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.\" Copyright (C) 2020 Jens Axboe <axboe@kernel.dk>
.\" Copyright (C) 2020 Red Hat, Inc.
.\"
.\" SPDX-License-Identifier: LGPL-2.0-or-later
.\"
.TH io_uring_queue_init 3 "July 10, 2020" "liburing-0.7" "liburing Manual"
.SH NAME
io_uring_queue_init - setup io_uring submission and completion queues
.SH SYNOPSIS
.nf
.BR "#include <liburing.h>"
.PP
.BI "int io_uring_queue_init(unsigned " entries ","
.BI "                        struct io_uring *" ring ","
.BI "                        unsigned " flags ");"
.BI "
.BI "int io_uring_queue_init_params(unsigned " entries ","
.BI "                               struct io_uring *" ring ","
.BI "                               struct io_uring_params *" params ");"
.fi
.PP
.SH DESCRIPTION
.PP
The io_uring_queue_init() function executes the
.BR io_uring_setup (2)
system call to initialize the submission and completion queues in the kernel
with at least
.I entries
entries in the submission queue and then maps the resulting file descriptor to
memory shared between the application and the kernel.

By default, the CQ ring will have twice the number of entries as specified by
.I entries
for the SQ ring. This is adequate for regular file or storage workloads, but
may be too small networked workloads. The SQ ring entries do not impose a limit
on the number of in-flight requests that the ring can support, it merely limits
the number that can be submitted to the kernel in one go (batch). if the CQ
ring overflows, eg more entries are generated than fits in the ring before the
application can reap them, then the ring enters a CQ ring overflow state. This
is indicated by
.B IORING_SQ_CQ_OVERFLOW
being set in the SQ ring flags. Unless the kernel runs out of available memory,
entries are not dropped, but it is a much slower completion path and will slow
down request processing. For that reason it should be avoided, and the CQ
ring sized appropriately for the workload. Setting
.I cq_entries
in
.I struct io_uring_params
will tell the kernel to allocate this many entries for the CQ ring, independent
of the SQ ring size in given in
.I entries.
If the value isn't a power of 2, it will be rounded up to the nearest power of
2.

On success io_uring_queue_init() returns 0 and
.I ring
will point to the shared memory containing the io_uring queues. On failure
.BR -errno
is returned.

.I flags
will be passed through to the io_uring_setup syscall (see 
.BR io_uring_setup (2)).

If the
.BR io_uring_queue_init_params (3)
variant is used, then the parameters indicated by
.I params
will be passed straight through to the
.BR io_uring_setup (2)
system call.

On success, the resources held by
.I ring
should be released via a corresponding call to
.BR io_uring_queue_exit (3).
.SH RETURN VALUE
.BR io_uring_queue_init (3)
returns 0 on success and -errno on failure.
.SH SEE ALSO
.BR io_uring_setup (2),
.BR io_uring_register_ring_fd (3),
.BR mmap (2),
.BR io_uring_queue_exit (3)