.\" Copyright (C) 2022 Jens Axboe <axboe@kernel.dk>
.\"
.\" SPDX-License-Identifier: LGPL-2.0-or-later
.\"
.TH io_uring_prep_splice 3 "March 13, 2022" "liburing-2.2" "liburing Manual"
.SH NAME
io_uring_prep_splice \- prepare an splice request
.SH SYNOPSIS
.nf
.B #include <fcntl.h>
.B #include <liburing.h>
.PP
.BI "void io_uring_prep_splice(struct io_uring_sqe *" sqe ","
.BI "                          int " fd_in ","
.BI "                          int64_t " off_in ","
.BI "                          int " fd_out ","
.BI "                          int64_t " off_out ","
.BI "                          unsigned int " nbytes ","
.BI "                          unsigned int " splice_flags ");"
.fi
.SH DESCRIPTION
.PP
The
.BR io_uring_prep_splice (3)
function prepares a splice request. The submission queue entry
.I sqe
is setup to use as input the file descriptor
.I fd_in
at offset
.IR off_in ,
splicing data to the file descriptor at
.I fd_out
and at offset
.IR off_out .
.I nbytes
bytes of data should be spliced between the two descriptors.
.I splice_flags
are modifier flags for the operation. See
.BR splice (2)
for the generic splice flags.

If the
.I fd_out
descriptor,
.B IOSQE_FIXED_FILE
can be set in the SQE to indicate that. For the input file, the io_uring
specific
.B SPLICE_F_FD_IN_FIXED
can be set in
.I splice_flags
and
.I fd_in
given as a registered file descriptor offset.

If
.I fd_in
refers to a pipe,
.I off_in
is ignored and must be set to -1.

If
.I fd_in
does not refer to a pipe and
.I off_in
is -1, then
.I nbytes
are read from
.I fd_in
starting from the file offset, which is incremented by the number of bytes read.

If
.I fd_in
does not refer to a pipe and
.I off_in
is not -1, then the starting offset of
.I fd_in
will be
.IR off_in .

The same rules apply to
.I fd_out
and
.IR off_out .

This function prepares an async
.BR splice (2)
request. See that man page for details.

.SH RETURN VALUE
None
.SH ERRORS
The CQE
.I res
field will contain the result of the operation. See the related man page for
details on possible values. Note that where synchronous system calls will return
.B -1
on failure and set
.I errno
to the actual error value, io_uring never uses
.IR errno .
Instead it returns the negated
.I errno
directly in the CQE
.I res
field.
.SH SEE ALSO
.BR io_uring_get_sqe (3),
.BR io_uring_submit (3),
.BR io_uring_register (2),
.BR splice (2)

.SH NOTES
Note that even if
.I fd_in
or
.I fd_out
refers to a pipe, the splice operation can still fail with
.B EINVAL
if one of the fd doesn't explicitly support splice operation, e.g. reading from
terminal is unsupported from kernel 5.7 to 5.11.