Ubuntu Manpage: io_uring_enter - initiate and/or complete asynchronous I/O

Provided by: liburing-dev_2.1-2build1_amd64

NAME

       io_uring_enter - initiate and/or complete asynchronous I/O

SYNOPSIS

       #include <linux/io_uring.h>

       int io_uring_enter(unsigned int fd, unsigned int to_submit,
                          unsigned int min_complete, unsigned int flags,
                          sigset_t *sig);

DESCRIPTION

io_uring_enter() is used to initiate and complete I/O using the shared submission and completion queues
setup by a call to 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 io_uring_enter().

fd is the file descriptor returned by io_uring_setup(2). to_submit specifies the number of I/Os to
submit from the submission queue. flags is a bitmask of the following values:

IORING_ENTER_GETEVENTS
If this flag is set, then the system call will wait for the specificied number of events in
min_complete before returning. This flag can be set along with to_submit to both submit and
complete events in a single system call.

IORING_ENTER_SQ_WAKEUP
If the ring has been created with IORING_SETUP_SQPOLL, then this flag asks the kernel to wakeup
the SQ kernel thread to submit IO.

IORING_ENTER_SQ_WAIT
If the ring has been created with IORING_SETUP_SQPOLL, then the application has no real insight
into when the SQ kernel thread has consumed entries from the SQ ring. This can lead to a situation
where the application can no longer get a free SQE entry to submit, without knowing when it one
becomes available as the SQ kernel thread consumes them. If the system call is used with this flag
set, then it will wait until at least one entry is free in the SQ ring.

IORING_ENTER_EXT_ARG
Since kernel 5.11, the system calls arguments have been modified to look like the following:

int io_uring_enter(unsigned int fd, unsigned int to_submit,
unsigned int min_complete, unsigned int flags,
const void *arg, size_t argsz);

which is behaves just like the original definition by default. However, if IORING_ENTER_EXT_ARG is
set, then instead of a sigset_t being passed in, a pointer to a struct io_uring_getevents_arg is
used instead and argsz must be set to the size of this structure. The definition is as follows:

struct io_uring_getevents_args {
__u64 sigmask;
__u32 sigmask_sz;
__u32 pad;
__u64 ts;
};

which allows passing in both a signal mask as well as pointer to a struct __kernel_timespec
timeout value. If ts is set to a valid pointer, then this time value indicates the timeout for
waiting on events. If an application is waiting on events and wishes to stop waiting after a
specified amount of time, then this can be accomplished directly in version 5.11 and newer by
using this feature.

If the io_uring instance was configured for polling, by specifying IORING_SETUP_IOPOLL in the call to
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 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 IORING_SETUP_IOPOLL was not specified in the call to
io_uring_setup(2)), an application may check the completion queue for event completions without entering
the kernel at all.

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.

sig is a pointer to a signal mask (see sigprocmask(2)); if sig is not NULL, io_uring_enter() first
replaces the current signal mask by the one pointed to by sig, then waits for events to become available
in the completion queue, and then restores the original signal mask. The following io_uring_enter()
call:

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

is equivalent to atomically executing the following calls:

pthread_sigmask(SIG_SETMASK, &sig, &orig);
ret = io_uring_enter(fd, 0, 1, IORING_ENTER_GETEVENTS, NULL);
pthread_sigmask(SIG_SETMASK, &orig, NULL);

See the description of pselect(2) for an explanation of why the sig parameter is necessary.

Submission queue entries are represented using the following data structure:

/*
* 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;
__u32 rename_flags;
__u32 unlink_flags;
__u32 hardlink_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;
union {
__s32 splice_fd_in;
__u32 file_index;
};
};
__u64 __pad2[3];
};
};

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

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

IORING_OP_READV

IORING_OP_WRITEV
Vectored read and write operations, similar to preadv2(2) and pwritev2(2). If the file is not
seekable, off must be set to zero.

IORING_OP_READ_FIXED

IORING_OP_WRITE_FIXED
Read from or write to pre-mapped buffers. See io_uring_register(2) for details on how to setup a
context for fixed reads and writes.

IORING_OP_FSYNC
File sync. See also 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.

IORING_OP_POLL_ADD
Poll the fd specified in the submission queue entry for the events specified in the poll_events
field. Unlike poll or epoll without EPOLLONESHOT, by default this interface always works in one
shot mode. That is, once the poll operation is completed, it will have to be resubmitted.

If IORING_POLL_ADD_MULTI is set in the SQE len field, then the poll will work in multi shot mode
instead. That means it'll repatedly trigger when the requested event becomes true, and hence
multiple CQEs can be generated from this single SQE. The CQE flags field will have
IORING_CQE_F_MORE set on completion if the application should expect further CQE entries from the
original request. If this flag isn't set on completion, then the poll request has been terminated
and no further events will be generated. This mode is available since 5.13.

If IORING_POLL_UPDATE_EVENTS is set in the SQE len field, then the request will update an existing
poll request with the mask of events passed in with this request. The lookup is based on the
user_data field of the original SQE submitted, and this values is passed in the addr field of the
SQE. This mode is available since 5.13.

If IORING_POLL_UPDATE_USER_DATA is set in the SQE len field, then the request will update the
user_data of an existing poll request based on the value passed in the off field. This mode is
available since 5.13.

This command works like an async poll(2) and the completion event result is the returned mask of
events. For the variants that update user_data or events , the completion result will be similar
to IORING_OP_POLL_REMOVE.

IORING_OP_POLL_REMOVE
Remove an existing poll request. If found, the res field of the struct io_uring_cqe will contain
0. If not found, res will contain -ENOENT, or -EALREADY if the poll request was in the process of
completing already.

IORING_OP_EPOLL_CTL
Add, remove or modify entries in the interest list of epoll(7). See epoll_ctl(2) for details of
the system call. fd holds the file descriptor that represents the epoll instance, addr holds the
file descriptor to add, remove or modify, len holds the operation (EPOLL_CTL_ADD, EPOLL_CTL_DEL,
EPOLL_CTL_MOD) to perform and, off holds a pointer to the epoll_events structure. Available since
5.6.

IORING_OP_SYNC_FILE_RANGE
Issue the equivalent of a sync_file_range (2) on the file descriptor. The fd field is the file
descriptor to sync, the off field holds the offset in bytes, the len field holds the length in
bytes, and the sync_range_flags field holds the flags for the command. See also sync_file_range(2)
for the general description of the related system call. Available since 5.2.

IORING_OP_SENDMSG
Issue the equivalent of a sendmsg(2) system call. fd must be set to the socket file descriptor,
addr must contain a pointer to the msghdr structure, and msg_flags holds the flags associated with
the system call. See also sendmsg(2) for the general description of the related system call.
Available since 5.3.

IORING_OP_RECVMSG
Works just like IORING_OP_SENDMSG, except for recvmsg(2) instead. See the description of
IORING_OP_SENDMSG. Available since 5.3.

IORING_OP_SEND
Issue the equivalent of a send(2) system call. fd must be set to the socket file descriptor, addr
must contain a pointer to the buffer, len denotes the length of the buffer to send, and msg_flags
holds the flags associated with the system call. See also send(2) for the general description of
the related system call. Available since 5.6.

IORING_OP_RECV
Works just like IORING_OP_SEND, except for recv(2) instead. See the description of IORING_OP_SEND.
Available since 5.6.

IORING_OP_TIMEOUT
This command will register a timeout operation. The addr field must contain a pointer to a struct
timespec64 structure, len must contain 1 to signify one timespec64 structure, timeout_flags may
contain IORING_TIMEOUT_ABS for an absolute timeout value, or 0 for a relative timeout. 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 CLOCK_MONOTONIC clock source. The request will complete with -ETIME if the timeout got
completed through expiration of the timer, or 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 -ECANCELED. Available since 5.4.

Since 5.15, this command also supports the following modifiers in timeout_flags:

IORING_TIMEOUT_BOOTTIME If set, then the clocksource used is CLOCK_BOOTTIME instead of
CLOCK_MONOTONIC. This clocksource differs in that it includes time elapsed if the system was
suspend while having a timeout request in-flight.

IORING_TIMEOUT_REALTIME If set, then the clocksource used is CLOCK_BOOTTIME instead of
CLOCK_MONOTONIC.

IORING_OP_TIMEOUT_REMOVE
If timeout_flags are zero, then it attempts to remove an existing timeout operation. addr must
contain the 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 0
If the timeout request was found but expiration was already in progress, this request will
terminate with a result value of -EBUSY If the timeout request wasn't found, the request will
terminate with a result value of -ENOENT Available since 5.5.

If timeout_flags contain IORING_TIMEOUT_UPDATE, instead of removing an existing operation, it
updates it. addr and return values are same as before. addr2 field must contain a pointer to a
struct timespec64 structure. timeout_flags may also contain IORING_TIMEOUT_ABS, in which case the
value given is an absolute one, not a relative one. Available since 5.11.

IORING_OP_ACCEPT
Issue the equivalent of an accept4(2) system call. fd must be set to the socket file descriptor,
addr must contain the pointer to the sockaddr structure, and addr2 must contain a pointer to the
socklen_t addrlen field. Flags can be passed using the accept_flags field. See also accept4(2) for
the general description of the related system call. Available since 5.5.

If the file_index field is set to a positive number, the file won't be installed into the normal
file table as usual but will be placed into the fixed file table at index file_index - 1. In this
case, instead of returning a file descriptor, the result will contain either 0 on success or an
error. If there is already a file registered at this index, the request will fail with -EBADF.
Only io_uring has access to such files and no other syscall can use them. See IOSQE_FIXED_FILE and
IORING_REGISTER_FILES.

Available since 5.15.

IORING_OP_ASYNC_CANCEL
Attempt to cancel an already issued request. addr must contain the 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 res field of the cqe will contain 0. If not found, res will contain -ENOENT.
If found and attempted cancelled, the 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.

IORING_OP_LINK_TIMEOUT
This request must be linked with another request through IOSQE_IO_LINK which is described below.
Unlike IORING_OP_TIMEOUT, IORING_OP_LINK_TIMEOUT acts on the linked request, not the completion
queue. The format of the command is otherwise like 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 -ETIME if the timer expired and the linked request was attempted
cancelled, or -ECANCELED if the timer got cancelled because of completion of the linked request.
Like IORING_OP_TIMEOUT the clock source used is CLOCK_MONOTONIC Available since 5.5.

IORING_OP_CONNECT
Issue the equivalent of a connect(2) system call. fd must be set to the socket file descriptor,
addr must contain the const pointer to the sockaddr structure, and off must contain the socklen_t
addrlen field. See also connect(2) for the general description of the related system call.
Available since 5.5.

IORING_OP_FALLOCATE
Issue the equivalent of a fallocate(2) system call. fd must be set to the file descriptor, len
must contain the mode associated with the operation, off must contain the offset on which to
operate, and addr must contain the length. See also fallocate(2) for the general description of
the related system call. Available since 5.6.

IORING_OP_FADVISE
Issue the equivalent of a posix_fadvise(2) system call. fd must be set to the file descriptor,
off must contain the offset on which to operate, len must contain the length, and fadvise_advice
must contain the advice associated with the operation. See also posix_fadvise(2) for the general
description of the related system call. Available since 5.6.

IORING_OP_MADVISE
Issue the equivalent of a madvise(2) system call. addr must contain the address to operate on,
len must contain the length on which to operate, and fadvise_advice must contain the advice
associated with the operation. See also madvise(2) for the general description of the related
system call. Available since 5.6.

IORING_OP_OPENAT
Issue the equivalent of a openat(2) system call. fd is the dirfd argument, addr must contain a
pointer to the *pathname argument, open_flags should contain any flags passed in, and len is
access mode of the file. See also openat(2) for the general description of the related system
call. Available since 5.6.

Available since 5.15.

IORING_OP_OPENAT2
Issue the equivalent of a openat2(2) system call. fd is the dirfd argument, addr must contain a
pointer to the *pathname argument, len should contain the size of the open_how structure, and off
should be set to the address of the open_how structure. See also openat2(2) for the general
description of the related system call. Available since 5.6.

Available since 5.15.

IORING_OP_CLOSE
Issue the equivalent of a close(2) system call. fd is the file descriptor to be closed. See also
close(2) for the general description of the related system call. Available since 5.6.

IORING_OP_STATX
Issue the equivalent of a statx(2) system call. fd is the dirfd argument, addr must contain a
pointer to the *pathname string, statx_flags is the flags argument, len should be the mask
argument, and off must contain a pointer to the statxbuf to be filled in. See also statx(2) for
the general description of the related system call. Available since 5.6.

IORING_OP_READ

IORING_OP_WRITE
Issue the equivalent of a pread(2) or pwrite(2) system call. fd is the file descriptor to be
operated on, addr contains the buffer in question, len contains the length of the IO operation,
and offs contains the read or write offset. If fd does not refer to a seekable file, off must be
set to zero. If offs is set to -1, the offset will use (and advance) the file position, like the
read(2) and write(2) system calls. These are non-vectored versions of the IORING_OP_READV and
IORING_OP_WRITEV opcodes. See also read(2) and write(2) for the general description of the related
system call. Available since 5.6.

IORING_OP_SPLICE
Issue the equivalent of a splice(2) system call. splice_fd_in is the file descriptor to read
from, splice_off_in is an offset to read from, fd is the file descriptor to write to, 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 splice(2). len contains the number of bytes to copy. 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 splice(2) for the general description of the
related system call. Available since 5.7.

IORING_OP_TEE
Issue the equivalent of a tee(2) system call. splice_fd_in is the file descriptor to read from,
fd is the file descriptor to write to, len contains the number of bytes to copy, and 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 tee(2) for the general description of the
related system call. Available since 5.8.

IORING_OP_FILES_UPDATE
This command is an alternative to using 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. addr must
contain a pointer to the array of file descriptors, len must contain the length of the array, and
off must contain the offset at which to operate. Note that the array of file descriptors pointed
to in addr must remain valid until this operation has completed. Available since 5.6.

IORING_OP_PROVIDE_BUFFERS
This command allows an application to register a group of buffers to be used by commands that
read/receive data. Using buffers in this manner can eliminate the need to separate the poll +
read, which provides a convenient point in time to allocate a buffer for a given request. It's
often infeasible to have as many buffers available as pending reads or receive. With this feature,
the application can have its pool of buffers ready in the kernel, and when the file or socket is
ready to read/receive data, a buffer can be selected for the operation. fd must contain the
number of buffers to provide, addr must contain the starting address to add buffers from, len must
contain the length of each buffer to add from the range, buf_group must contain the group ID of
this range of buffers, and off must contain the starting buffer ID of this range of buffers. With
that set, the kernel adds buffers starting with the memory address in addr, each with a length of
len. Hence the application should provide len * fd worth of memory in addr. Buffers are grouped
by the group ID, and each buffer within this group will be identical in size according to the
above arguments. This allows the application to provide different groups of buffers, and this is
often used to have differently sized buffers available depending on what the expectations are of
the individual request. When submitting a request that should use a provided buffer, the
IOSQE_BUFFER_SELECT flag must be set, and buf_group must be set to the desired buffer group ID
where the buffer should be selected from. Available since 5.7.

IORING_OP_REMOVE_BUFFERS
Remove buffers previously registered with IORING_OP_PROVIDE_BUFFERS. fd must contain the number
of buffers to remove, and buf_group must contain the buffer group ID from which to remove the
buffers. Available since 5.7.

IORING_OP_SHUTDOWN
Issue the equivalent of a shutdown(2) system call. fd is the file descriptor to the socket being
shutdown, no other fields should be set. Available since 5.11.

IORING_OP_RENAMEAT
Issue the equivalent of a renameat2(2) system call. fd should be set to the olddirfd, addr should
be set to the oldpath, len should be set to the newdirfd, addr should be set to the oldpath, addr2
should be set to the newpath, and finally rename_flags should be set to the flags passed in to
renameat2(2). Available since 5.11.

IORING_OP_UNLINKAT
Issue the equivalent of a unlinkat2(2) system call. fd should be set to the dirfd, addr should be
set to the pathname, and unlink_flags should be set to the flags being passed in to unlinkat(2).
Available since 5.11.

IORING_OP_MKDIRAT
Issue the equivalent of a mkdirat2(2) system call. fd should be set to the dirfd, addr should be
set to the pathname, and len should be set to the mode being passed in to mkdirat(2). Available
since 5.15.

IORING_OP_SYMLINKAT
Issue the equivalent of a symlinkat2(2) system call. fd should be set to the newdirfd, addr
should be set to the target and addr2 should be set to the linkpath being passed in to
symlinkat(2). Available since 5.15.

IORING_OP_LINKAT
Issue the equivalent of a linkat2(2) system call. fd should be set to the olddirfd, addr should
be set to the oldpath, len should be set to the newdirfd, addr2 should be set to the newpath, and
hardlink_flags should be set to the flags being passed in to linkat(2). Available since 5.15.

The flags field is a bit mask. The supported flags are:

IOSQE_FIXED_FILE
When this flag is specified, fd is an index into the files array registered with the io_uring
instance (see the IORING_REGISTER_FILES section of the io_uring_register(2) man page). Note that
this isn't always available for all commands. If used on a command that doesn't support fixed
files, the SQE will error with -EBADF. Available since 5.1.

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.

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 -ECANCELED as the error code. Available since 5.3.

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.

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.

IOSQE_BUFFER_SELECT
Used in conjunction with the IORING_OP_PROVIDE_BUFFERS command, which registers a pool of buffers
to be used by commands that read or receive data. When buffers are registered for this use case,
and this flag is set in the command, io_uring will grab a buffer from this pool when the request
is ready to receive or read data. If successful, the resulting CQE will have IORING_CQE_F_BUFFER
set in the flags part of the struct, and the upper IORING_CQE_BUFFER_SHIFT bits will contain the
ID of the selected buffers. This allows the application to know exactly which buffer was selected
for the operation. If no buffers are available and this flag is set, then the request will fail
with -ENOBUFS as the error code. Once a buffer has been used, it is no longer available in the
kernel pool. The application must re-register the given buffer again when it is ready to recycle
it (eg has completed using it). Available since 5.7.

ioprio specifies the I/O priority. See ioprio_get(2) for a description of Linux I/O priorities.

fd specifies the file descriptor against which the operation will be performed, with the exception noted
above.

If the operation is one of IORING_OP_READ_FIXED or IORING_OP_WRITE_FIXED, addr and len must fall within
the buffer located at buf_index in the fixed buffer array. If the operation is either IORING_OP_READV or
IORING_OP_WRITEV, then addr points to an iovec array of len entries.

rw_flags, specified for read and write operations, contains a bitwise OR of per-I/O flags, as described
in the preadv2(2) man page.

The fsync_flags bit mask may contain either 0, for a normal file integrity sync, or IORING_FSYNC_DATASYNC
to provide data sync only semantics. See the descriptions of O_SYNC and O_DSYNC in the open(2) manual
page for more information.

The bits that may be set in poll_events are defined in <poll.h>, and documented in poll(2).

user_data is an application-supplied value that will be copied into the completion queue entry (see
below). buf_index is an index into an array of fixed buffers, and is only valid if fixed buffers were
registered. personality is the credentials id to use for this operation. See 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.

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 io_uring_enter(2) system call can be invoked to initiate the I/O.

Completions use the following data structure:

/*
* 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;
};

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
flags is used for certain commands, like IORING_OP_POLL_ADD or in conjunction with IOSQE_BUFFER_SELECT ,
see those entries. res is the operation-specific result, but io_uring-specific errors (e.g. flags or
opcode invalid) are returned through this field. They are described in section CQE ERRORS.

For read and write opcodes, the return values match errno values documented in the preadv2(2) and
pwritev2(2) man pages, with res holding the equivalent of -errno for error cases, or the transferred
number of bytes in case the operation is successful. Hence both error and success return can be found in
that field in the CQE. For other request types, the return values are documented in the matching man page
for that type, or in the opcodes section above for io_uring-specific opcodes.

RETURN VALUE

       io_uring_enter() returns the number of I/Os successfully consumed.  This can be  zero  if  to_submit  was
       zero  or  if  the  submission queue was empty. Note that if the ring was created with IORING_SETUP_SQPOLL
       specified, then the return value will generally be the same as to_submit as  submission  happens  outside
       the context of the system call.

       The  errors  related  to  a submission queue entry will be returned through a completion queue entry (see
       section CQE ERRORS), 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 errno is set appropriately.

ERRORS

       These are the errors returned by io_uring_enter() system call.

       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.

       EBADF  fd is not a valid file descriptor.

       EBADFD fd  is  a  valid  file  descriptor, but the io_uring ring is not in the right state (enabled). See
              io_uring_register(2) for details on how to enable the ring.

       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, or if the application attempts  to  wait
              for more events without having reaped the ones already present in the CQ ring.

       EINVAL Some bits in the flags argument are invalid.

       EFAULT An invalid user space address was specified for the sig argument.

       ENXIO  The io_uring instance is in the process of being torn down.

       EOPNOTSUPP
              fd does not refer to an io_uring instance.

       EINTR  The  operation  was interrupted by a delivery of a signal before it could complete; see signal(7).
              Can happen while waiting for events with IORING_ENTER_GETEVENTS.

CQE ERRORS

These io_uring-specific errors are returned as a negative value in the res field of the completion queue
entry.

EACCES The flags field or opcode in a submission queue entry is not allowed due to registered
restrictions. See io_uring_register(2) for details on how restrictions work.

EBADF The fd field in the submission queue entry is invalid, or the IOSQE_FIXED_FILE flag was set in the
submission queue entry, but no files were registered with the io_uring instance.

EFAULT buffer is outside of the process' accessible address space

EFAULT IORING_OP_READ_FIXED or IORING_OP_WRITE_FIXED was specified in the opcode field of the submission
queue entry, but either buffers were not registered for this io_uring instance, or the address
range described by addr and len does not fit within the buffer registered at buf_index.

EINVAL The flags field or opcode in a submission queue entry is invalid.

EINVAL The buf_index member of the submission queue entry is invalid.

EINVAL The personality field in a submission queue entry is invalid.

EINVAL IORING_OP_NOP was specified in the submission queue entry, but the io_uring context was setup for
polling (IORING_SETUP_IOPOLL was specified in the call to io_uring_setup).

EINVAL IORING_OP_READV or IORING_OP_WRITEV was specified in the submission queue entry, but the io_uring
instance has fixed buffers registered.

EINVAL IORING_OP_READ_FIXED or IORING_OP_WRITE_FIXED was specified in the submission queue entry, and the
buf_index is invalid.

EINVAL IORING_OP_READV, IORING_OP_WRITEV, IORING_OP_READ_FIXED, IORING_OP_WRITE_FIXED or IORING_OP_FSYNC
was specified in the submission queue entry, but the io_uring instance was configured for
IOPOLLing, or any of addr, ioprio, off, len, or buf_index was set in the submission queue entry.

EINVAL IORING_OP_POLL_ADD or IORING_OP_POLL_REMOVE was specified in the opcode field of the submission
queue entry, but the io_uring instance was configured for busy-wait polling (IORING_SETUP_IOPOLL),
or any of ioprio, off, len, or buf_index was non-zero in the submission queue entry.

EINVAL IORING_OP_POLL_ADD was specified in the opcode field of the submission queue entry, and the addr
field was non-zero.

EOPNOTSUPP
opcode is valid, but not supported by this kernel.

EOPNOTSUPP
IOSQE_BUFFER_SELECT was set in the flags field of the submission queue entry, but the opcode
doesn't support buffer selection.

Linux 2019-01-22 IO_URING_ENTER(2)