man/io_uring_sqe_set_flags.3 - master - githubmirror/liburing

man/io_uring_sqe_set_flags.3
87 строк3 KB
Jens Axboe
man/io_uring_sqe_set_flags.3: clarify IOSQE_ASYNC
25 сен 2022, 23:05
25 сен 2022, 23:053228b1d
.\" Copyright (C) 2022 Stefan Roesch <shr@fb.com>
.\"
.\" SPDX-License-Identifier: LGPL-2.0-or-later
.\"
.TH io_uring_sqe_set_flags 3 "January 25, 2022" "liburing-2.1" "liburing Manual"
.SH NAME
io_uring_sqe_set_flags \- set flags for submission queue entry
.SH SYNOPSIS
.nf
.B #include <liburing.h>
.PP
.BI "void io_uring_sqe_set_flags(struct io_uring_sqe *" sqe ","
.BI "                            unsigned " flags ");"
.fi
.SH DESCRIPTION
.PP
The
.BR io_uring_sqe_set_flags (3)
function allows the caller to change the behavior of the submission queue entry
by specifying flags. It enables the
.I flags
belonging to the
.I sqe
submission queue entry param.

.I flags
is a bit mask of 0 or more of the following values ORed together:
.TP
.B IOSQE_FIXED_FILE
The file descriptor in the SQE refers to the index of a previously registered
file or direct file descriptor, not a normal file descriptor.
.TP
.B IOSQE_ASYNC
Normal operation for io_uring is to try and issue an sqe as non-blocking first,
and if that fails, execute it in an async manner. To support more efficient
overlapped operation of requests that the application knows/assumes will
always (or most of the time) block, the application can ask for an sqe to be
issued async from the start. Note that this flag immediately causes the SQE
to be offloaded to an async helper thread with no initial non-blocking attempt.
This may be less efficient and should not be used liberally or without
understanding the performance and efficiency tradeoffs.
.TP
.B IOSQE_IO_LINK
When this flag is specified, the SQE forms a link with the next SQE in the
submission ring. That next SQE will not be started before the previous request
completes. This, in effect, forms a chain of SQEs, which can be arbitrarily
long. The tail of the chain is denoted by the first SQE that does not have this
flag set. Chains are not supported across submission boundaries. Even if the
last SQE in a submission has this flag set, it will still terminate the current
chain. This flag has no effect on previous SQE submissions, nor does it impact
SQEs that are outside of the chain tail. This means that multiple chains can be
executing in parallel, or chains and individual SQEs. Only members inside the
chain are serialized. A chain of SQEs will be broken if any request in that
chain ends in error.
.TP
.B IOSQE_IO_HARDLINK
Like
.B IOSQE_IO_LINK ,
except the links aren't severed if an error or unexpected result occurs.
.TP
.B IOSQE_IO_DRAIN
When this flag is specified, the SQE will not be started before previously
submitted SQEs have completed, and new SQEs will not be started before this
one completes.
.TP
.B IOSQE_CQE_SKIP_SUCCESS
Request that no CQE be generated for this request, if it completes successfully.
This can be useful in cases where the application doesn't need to know when
a specific request completed, if it completed successfully.
.TP
.B IOSQE_BUFFER_SELECT
If set, and if the request types supports it, select an IO buffer from the
indicated buffer group. This can be used with requests that read or receive
data from a file or socket, where buffer selection is deferred until the kernel
is ready to transfer data, instead of when the IO is originally submitted. The
application must also set the
.I buf_group
field in the SQE, indicating which previously registered buffer group to select
a buffer from.

.SH RETURN VALUE
None
.SH SEE ALSO
.BR io_uring_submit (3),
.BR io_uring_register (3)
.BR io_uring_register_buffers (3)
.BR io_uring_register_buf_ring (3)