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

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 specified 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.

IORING_ENTER_REGISTERED_RING
If the ring file descriptor has been registered through use of
IORING_REGISTER_RING_FDS, then setting this flag will tell the kernel that the
ring_fd passed in is the registered ring offset rather than a normal file
descriptor.

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 canceled 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 canceled 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 the index points to a valid
empty slot, the installation is guaranteed to not fail. If there is already a file
in the slot, it will be replaced, similar to IORING_OP_FILES_UPDATE. Please note
that 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 5.5.

IORING_OP_ASYNC_CANCEL
Attempt to cancel an already issued request. addr must contain the user_data field
of the request that should be canceled. The cancelation 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 canceled, 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 canceled,
while disk IO requests cannot be canceled 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
canceled, or -ECANCELED if the timer got canceled 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. If the file_index field is set to a positive number, this
command can be used to close files that were direct opened through IORING_OP_OPENAT
, IORING_OP_OPENAT2 , or IORING_OP_ACCEPT using the io_uring specific direct
descriptors. Note that only one of the descriptor fields may be set. The direct
close feature is available since the 5.15 kernel, where direct descriptors were
introduced.

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, and len must be set to the how argument. No 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.

IORING_OP_MSG_RING
Send a message to an io_uring. fd must be set to a file descriptor of a ring that
the application has access to, len can be set to any 32-bit value that the
application wishes to pass on, and off should be set any 64-bit value that the
application wishes to send. On the target ring, a CQE will be posted with the res
field matching the len set, and a user_data field matching the off value being
passed in. This request type can be used to either just wake or interrupt anyone
waiting for completions on the target ring, or it can be used to pass messages via
the two fields. Available since 5.18.

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, 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. 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.

IOSQE_CQE_SKIP_SUCCESS
Don't generate a CQE if the request completes successfully. If the request fails,
an appropriate CQE will be posted as usual and if there is no IOSQE_IO_HARDLINK,
CQEs for all linked requests will be omitted. The notion of failure/success is
opcode specific and is the same as with breaking chains of IOSQE_IO_LINK. One
special case is when the request has a linked timeout, then the CQE generation for
the linked timeout is decided solely by whether it has IOSQE_CQE_SKIP_SUCCESS set,
regardless whether it timed out or was canceled. In other words, if a linked
timeout has the flag set, it's guaranteed to not post a CQE.

The semantics are chosen to accommodate several use cases. First, when all but the
last request of a normal link without linked timeouts are marked with the flag,
only one CQE per lin is posted. Additionally, it enables suppression of CQEs in
cases where the side effects of a successfully executed operation is enough for
userspace to know the state of the system. One such example would be writing to a
synchronisation file.

This flag is incompatible with IOSQE_IO_DRAIN. Using both of them in a single ring
is undefined behavior, even when they are not used together in a single request.
Currently, after the first request with IOSQE_CQE_SKIP_SUCCESS, all subsequent
requests marked with drain will be failed at submission time. Note that the error
reporting is best effort only, and restrictions may change in the future.

Available since 5.17.

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 or IORING_OP_MSG_RING, , see those entries for
details. 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.