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[thirdparty/man-pages.git] / man2 / sync_file_range.2

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.\" 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
.\"
.TH SYNC_FILE_RANGE 2 2006-07-05 "Linux" "Linux Programmer's Manual"
.SH NAME
sync_file_range \- sync a file segment with disk
.SH SYNOPSIS
.nf
.B #define _GNU_SOURCE
.B #include <fcntl.h>

.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 synchronising the open file referred to by the
file descriptor
.I fd
with disk.

.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.

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.
.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 Some details
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.

.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.

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 traditional
.BR 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
.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,
a directory, or a symbolic link.
.\" FIXME . (bug?) Actually, how can 'fd' refer to a symbolic link (S_ISLNK)?
.\" (In userspace at least) it isn't possible to obtain a file descriptor
.\" for a symbolic link.
.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 "SEE ALSO"
.BR fdatasync (2),
.BR fsync (2),
.BR msync (2),
.BR sync (2),
.BR feature_test_macros (7)