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man/io_uring.7
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.\" Copyright (C) 2020 Shuveb Hussain <shuveb@gmail.com>
.\" SPDX-License-Identifier: LGPL-2.0-or-later
.\"

.TH io_uring 7 "July 26, 2020" "Linux" "Linux Programmer's Manual"
.SH NAME
io_uring \- Asynchronous I/O facility
.SH SYNOPSIS
.nf
.B "#include <linux/io_uring.h>"
.fi
.PP
.SH DESCRIPTION
.PP
.B io_uring
is a Linux-specific API for asynchronous I/O.
It allows the user to submit one or more I/O requests,
which are processed asynchronously without blocking the calling process.
.B io_uring
gets its name from ring buffers which are shared between user space and
kernel space. This arrangement allows for efficient I/O,
while avoiding the overhead of copying buffers between them,
where possible.
This interface makes
.B io_uring
different from other UNIX I/O APIs,
wherein,
rather than just communicate between kernel and user space with system calls, 
ring buffers are used as the main mode of communication.
This arrangement has various performance benefits which are discussed in a
separate section below.
This man page uses the terms shared buffers, shared ring buffers and
queues interchangeably.
.PP
The general programming model you need to follow for
.B io_uring
is outlined below
.IP \(bu
Set up shared buffers with
.BR io_uring_setup (2)
and
.BR mmap (2),
mapping into user space shared buffers for the submission queue (SQ) and the 
completion queue (CQ).
You place I/O requests you want to make on the SQ,
while the kernel places the results of those operations on the CQ.
.IP \(bu
For every I/O request you need to make (like to read a file, write a file, 
accept a socket connection, etc), you create a submission queue entry,
or SQE,
describe the I/O operation you need to get done and add it to the tail of
the submission queue (SQ).
Each I/O operation is,
in essence,
the equivalent of a system call you would have made otherwise,
if you were not using
.BR io_uring .
For instance,
a SQE with the
.I opcode
set to
.B IORING_OP_READ
will request a read operation to be issued that is similar to the
.BR read (2)
system call. Refer to the opcode documentation in
.BR io_uring_enter (2)
for all supported opcodes and their properties.
You can add more than one SQE to the queue depending on the number of
operations you want to request.
.IP \(bu
After you add one or more SQEs,
you need to call
.BR io_uring_enter (2)
to tell the kernel to dequeue your I/O requests off the SQ and begin
processing them.
.IP \(bu
For each SQE you submit,
once it is done processing the request,
the kernel places a completion queue event or CQE at the tail of the
completion queue or CQ.
The kernel places exactly one matching CQE in the CQ for every SQE you
submit on the SQ.
After you retrieve a CQE,
minimally,
you might be interested in checking the
.I res
field of the CQE structure,
which corresponds to the return value of the system
call's equivalent,
had you used it directly without using 
.BR io_uring .
Given that
.B io_uring
is an async interface,
.I errno
is never used for passing back error information. Instead,
.I res
will contain what the equivalent system call would have returned in case
of success, and in case of error
.I res
will contain
.IR -errno .
For example, if the normal read system call would have returned -1 and set
.I errno
to
.BR EINVAL ,
then
.I res
would contain
.BR -EINVAL .
If the normal system call would have returned a read size of 1024, then
.I res
would contain 1024.
.IP \(bu
Optionally, 
.BR io_uring_enter (2)
can also wait for a specified number of requests to be processed by the kernel
before it returns.
If you specified a certain number of completions to wait for,
the kernel would have placed at least those many number of CQEs on the CQ,
which you can then readily read,
right after the return from
.BR io_uring_enter (2).
.IP \(bu
It is important to remember that I/O requests submitted to the kernel can
complete in any order.
It is not necessary for the kernel to process one request after another,
in the order you placed them.
Given that the interface is a ring,
the requests are attempted in order,
however that doesn't imply any sort of ordering on their execution or 
completion.
When more than one request is in flight,
it is not possible to determine which one will execute or complete first.
When you dequeue CQEs off the CQ,
you should always check which submitted request it corresponds to.
The most common method for doing so is utilizing the
.I user_data
field in the request, which is passed back on the completion side.
.IP \(bu
Concretely, for operations where strict ordering is required, 
such as for sends and receives on a stream-oriented TCP socket, 
it is generally unsafe to have more than one outstanding send,
or more than one outstanding receive (the two directions are independent)
on a given socket at a time, as the kernel may reorder their execution 
if poll arming or other background kernel activities are involved.
However,
.B io_uring
provides various facilities to enable applications to efficiently 
pipeline their operations safely. If the requests are submitted in a
single batch, the application may use 
.B IOSQE_IO_LINK
to enforce an execution order in the kernel. Otherwise,
.B io_uring
provides advanced features like 
.I multi shot
and send/receive 
.I bundles
to allow applications to provide more data in fewer, more efficient trips 
to the kernel. Even if these features are used, applications must still
ensure they do not overlap different sends or different receives on a
given file.
.PP
Adding to and reading from the queues:
.IP \(bu
You add SQEs to the tail of the SQ.
The kernel reads SQEs off the head of the queue.
.IP \(bu
The kernel adds CQEs to the tail of the CQ.
You read CQEs off the head of the queue.
.PP
It should be noted that depending on the configuration io_uring's behavior
can deviate from the behavior outlined above, like not posting a CQE for
every SQE when setting
.B IOSQE_CQE_SKIP_SUCCESS
in the SQE or posting multiple CQEs for a single SQE for multi shot operations
or requiring an
.BR io_uring_enter (2)
syscall to make the kernel begin processing newly added SQEs when using
submission queue polling.

.SS Submission queue polling
One of the goals of 
.B io_uring
is to provide a means for efficient I/O.
To this end,
.B io_uring
supports a polling mode that lets you avoid the call to
.BR io_uring_enter (2),
which you use to inform the kernel that you have queued SQEs on to the SQ.
With SQ Polling,
.B io_uring
starts a kernel thread that polls the submission queue for any I/O
requests you submit by adding SQEs.
With SQ Polling enabled,
there is no need for you to call 
.BR io_uring_enter (2),
letting you avoid the overhead of system calls.
A designated kernel thread dequeues SQEs off the SQ as you add them and
dispatches them for asynchronous processing.
.SS Setting up io_uring
.PP
The main steps in setting up
.B io_uring
consist of mapping in the shared buffers with
.BR mmap (2)
calls.
In the example program included in this man page, 
the function
.BR app_setup_uring ()
sets up 
.B io_uring
with a QUEUE_DEPTH deep submission queue.
Pay attention to the 2 
.BR mmap (2)
calls that set up the shared submission and completion queues.
If your kernel is older than version 5.4,
three 
.BR mmap(2) 
calls are required.
.PP
.SS Submitting I/O requests
The process of submitting a request consists of describing the I/O
operation you need to get done using an 
.B io_uring_sqe
structure instance.
These details describe the equivalent system call and its parameters.
Because the range of I/O operations Linux supports are very varied and the
.B io_uring_sqe
structure needs to be able to describe them, 
it has several fields,
some packed into unions for space efficiency.
Here is a simplified version of struct 
.B io_uring_sqe 
with some of the most often used fields:
.PP
.in +4n
.EX
struct io_uring_sqe {
        __u8    opcode;         /* type of operation for this sqe */
        __s32   fd;             /* file descriptor to do IO on */
        __u64   off;            /* offset into file */
        __u64   addr;           /* pointer to buffer or iovecs */
        __u32   len;            /* buffer size or number of iovecs */
        __u64   user_data;      /* data to be passed back at completion time */
        __u8    flags;          /* IOSQE_ flags */
        ...
};
.EE
.in

Here is struct 
.B io_uring_sqe
in full:

.in +4n
.EX
struct io_uring_sqe {
	__u8	opcode;		/* type of operation for this sqe */
	__u8	flags;		/* IOSQE_ flags */
	__u16	ioprio;		/* ioprio for the request */
	__s32	fd;		/* file descriptor to do IO on */
	union {
		__u64	off;	/* offset into file */
		__u64	addr2;
		struct {
			__u32	cmd_op;
			__u32	__pad1;
		};
	};
	union {
		__u64	addr;	/* pointer to buffer or iovecs */
		__u64	splice_off_in;
		struct {
			__u32	level;
			__u32	optname;
		};
	};
	__u32	len;		/* buffer size or number of iovecs */
	union {
		__kernel_rwf_t	rw_flags;
		__u32		fsync_flags;
		__u16		poll_events;	/* compatibility */
		__u32		poll32_events;	/* word-reversed for BE */
		__u32		sync_range_flags;
		__u32		msg_flags;
		__u32		timeout_flags;
		__u32		accept_flags;
		__u32		cancel_flags;
		__u32		open_flags;
		__u32		statx_flags;
		__u32		fadvise_advice;
		__u32		splice_flags;
		__u32		rename_flags;
		__u32		unlink_flags;
		__u32		hardlink_flags;
		__u32		xattr_flags;
		__u32		msg_ring_flags;
		__u32		uring_cmd_flags;
		__u32		waitid_flags;
		__u32		futex_flags;
		__u32		install_fd_flags;
		__u32		nop_flags;
	};
	__u64	user_data;	/* data to be passed back at completion time */
	/* pack this to avoid bogus arm OABI complaints */
	union {
		/* index into fixed buffers, if used */
		__u16	buf_index;
		/* for grouped buffer selection */
		__u16	buf_group;
	} __attribute__((packed));
	/* personality to use, if used */
	__u16	personality;
	union {
		__s32	splice_fd_in;
		__u32	file_index;
		__u32	optlen;
		struct {
			__u16	addr_len;
			__u16	__pad3[1];
		};
	};
	union {
		struct {
			__u64	addr3;
			__u64	__pad2[1];
		};
		__u64	optval;
		/*
		 * If the ring is initialized with IORING_SETUP_SQE128, then
		 * this field is used for 80 bytes of arbitrary command data
		 */
		__u8	cmd[0];
	};
};
.EE
.in
.PP
To submit an I/O request to 
.BR io_uring ,
you need to acquire a submission queue entry (SQE) from the submission
queue (SQ),
fill it up with details of the operation you want to submit and call 
.BR io_uring_enter (2). 
There are helper functions of the form io_uring_prep_X to enable proper
setup of the SQE. If you want to avoid calling
.BR io_uring_enter (2),
you have the option of setting up Submission Queue Polling.
.PP
SQEs are added to the tail of the submission queue.
The kernel picks up SQEs off the head of the SQ.
The general algorithm to get the next available SQE and update the tail is
as follows.
.PP
.in +4n
.EX
struct io_uring_sqe *sqe;
unsigned tail, index;
tail = *sqring->tail;
index = tail & (*sqring->ring_mask);
sqe = &sqring->sqes[index];
/* fill up details about this I/O request */
describe_io(sqe);
/* fill the sqe index into the SQ ring array */
sqring->array[index] = index;
tail++;
atomic_store_explicit(sqring->tail, tail, memory_order_release);
.EE
.in
.PP
To get the index of an entry,
the application must mask the current tail index with the size mask of the
ring.
This holds true for both SQs and CQs.
Once the SQE is acquired,
the necessary fields are filled in,
describing the request.
While the CQ ring directly indexes the shared array of CQEs,
the submission side has an indirection array between them.
The submission side ring buffer is an index into this array,
which in turn contains the index into the SQEs.
.PP
The following code snippet demonstrates how a read operation,
an equivalent of a
.BR preadv2 (2)
system call is described by filling up an SQE with the necessary
parameters.
.PP
.in +4n
.EX
struct iovec iovecs[16];
 ...
sqe->opcode = IORING_OP_READV;
sqe->fd = fd;
sqe->addr = (unsigned long) iovecs;
sqe->len = 16;
sqe->off = offset;
sqe->flags = 0;
.EE
.in
.TP 
.B Memory ordering
Modern compilers and CPUs freely reorder reads and writes without 
affecting the program's outcome to optimize performance. 
Some aspects of this need to be kept in mind on SMP systems since 
.B io_uring
involves buffers shared between kernel and user space.
These buffers are both visible and modifiable from kernel and user space.
As heads and tails belonging to these shared buffers are updated by kernel
and user space,
changes need to be coherently visible on either side,
irrespective of whether a CPU switch took place after the kernel-user mode
switch happened.
We use memory barriers to enforce this coherency.
Being significantly large subjects on their own,
memory barriers are out of scope for further discussion on this man page.
For more information on modern memory models the reader may refer to the
Documentation/memory-barriers.txt in the kernel tree or to the documentation
of the formal C11 or kernel memory model.
.TP
.B Letting the kernel know about I/O submissions
Once you place one or more SQEs on to the SQ,
you need to let the kernel know that you've done so.
You can do this by calling the
.BR io_uring_enter (2) 
system call.
This system call is also capable of waiting for a specified count of
events to complete.
This way,
you can be sure to find completion events in the completion queue without
having to poll it for events later.
.SS SQE pointer lifetimes & data stability
Due to the fixed size of the submission queue entry (SQE) some data you
provide in order to perform a desired operation will be passed in the
form of a pointer rather than value. In this situation, you may free
the memory backing the pointer once the succeeding
.BR io_uring_enter (2)
call has completed; providing it is only required by the operation when submitting.

When
.B IORING_SETUP_SQPOLL
is not enabled, this is done when you call
.BR io_uring_submit (3)
In The event
.B IORING_SETUP_SQPOLL
is enabled, you must ensure any provided pointers remain valid until completion.

However, very early kernels (5.4 and earlier) required state to be
 stable until the completion occurred regardless. Applications can test for this
 behavior by inspecting the
.B IORING_FEAT_SUBMIT_STABLE
flag passed back from
.BR io_uring_queue_init_params (3).

As an example, the
.B IORING_OP_TIMEOUT
operation takes a pointer to a __kernel_timespec struct. This struct is
then read by the kernel when you submit the submission queue entries,
once submitted, you may free the backing memory of the __kernel_timespec
as it will not be read again by the kernel.

It should be noted that this behaviour does not apply to data that is read
or written while the operation is inflight. For example, the pointers to
a buffer used as part of a
.B IORING_OP_WRITE
or
.B IORING_OP_READ
operation must remain valid until completion.
.SS Reading completion events
Similar to the submission queue (SQ),
the completion queue (CQ) is a shared buffer between the kernel and user
space.
Whereas you placed submission queue entries on the tail of the SQ and the
kernel read off the head,
when it comes to the CQ,
the kernel places completion queue events or CQEs on the tail of the CQ and
you read off its head.
.PP
Submission is flexible (and thus a bit more complicated) since it needs to
be able to encode different types of system calls that take various
parameters.
Completion,
on the other hand is simpler since we're looking only for a return value
back from the kernel.
This is easily understood by looking at the completion queue event
structure,
struct 
.BR io_uring_cqe :
.PP
.in +4n
.EX
struct io_uring_cqe {
	__u64	user_data;  /* sqe->data submission passed back */
	__s32	res;        /* result code for this event */
	__u32	flags;
};
.EE
.in
.PP
Here,
.I user_data
is custom data that is passed unchanged from submission to completion.
That is,
from SQEs to CQEs.
This field can be used to set context,
uniquely identifying submissions that got completed.
Given that I/O requests can complete in any order,
this field can be used to correlate a submission with a completion.
.I res
is the result from the system call that was performed as part of the
submission;
its return value.

The
.I flags
field carries request-specific information. As of the 6.12 kernel,
the following flags are defined:

.TP
.B IORING_CQE_F_BUFFER
If set, the upper 16 bits of the flags field carries the buffer ID that was
chosen for this request. The request must have been issued with
.B IOSQE_BUFFER_SELECT
set, and used with a request type that supports buffer selection. Additionally,
buffers must have been provided upfront either via the
.B IORING_OP_PROVIDE_BUFFERS
or the
.B IORING_REGISTER_PBUF_RING
methods.
.TP
.B IORING_CQE_F_MORE
If set, the application should expect more completions from the request. This
is used for requests that can generate multiple completions, such as multi-shot
requests, receive, or accept.
.TP
.B IORING_CQE_F_SOCK_NONEMPTY
If set, upon receiving the data from the socket in the current request, the
socket still had data left on completion of this request.
.TP
.B IORING_CQE_F_NOTIF
Set for notification CQEs, as seen with the zero-copy networking send and
receive support.
.TP
.B IORING_CQE_F_BUF_MORE
If set, the buffer ID set in the completion will get more completions. This
means that the provided buffer has been partially consumed and there's more
buffer space left, and hence the application should expect more completions
with this buffer ID. Each completion will continue where the previous one
left off. This can only happen if the provided buffer ring has been setup
with
.B IOU_PBUF_RING_INC
to allow for incremental / partial consumption of buffers.
.TP
.B IORING_CQE_F_SKIP
If the ring has been configured with
.B IORING_SETUP_CQE_MIXED ,
then CQEs may be posted which has this flag set. This can happen if the ring
is a single 16b CQE entry away from wrapping, but someone needs to post a 32b
CQE. As CQEs must be contiguous in memory, a filler/pad CQE needs to get
posted to allow posting of the 32b CQE. CQEs with this flag set should simply
be skipped and ignored, they serve no other purpose than to fill a gap in the
CQ ring.
.TP
.B IORING_CQE_F_32
If the ring has been configured with
.B IORING_SETUP_CQE_MIXED ,
this flag is set when the CQE is of the 32b type. This tells the application
that there's an extra 16b of space in this CQE, and that to get to the next
CQE the CQ ring must be advanced by twice as much as for a normal 16b CQE.
.PP
The general sequence to read completion events off the completion queue is
as follows:
.PP
.in +4n
.EX
unsigned head;
head = *cqring->head;
if (head != atomic_load_acquire(cqring->tail)) {
    struct io_uring_cqe *cqe;
    unsigned index;
    index = head & (cqring->mask);
    cqe = &cqring->cqes[index];
    /* process completed CQE */
    process_cqe(cqe);
    /* CQE consumption complete */
    head++;
}
atomic_store_explicit(cqring->head, head, memory_order_release);
.EE
.in
.PP
It helps to be reminded that the kernel adds CQEs to the tail of the CQ,
while you need to dequeue them off the head.
To get the index of an entry at the head,
the application must mask the current head index with the size mask of the
ring.
Once the CQE has been consumed or processed,
the head needs to be updated to reflect the consumption of the CQE.
Attention should be paid to the read and write barriers to ensure
successful read and update of the head.
.SS io_uring performance
Because of the shared ring buffers between kernel and user space,
.B io_uring
can be a zero-copy system.
Copying buffers to and from becomes necessary when system calls that
transfer data between kernel and user space are involved.
But since the bulk of the communication in 
.B io_uring
is via buffers shared between the kernel and user space,
this huge performance overhead is completely avoided.
.PP
While system calls may not seem like a significant overhead,
in high performance applications,
making a lot of them will begin to matter.
While workarounds the operating system has in place to deal with Spectre
and Meltdown are ideally best done away with,
unfortunately,
some of these workarounds are around the system call interface,
making system calls not as cheap as before on affected hardware.
While newer hardware should not need these workarounds,
hardware with these vulnerabilities can be expected to be in the wild for a
long time.
While using synchronous programming interfaces or even when using
asynchronous programming interfaces under Linux,
there is at least one system call involved in the submission of each
request.
In
.BR io_uring ,
on the other hand,
you can batch several requests in one go,
simply by queueing up multiple SQEs,
each describing an I/O operation you want and make a single call to 
.BR io_uring_enter (2). 
This is possible due to
.BR io_uring 's
shared buffers based design.
.PP
While this batching in itself can avoid the overhead associated with
potentially multiple and frequent system calls,
you can reduce even this overhead further with Submission Queue Polling,
by having the kernel poll and pick up your SQEs for processing as you add
them to the submission queue. This avoids the
.BR io_uring_enter (2)
call you need to make to tell the kernel to pick SQEs up.
For high-performance applications,
this means even fewer system call overheads.
.SH CONFORMING TO
.B io_uring
is Linux-specific.
.SH EXAMPLES
The following example uses
.B io_uring
to copy stdin to stdout.
Using shell redirection,
you should be able to copy files with this example.
Because it uses a queue depth of only one,
this example processes I/O requests one after the other.
It is purposefully kept this way to aid understanding.
In real-world scenarios however,
you'll want to have a larger queue depth to parallelize I/O request
processing so as to gain the kind of performance benefits
.B io_uring
provides with its asynchronous processing of requests.
.PP
.EX
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <sys/syscall.h>
#include <sys/mman.h>
#include <sys/uio.h>
#include <linux/fs.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdatomic.h>
#include <errno.h>

#include <linux/io_uring.h>

#define QUEUE_DEPTH 1
#define BLOCK_SZ    1024

/* Macros for barriers needed by io_uring */
#define io_uring_smp_store_release(p, v)            \\
    atomic_store_explicit((_Atomic typeof(*(p)) *)(p), (v), \\
                  memory_order_release)
#define io_uring_smp_load_acquire(p)                \\
    atomic_load_explicit((_Atomic typeof(*(p)) *)(p),   \\
                 memory_order_acquire)

int ring_fd;
unsigned *sring_tail, *sring_mask, *sring_array, 
            *cring_head, *cring_tail, *cring_mask;
struct io_uring_sqe *sqes;
struct io_uring_cqe *cqes;
char buff[BLOCK_SZ];
off_t offset;

/*
 * System call wrappers provided since glibc does not yet
 * provide wrappers for io_uring system calls.
* */

int io_uring_setup(unsigned entries, struct io_uring_params *p)
{
    int ret;
    ret = syscall(__NR_io_uring_setup, entries, p);
    return (ret < 0) ? -errno : ret;
}

int io_uring_enter(int ring_fd, unsigned int to_submit,
                   unsigned int min_complete, unsigned int flags)
{
    int ret;
    ret = syscall(__NR_io_uring_enter, ring_fd, to_submit,
                  min_complete, flags, NULL, 0);
    return (ret < 0) ? -errno : ret;
}

int app_setup_uring(void) {
    struct io_uring_params p;
    void *sq_ptr, *cq_ptr;

    /* See io_uring_setup(2) for io_uring_params.flags you can set */
    memset(&p, 0, sizeof(p));
    ring_fd = io_uring_setup(QUEUE_DEPTH, &p);
    if (ring_fd < 0) {
        perror("io_uring_setup");
        return 1;
    }

    /*
     * io_uring communication happens via 2 shared kernel-user space ring
     * buffers, which can be jointly mapped with a single mmap() call in
     * kernels >= 5.4.
     */

    int sring_sz = p.sq_off.array + p.sq_entries * sizeof(unsigned);
    int cring_sz = p.cq_off.cqes + p.cq_entries * sizeof(struct io_uring_cqe);

    /* Rather than check for kernel version, the recommended way is to
     * check the features field of the io_uring_params structure, which is a 
     * bitmask. If IORING_FEAT_SINGLE_MMAP is set, we can do away with the
     * second mmap() call to map in the completion ring separately.
     */
    if (p.features & IORING_FEAT_SINGLE_MMAP) {
        if (cring_sz > sring_sz)
            sring_sz = cring_sz;
        cring_sz = sring_sz;
    }

    /* Map in the submission and completion queue ring buffers.
     *  Kernels < 5.4 only map in the submission queue, though.
     */
    sq_ptr = mmap(0, sring_sz, PROT_READ | PROT_WRITE,
                  MAP_SHARED | MAP_POPULATE,
                  ring_fd, IORING_OFF_SQ_RING);
    if (sq_ptr == MAP_FAILED) {
        perror("mmap");
        return 1;
    }

    if (p.features & IORING_FEAT_SINGLE_MMAP) {
        cq_ptr = sq_ptr;
    } else {
        /* Map in the completion queue ring buffer in older kernels separately */
        cq_ptr = mmap(0, cring_sz, PROT_READ | PROT_WRITE,
                      MAP_SHARED | MAP_POPULATE,
                      ring_fd, IORING_OFF_CQ_RING);
        if (cq_ptr == MAP_FAILED) {
            perror("mmap");
            return 1;
        }
    }
    /* Save useful fields for later easy reference */
    sring_tail = sq_ptr + p.sq_off.tail;
    sring_mask = sq_ptr + p.sq_off.ring_mask;
    sring_array = sq_ptr + p.sq_off.array;

    /* Map in the submission queue entries array */
    sqes = mmap(0, p.sq_entries * sizeof(struct io_uring_sqe),
                   PROT_READ | PROT_WRITE, MAP_SHARED | MAP_POPULATE,
                   ring_fd, IORING_OFF_SQES);
    if (sqes == MAP_FAILED) {
        perror("mmap");
        return 1;
    }

    /* Save useful fields for later easy reference */
    cring_head = cq_ptr + p.cq_off.head;
    cring_tail = cq_ptr + p.cq_off.tail;
    cring_mask = cq_ptr + p.cq_off.ring_mask;
    cqes = cq_ptr + p.cq_off.cqes;

    return 0;
}

/*
* Read from completion queue.
* In this function, we read completion events from the completion queue.
* We dequeue the CQE, update and head and return the result of the operation.
* */

int read_from_cq() {
    struct io_uring_cqe *cqe;
    unsigned head;

    /* Read barrier */
    head = io_uring_smp_load_acquire(cring_head);
    /*
    * Remember, this is a ring buffer. If head == tail, it means that the
    * buffer is empty.
    * */
    if (head == *cring_tail)
        return -1;

    /* Get the entry */
    cqe = &cqes[head & (*cring_mask)];
    if (cqe->res < 0)
        fprintf(stderr, "Error: %s\\n", strerror(abs(cqe->res)));

    head++;

    /* Write barrier so that update to the head are made visible */
    io_uring_smp_store_release(cring_head, head);

    return cqe->res;
}

/*
* Submit a read or a write request to the submission queue.
* */

int submit_to_sq(int fd, int op) {
    unsigned index, tail;

    /* Add our submission queue entry to the tail of the SQE ring buffer */
    tail = *sring_tail;
    index = tail & *sring_mask;
    struct io_uring_sqe *sqe = &sqes[index];
    /* Fill in the parameters required for the read or write operation */
    sqe->opcode = op;
    sqe->fd = fd;
    sqe->addr = (unsigned long) buff;
    if (op == IORING_OP_READ) {
        memset(buff, 0, sizeof(buff));
        sqe->len = BLOCK_SZ;
    }
    else {
        sqe->len = strlen(buff);
    }
    sqe->off = offset;

    sring_array[index] = index;
    tail++;

    /* Update the tail */
    io_uring_smp_store_release(sring_tail, tail);

    /*
    * Tell the kernel we have submitted events with the io_uring_enter()
    * system call. We also pass in the IORING_ENTER_GETEVENTS flag which
    * causes the io_uring_enter() call to wait until min_complete
    * (the 3rd param) events complete.
    * */
    int ret =  io_uring_enter(ring_fd, 1,1,
                              IORING_ENTER_GETEVENTS);
    if(ret < 0) {
        perror("io_uring_enter");
        return -1;
    }

    return ret;
}

int main(int argc, char *argv[]) {
    int res;

    /* Setup io_uring for use */
    if(app_setup_uring()) {
        fprintf(stderr, "Unable to setup uring!\\n");
        return 1;
    }

    /* 
    * A while loop that reads from stdin and writes to stdout.
    * Breaks on EOF.
    */
    while (1) {
        /* Initiate read from stdin and wait for it to complete */
        submit_to_sq(STDIN_FILENO, IORING_OP_READ);
        /* Read completion queue entry */
        res = read_from_cq();
        if (res > 0) {
            /* Read successful. Write to stdout. */
            submit_to_sq(STDOUT_FILENO, IORING_OP_WRITE);
            read_from_cq();
        } else if (res == 0) {
            /* reached EOF */
            break;
        }
        else if (res < 0) {
            /* Error reading file */
            fprintf(stderr, "Error: %s\\n", strerror(abs(res)));
            break;
        }
        offset += res;
    }

    return 0;
}
.EE
.SH SEE ALSO
.BR io_uring_enter (2)
.BR io_uring_register (2)
.BR io_uring_setup (2)