I nbytes
specifies the length of the range to be synchronized, in bytes; if
.I nbytes
is zero, then all bytes from
.I offset
through to the end of file are synchronized.
Synchronization is in units of the system page size:
.I offset
is rounded down to a page boundary;
.I (offset+nbytes\-1)
is rounded up to a page boundary.
.PP
The
.I flags
bit-mask argument can include any of the following values:
.TP
.B 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.
.TP
.B 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.
.TP
.B SYNC_FILE_RANGE_WAIT_AFTER
Wait upon write-out of all pages in the range
after performing any write.
.PP
Specifying
.I flags
as 0 is permitted, as a no-op.
.SS 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.,
.IR 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.
.SS Some details
.B SYNC_FILE_RANGE_WAIT_BEFORE
and
.B SYNC_FILE_RANGE_WAIT_AFTER
will detect any
I/O errors or
.B ENOSPC
conditions and will return these to the caller.
.PP
Useful combinations of the
.I flags
bits are:
.TP
.B SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE
Ensures that all pages
in the specified range which were dirty when
.BR sync_file_range ()
was called are placed
under write-out.
This is a start-write-for-data-integrity operation.
.TP
.B 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.
.TP
.BR 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
.B SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE
operation to wait for completion of that operation, and obtain its result.
.TP
.B 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
.BR sync_file_range ()
was called are committed to disk.
.SH RETURN VALUE
On success,
.BR sync_file_range ()
returns 0; on failure \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I fd
is not a valid file descriptor.
.TP
.B EINVAL
.I flags
specifies an invalid bit; or
.I offset
or
.I nbytes
is invalid.
.TP
.B EIO
I/O error.
.TP
.B ENOMEM
Out of memory.
.TP
.B ENOSPC
Out of disk space.
.TP
.B ESPIPE
.I fd
refers to something other than a regular file, a block device, or
a directory.
.SH VERSIONS
.BR sync_file_range ()
appeared on Linux in kernel 2.6.17.
.SH CONFORMING TO
This system call is Linux-specific, and should be avoided
in portable programs.
.SH NOTES
.SS sync_file_range2()
Some architectures (e.g., PowerPC, ARM)
need 64-bit arguments to be aligned in a suitable pair of registers.
.\" See kernel commit edd5cd4a9424f22b0fa08bef5e299d41befd5622
On such architectures, the call signature of
.BR sync_file_range ()
shown in the SYNOPSIS would force
a register to be wasted as padding between the
.I fd
and
.I offset
arguments.
(See
.BR syscall (2)
for details.)
Therefore, these architectures define a different
system call that orders the arguments suitably:
.PP
.in +4n
.EX
.BI "int sync_file_range2(int " fd ", unsigned int " flags ,
.BI "                     off64_t " offset ", off64_t " nbytes );
.EE
.in
.PP
The behavior of this system call is otherwise exactly the same as
.BR sync_file_range ().
.PP
A system call with this signature first appeared on the ARM architecture
in Linux 2.6.20, with the name
.BR 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
.BR sync_file_range2 ()
under the name
.BR sync_file_range ().
.SH SEE ALSO
.BR fdatasync (2),
.BR fsync (2),
.BR msync (2),
.BR sync (2)