.\" Copyright (C) 2019 Jens Axboe <axboe@kernel.dk>

.\" Copyright (C) 2019 Red Hat, Inc.

.\"

.\" SPDX-License-Identifier: LGPL-2.0-or-later

.\"

.TH IO_URING_ENTER 2 2019-01-22 "Linux" "Linux Programmer's Manual"

.SH NAME

io_uring_enter \- initiate and/or complete asynchronous I/O

.SH SYNOPSIS

.nf

.BR "#include <linux/io_uring.h>"

.PP

.BI "int io_uring_enter(unsigned int " fd ", unsigned int " to_submit ,

.BI "                   unsigned int " min_complete ", unsigned int " flags ,

.BI "                   sigset_t *" sig );

.fi

.PP

.SH DESCRIPTION

.PP

.BR io_uring_enter ()

is used to initiate and complete I/O using the shared submission and

completion queues setup by a call to

.BR io_uring_setup (2).

A single call can both submit new I/O and wait for completions of I/O

initiated by this call or previous calls to

.BR io_uring_enter ().

.I fd

is the file descriptor returned by

.BR io_uring_setup (2).

.I to_submit

specifies the number of I/Os to submit from the submission queue.  If

the

.B IORING_ENTER_GETEVENTS

bit is set in

.IR flags ,

then the system call will attempt to wait for

.I min_complete

event completions before returning.  If the io_uring instance was

configured for polling, by specifying

.B IORING_SETUP_IOPOLL

in the call to

.BR io_uring_setup (2),

then min_complete has a slightly different meaning.  Passing a value

of 0 instructs the kernel to return any events which are already complete,

without blocking.  If

.I min_complete

is a non-zero value, the kernel will still return immediately if any

completion events are available.  If no event completions are

available, then the call will poll either until one or more

completions become available, or until the process has exceeded its

scheduler time slice.

Note that, for interrupt driven I/O (where

.B IORING_SETUP_IOPOLL

was not specified in the call to

.BR io_uring_setup (2)),

an application may check the completion queue for event completions

without entering the kernel at all.

.PP

When the system call returns that a certain amount of SQEs have been

consumed and submitted, it's safe to reuse SQE entries in the ring. This is

true even if the actual IO submission had to be punted to async context,

which means that the SQE may in fact not have been submitted yet. If the

kernel requires later use of a particular SQE entry, it will have made a

private copy of it.

.I sig

is a pointer to a signal mask (see

.BR sigprocmask (2));

if

.I sig

is not NULL,

.BR io_uring_enter ()

first replaces the current signal mask by the one pointed to by

.IR sig ,

then waits for events to become available in the completion queue, and

then restores the original signal mask.  The following

.BR io_uring_enter ()

call:

.PP

.in +4n

.EX

ret = io_uring_enter(fd, 0, 1, IORING_ENTER_GETEVENTS, &sig);

.EE

.in

.PP

is equivalent to

.I atomically

executing the following calls:

.PP

.in +4n

.EX

pthread_sigmask(SIG_SETMASK, &sig, &orig);

ret = io_uring_enter(fd, 0, 1, IORING_ENTER_GETEVENTS, NULL);

pthread_sigmask(SIG_SETMASK, &orig, NULL);

.EE

.in

.PP

See the description of

.BR pselect (2)

for an explanation of why the

.I sig

parameter is necessary.

Submission queue entries are represented using the following data

structure:

.PP

.in +4n

.EX

/*

 * IO submission data structure (Submission Queue Entry)

 */

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;

    };

    union {

        __u64   addr;       /* pointer to buffer or iovecs */

        __u64   splice_off_in;

    }

    __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;

    };

    __u64    user_data;     /* data to be passed back at completion time */

    union {

	struct {

	    /* index into fixed buffers, if used */

            union {

                /* index into fixed buffers, if used */

                __u16    buf_index;

                /* for grouped buffer selection */

                __u16    buf_group;

            }

	    /* personality to use, if used */

	    __u16    personality;

            __s32    splice_fd_in;

	};

        __u64    __pad2[3];

    };

};

.EE

.in

.PP

The

.I opcode

describes the operation to be performed.  It can be one of:

.TP

.B IORING_OP_NOP

Do not perform any I/O.  This is useful for testing the performance of

the io_uring implementation itself.

.TP

.B IORING_OP_READV

.TP

.B IORING_OP_WRITEV

Vectored read and write operations, similar to

.BR preadv2 (2)

and

.BR pwritev2 (2).

.TP

.B IORING_OP_READ_FIXED

.TP

.B IORING_OP_WRITE_FIXED

Read from or write to pre-mapped buffers.  See

.BR io_uring_register (2)

for details on how to setup a context for fixed reads and writes.

.TP

.B IORING_OP_FSYNC

File sync.  See also

.BR fsync (2).

Note that, while I/O is initiated in the order in which it appears in

the submission queue, completions are unordered.  For example, an

application which places a write I/O followed by an fsync in the

submission queue cannot expect the fsync to apply to the write.  The

two operations execute in parallel, so the fsync may complete before

the write is issued to the storage.  The same is also true for

previously issued writes that have not completed prior to the fsync.

.TP

.B IORING_OP_POLL_ADD

Poll the

.I fd

specified in the submission queue entry for the events

specified in the

.I poll_events

field.  Unlike poll or epoll without

.BR EPOLLONESHOT ,

this interface always works in one shot mode.  That is, once the poll

operation is completed, it will have to be resubmitted.

.TP

.B IORING_OP_POLL_REMOVE

Remove an existing poll request.  If found, the

.I res

field of the

.I "struct io_uring_cqe"

will contain 0.  If not found,

.I res

will contain

.B -ENOENT.

.TP

.B IORING_OP_EPOLL_CTL

Add, remove or modify entries in the interest list of

.BR epoll (7).

See

.BR epoll_ctl (2)

for details of the system call.

.I fd

holds the file descriptor that represents the epoll instance,

.I addr

holds the file descriptor to add, remove or modify,

.I len

holds the operation (EPOLL_CTL_ADD, EPOLL_CTL_DEL, EPOLL_CTL_MOD) to perform and,

.I off

holds a pointer to the

.I epoll_events

structure. Available since 5.6.

.TP

.B IORING_OP_SYNC_FILE_RANGE

Issue the equivalent of a \fBsync_file_range\fR (2) on the file descriptor. The

.I fd

field is the file descriptor to sync, the

.I off

field holds the offset in bytes, the

.I len

field holds the length in bytes, and the

.I sync_range_flags

field holds the flags for the command. See also

.BR sync_file_range (2)

for the general description of the related system call. Available since 5.2.

.TP

.B IORING_OP_SENDMSG

Issue the equivalent of a

.BR sendmsg(2)

system call.

.I fd

must be set to the socket file descriptor,

.I addr

must contain a pointer to the msghdr structure, and

.I msg_flags

holds the flags associated with the system call. See also

.BR sendmsg (2)

for the general description of the related system call. Available since 5.3.

.TP

.B IORING_OP_RECVMSG

Works just like IORING_OP_SENDMSG, except for

.BR recvmsg(2)

instead. See the description of IORING_OP_SENDMSG. Available since 5.3.

.TP

.B IORING_OP_SEND

Issue the equivalent of a

.BR send(2)

system call.

.I fd

must be set to the socket file descriptor,

.I addr

must contain a pointer to the buffer,

.I len

denotes the length of the buffer to send, and

.I msg_flags

holds the flags associated with the system call. See also

.BR send(2)

for the general description of the related system call. Available since 5.6.

.TP

.B IORING_OP_RECV

Works just like IORING_OP_SEND, except for

.BR recv(2)

instead. See the description of IORING_OP_SEND. Available since 5.6.

.TP

.B IORING_OP_TIMEOUT

This command will register a timeout operation. The

.I addr

field must contain a pointer to a struct timespec64 structure,

.I len

must contain 1 to signify one timespec64 structure,

.I timeout_flags

may contain IORING_TIMEOUT_ABS

for an absolute timeout value, or 0 for a relative timeout.

.I off

may contain a completion event count. A timeout

will trigger a wakeup event on the completion ring for anyone waiting for

events. A timeout condition is met when either the specified timeout expires,

or the specified number of events have completed. Either condition will

trigger the event. If set to 0, completed events are not counted, which

effectively acts like a timer. io_uring timeouts use the

.B CLOCK_MONOTONIC

clock source. The request will complete with

.I -ETIME

if the timeout got completed through expiration of the timer, or

.I 0

if the timeout got completed through requests completing on their own. If

the timeout was cancelled before it expired, the request will complete with

.I -ECANCELED.

Available since 5.4.

.TP

.B IORING_OP_TIMEOUT_REMOVE

Attempt to remove an existing timeout operation.

.I addr

must contain the

.I user_data

field of the previously issued timeout operation. If the specified timeout

request is found and cancelled successfully, this request will terminate

with a result value of

.I 0

If the timeout request was found but expiration was already in progress,

this request will terminate with a result value of

.I -EBUSY

If the timeout request wasn't found, the request will terminate with a result

value of

.I -ENOENT

Available since 5.5.

.TP

.B IORING_OP_ACCEPT

Issue the equivalent of an

.BR accept4(2)

system call.

.I fd

must be set to the socket file descriptor,

.I addr

must contain the pointer to the sockaddr structure, and

.I addr2

must contain a pointer to the socklen_t addrlen field. See also

.BR accept4(2)

for the general description of the related system call. Available since 5.5.

.TP

.B IORING_OP_ASYNC_CANCEL

Attempt to cancel an already issued request.

.I addr

must contain the

.I user_data

field of the request that should be cancelled. The cancellation request will

complete with one of the following results codes. If found, the

.I res

field of the cqe will contain 0. If not found,

.I res

will contain -ENOENT. If found and attempted cancelled, the

.I res

field will contain -EALREADY. In this case, the request may or may not

terminate. In general, requests that are interruptible (like socket IO) will

get cancelled, while disk IO requests cannot be cancelled if already started.

Available since 5.5.

.TP

.B IORING_OP_LINK_TIMEOUT

This request must be linked with another request through

.I IOSQE_IO_LINK

which is described below. Unlike

.I IORING_OP_TIMEOUT,

.I IORING_OP_LINK_TIMEOUT

acts on the linked request, not the completion queue. The format of the command

is otherwise like

.I IORING_OP_TIMEOUT,

except there's no completion event count as it's tied to a specific request.

If used, the timeout specified in the command will cancel the linked command,

unless the linked command completes before the timeout. The timeout will

complete with

.I -ETIME

if the timer expired and the linked request was attempted cancelled, or

.I -ECANCELED

if the timer got cancelled because of completion of the linked request. Like

.B IORING_OP_TIMEOUT

the clock source used is

.B CLOCK_MONOTONIC

Available since 5.5.

.TP

.B IORING_OP_CONNECT

Issue the equivalent of a

.BR connect(2)

system call.

.I fd

must be set to the socket file descriptor,

.I addr

must contain the const pointer to the sockaddr structure, and

.I off

must contain the socklen_t addrlen field. See also

.BR connect(2)

for the general description of the related system call. Available since 5.5.

.TP

.B IORING_OP_FALLOCATE

Issue the equivalent of a

.BR fallocate(2)

system call.

.I fd

must be set to the file descriptor,

.I off

must contain the offset on which to operate, and

.I len

must contain the length. See also

.BR fallocate(2)

for the general description of the related system call. Available since 5.6.

.TP

.B IORING_OP_FADVISE

Issue the equivalent of a

.BR posix_fadvise(2)

system call.

.I fd

must be set to the file descriptor,

.I off

must contain the offset on which to operate,

.I len

must contain the length, and

.I fadvise_advice

must contain the advice associated with the operation. See also

.BR posix_fadvise(2)

for the general description of the related system call. Available since 5.6.

.TP

.B IORING_OP_MADVISE

Issue the equivalent of a

.BR madvise(2)

system call.

.I addr

must contain the address to operate on,

.I len

must contain the length on which to operate,

and

.I fadvise_advice

must contain the advice associated with the operation. See also

.BR madvise(2)

for the general description of the related system call. Available since 5.6.

.TP

.B IORING_OP_OPENAT

Issue the equivalent of a

.BR openat(2)

system call.

.I fd

is the

.I dirfd

argument,

.I addr

must contain a pointer to the

.I *pathname

argument,

.I open_flags

should contain any flags passed in, and

.I mode

is access mode of the file. See also

.BR openat(2)

for the general description of the related system call. Available since 5.6.

.TP

.B IORING_OP_OPENAT2

Issue the equivalent of a

.BR openat2(2)

system call.

.I fd

is the

.I dirfd

argument,

.I addr

must contain a pointer to the

.I *pathname

argument,

.I len

should contain the size of the open_how structure, and

.I off

should be set to the address of the open_how structure. See also

.BR openat2(2)

for the general description of the related system call. Available since 5.6.

.TP

.B IORING_OP_CLOSE

Issue the equivalent of a

.BR close(2)

system call.

.I fd

is the file descriptor to be closed. See also

.BR close(2)

for the general description of the related system call. Available since 5.6.

.TP

.B IORING_OP_STATX

Issue the equivalent of a

.BR statx(2)

system call.

.I fd

is the

.I dirfd

argument,

.I addr

must contain a pointer to the

.I *pathname

string,

.I statx_flags

is the

.I flags

argument,

.I len

should be the

.I mask

argument, and

.I off

must contain a pointer to the

.I statxbuf

to be filled in. See also

.BR statx(2)

for the general description of the related system call. Available since 5.6.

.TP

.B IORING_OP_READ

.TP

.B IORING_OP_WRITE

Issue the equivalent of a

.BR read(2)

or

.BR write(2)

system call.

.I fd

is the file descriptor to be operated on,

.I addr

contains the buffer in question, and

.I len

contains the length of the IO operation. These are non-vectored versions of the

.B IORING_OP_READV

and

.B IORING_OP_WRITEV

opcodes. See also

.BR read(2)

and

.BR write(2)

for the general description of the related system call. Available since 5.6.

.TP

.B IORING_OP_SPLICE

Issue the equivalent of a

.BR splice(2)

system call.

.I splice_fd_in

is the file descriptor to read from,

.I splice_off_in

is an offset to read from,

.I fd

is the file descriptor to write to,

.I off

is an offset from which to start writing to. A sentinel value of -1 is used

to pass the equivalent of a NULL for the offsets to

.BR splice(2).

.I len

contains the number of bytes to copy.

.I splice_flags

contains a bit mask for the flag field associated with the system call.

Please note that one of the file descriptors must refer to a pipe.

See also

.BR splice(2)

for the general description of the related system call. Available since 5.7.

.TP

.B IORING_OP_TEE

Issue the equivalent of a

.BR tee(2)

system call.

.I splice_fd_in

is the file descriptor to read from,

.I fd

is the file descriptor to write to,

.I len

contains the number of bytes to copy, and

.I splice_flags

contains a bit mask for the flag field associated with the system call.

Please note that both of the file descriptors must refer to a pipe.

See also

.BR tee(2)

for the general description of the related system call. Available since 5.8.

.TP

.B IORING_OP_FILES_UPDATE

This command is an alternative to using

.B IORING_REGISTER_FILES_UPDATE

which then works in an async fashion, like the rest of the io_uring commands.

The arguments passed in are the same.

.I addr

must contain a pointer to the array of file descriptors,

.I len

must contain the length of the array, and

.I off

must contain the offset at which to operate. Note that the array of file

descriptors pointed to in

.I addr

must remain valid until this operation has completed. Available since 5.6.

.PP

The

.I flags

field is a bit mask. The supported flags are:

.TP

.B IOSQE_FIXED_FILE

When this flag is specified,

.I fd

is an index into the files array registered with the io_uring instance (see the

.B IORING_REGISTER_FILES

section of the

.BR io_uring_register (2)

man page). Available since 5.1.

.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. Available since 5.2.

.TP

.B IOSQE_IO_LINK

When this flag is specified, it forms a link with the next SQE in the

submission ring. That next SQE will not be started before this one 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.

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. io_uring considers any unexpected result an error. This

means that, eg, a short read will also terminate the remainder of the chain.

If a chain of SQE links is broken, the remaining unstarted part of the chain

will be terminated and completed with

.B -ECANCELED

as the error code. Available since 5.3.

.TP

.B IOSQE_IO_HARDLINK

Like IOSQE_IO_LINK, but it doesn't sever regardless of the completion result.

Note that the link will still sever if we fail submitting the parent request,

hard links are only resilient in the presence of completion results for

requests that did submit correctly. IOSQE_IO_HARDLINK implies IOSQE_IO_LINK.

Available since 5.5.

.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. Available since 5.6.

.PP

.I ioprio

specifies the I/O priority.  See

.BR ioprio_get (2)

for a description of Linux I/O priorities.

.I fd

specifies the file descriptor against which the operation will be

performed, with the exception noted above.

If the operation is one of

.B IORING_OP_READ_FIXED

or

.BR IORING_OP_WRITE_FIXED ,

.I addr

and

.I len

must fall within the buffer located at

.I buf_index

in the fixed buffer array.  If the operation is either

.B IORING_OP_READV

or

.BR IORING_OP_WRITEV ,

then

.I addr

points to an iovec array of

.I len

entries.

.IR rw_flags ,

specified for read and write operations, contains a bitwise OR of

per-I/O flags, as described in the

.BR preadv2 (2)

man page.

The

.I fsync_flags

bit mask may contain either 0, for a normal file integrity sync, or

.B IORING_FSYNC_DATASYNC

to provide data sync only semantics.  See the descriptions of

.B O_SYNC

and

.B O_DSYNC

in the

.BR open (2)

manual page for more information.

The bits that may be set in

.I poll_events

are defined in \fI<poll.h>\fP, and documented in

.BR poll (2).

.I user_data

is an application-supplied value that will be copied into

the completion queue entry (see below).

.I buf_index

is an index into an array of fixed buffers, and is only valid if fixed

buffers were registered.

.I personality

is the credentials id to use for this operation. See

.BR io_uring_register(2)

for how to register personalities with io_uring. If set to 0, the current

personality of the submitting task is used.

.PP

Once the submission queue entry is initialized, I/O is submitted by

placing the index of the submission queue entry into the tail of the

submission queue.  After one or more indexes are added to the queue,

and the queue tail is advanced, the

.BR io_uring_enter (2)

system call can be invoked to initiate the I/O.

Completions use the following data structure:

.PP

.in +4n

.EX

/*

 * IO completion data structure (Completion Queue Entry)

 */

struct io_uring_cqe {

    __u64    user_data; /* sqe->data submission passed back */

    __s32    res;       /* result code for this event */

    __u32    flags;

};

.EE

.in

.PP

.I user_data

is copied from the field of the same name in the submission queue

entry.  The primary use case is to store data that the application

will need to access upon completion of this particular I/O.  The

.I flags

is reserved for future use.

.I res

is the operation-specific result.

.PP

For read and write opcodes, the

return values match those documented in the

.BR preadv2 (2)

and

.BR pwritev2 (2)

man pages.

Return codes for the io_uring-specific opcodes are documented in the

description of the opcodes above.

.PP

.SH RETURN VALUE

.BR io_uring_enter ()

returns the number of I/Os successfully consumed.  This can be zero

if

.I to_submit

was zero or if the submission queue was empty. The errors below that refer to

an error in a submission queue entry will be returned though a completion queue

entry, rather than through the system call itself.

Errors that occur not on behalf of a submission queue entry are returned via the

system call directly. On such an error, -1 is returned and

.I errno

is set appropriately.

.PP

.SH ERRORS

.TP

.B EAGAIN

The kernel was unable to allocate memory for the request, or otherwise ran out

of resources to handle it. The application should wait for some completions and

try again.

.TP

.B EBUSY

The application is attempting to overcommit the number of requests it can have

pending. The application should wait for some completions and try again. May

occur if the application tries to queue more requests than we have room for in

the CQ ring.

.TP

.B EBADF

The

.I fd

field in the submission queue entry is invalid, or the

.B IOSQE_FIXED_FILE

flag was set in the submission queue entry, but no files were registered

with the io_uring instance.

.TP

.B EFAULT

buffer is outside of the process' accessible address space

.TP

.B EFAULT

.B IORING_OP_READ_FIXED

or

.B IORING_OP_WRITE_FIXED

was specified in the

.I opcode

field of the submission queue entry, but either buffers were not

registered for this io_uring instance, or the address range described

by

.I addr

and

.I len

does not fit within the buffer registered at

.IR buf_index .

.TP

.B EINVAL

The

.I index

member of the submission queue entry is invalid.

.TP

.B EINVAL

The

.I flags

field or

.I opcode

in a submission queue entry is invalid.

.TP

.B EINVAL

.B IORING_OP_NOP

was specified in the submission queue entry, but the io_uring context

was setup for polling

.RB ( IORING_SETUP_IOPOLL

was specified in the call to io_uring_setup).

.TP

.B EINVAL

.B IORING_OP_READV

or

.B IORING_OP_WRITEV

was specified in the submission queue entry, but the io_uring instance

has fixed buffers registered.

.TP

.B EINVAL

.B IORING_OP_READ_FIXED

or

.B IORING_OP_WRITE_FIXED

was specified in the submission queue entry, and the

.I buf_index

is invalid.

.TP

.B EINVAL

.BR IORING_OP_READV ,

.BR IORING_OP_WRITEV ,

.BR IORING_OP_READ_FIXED ,

.B IORING_OP_WRITE_FIXED

or

.B IORING_OP_FSYNC

was specified in the submission queue entry, but the io_uring instance

was configured for IOPOLLing, or any of

.IR addr ,

.IR ioprio ,

.IR off ,

.IR len ,

or

.I buf_index

was set in the submission queue entry.

.TP

.B EINVAL

.B IORING_OP_POLL_ADD

or

.B IORING_OP_POLL_REMOVE

was specified in the

.I opcode

field of the submission queue entry, but the io_uring instance was