# io_uring_enter

## NAME



           io_uring_enter - initiate and/or complete asynchronous I/O
    


## SYNOPSIS



           **#include <liburing.h>**
    
           **int io_uring_enter(unsigned int** _fd_**, unsigned int** _to_submit_**,**
                              **unsigned int** _min_complete_**, unsigned int** _flags_**,**
                              **sigset_t ***_sig_**);**
    
           **int io_uring_enter2(unsigned int** _fd_**, unsigned int** _to_submit_**,**
                               **unsigned int** _min_complete_**, unsigned int** _flags_**,**
                               **void ***_arg_**, size_t** _sz_**);**
    


## DESCRIPTION



           io_uring_enter(2) 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(2).
    
           _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.
                  If this flag is set either the flag
                  **IORING_SETUP_DEFER_TASKRUN** must not be set or the thread
                  issuing the syscall must be the thread that created the
                  io_uring associated with _fd,_ or be the thread that enabled
                  the ring originally created with **IORING_SETUP_R_DISABLED**
                  via io_uring_register(2) or io_uring_enable_rings(3).
    
           **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 one will
                  become 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**
                  By default, _arg_ is a _sigset_t_ pointer. If
                  **IORING_ENTER_EXT_ARG** is set (supported since kernel 5.11),
                  then _arg_ is instead a pointer to a _struct_
                  _io_uring_getevents_arg_ and _argsz_ must be set to the size of
                  this structure. The definition is as follows:
    
                  **struct io_uring_getevents_arg {**
                          **__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.
    
           **IORING_ENTER_ABS_TIMER**
    
                  When this flag is set, the timeout argument passed in
                  _struct io_uring_getevents_arg_ will be interpreted as an
                  absolute time of the registered clock (see
                  **IORING_REGISTER_CLOCK)** until which the waiting should end.
    
                  Available since 6.12
    
           **IORING_ENTER_EXT_ARG_REG**
    
                  When this flag is set, _arg_ is not a pointer to a
                  _struct_ io_uring_getevents_arg _,_ but merely an offset into an
                  area of wait regions previously registered with
                  io_uring_register(2) using the **IORING_REGISTER_MEM_REGION**
                  operation.
    
                  Available since 6.13
    
           **IORING_ENTER_NO_IOWAIT**
                  When this flag is set, the system call will not mark the
                  waiting task as being in iowait if it is sleeping waiting
                  on events and there are pending requests.  This is useful
                  if iowait isn't expected when waiting for events. It can
                  also prevent extra power usage by allowing the CPU to enter
                  lower sleep states.  This flag is only available if the
                  kernel supports the **IORING_FEAT_NO_IOWAIT** feature.
    
                  Available since 6.15.
    
           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(2) 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(2) 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;
                         struct {
                              __u32     cmd_op;
                              __u32     __pad1;
                         };
                    };
                    union {
                         __u64     addr;     /* pointer to buffer or iovecs */
                         __u64     splice_off_in;
                         struct {
                              __u32     level;
                              __u32     optname;
                         };
                    };
                    __u32     len;      /* buffer size or number of iovecs */
                    union {
                         __kernel_rwf_t rw_flags;
                         __u32          fsync_flags;
                         __u16          poll_events;   /* compatibility */
                         __u32          poll32_events; /* word-reversed for BE */
                         __u32          sync_range_flags;
                         __u32          msg_flags;
                         __u32          timeout_flags;
                         __u32          accept_flags;
                         __u32          cancel_flags;
                         __u32          open_flags;
                         __u32          statx_flags;
                         __u32          fadvise_advice;
                         __u32          splice_flags;
                         __u32          rename_flags;
                         __u32          unlink_flags;
                         __u32          hardlink_flags;
                         __u32          xattr_flags;
                         __u32          msg_ring_flags;
                         __u32          uring_cmd_flags;
                         __u32          waitid_flags;
                         __u32          futex_flags;
                         __u32          install_fd_flags;
                         __u32          nop_flags;
                    };
                    __u64     user_data;     /* data to be passed back at completion time */
                    /* pack this to avoid bogus arm OABI complaints */
                    union {
                         /* index into fixed buffers, if used */
                         __u16     buf_index;
                         /* for grouped buffer selection */
                         __u16     buf_group;
                    } __attribute__((packed));
                    /* personality to use, if used */
                    __u16     personality;
                    union {
                         __s32     splice_fd_in;
                         __u32     file_index;
                         __u32     optlen;
                         struct {
                              __u16     addr_len;
                              __u16     __pad3[1];
                         };
                    };
                    union {
                         struct {
                              __u64     addr3;
                              __u64     __pad2[1];
                         };
                         __u64     optval;
                         /*
                          * If the ring is initialized with IORING_SETUP_SQE128, then
                          * this field is used for 80 bytes of arbitrary command data
                          */
                         __u8 cmd[0];
                    };
               };
    
           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 or -1.
    
           **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).  Optionally _off_ and _len_ can
                  be used to specify a range within the file to be synced
                  rather than syncing the entire file, which is the default
                  behavior.  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.  To enforce ordering one may
                  utilize linked SQEs, **IOSQE_IO_DRAIN** or wait for the arrival
                  of CQEs of requests which have to be ordered before a given
                  request before submitting its SQE.
    
           **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.
    
                  This command works like an async poll(2) and the completion
                  event result is the returned mask of events.
    
                  Without **IORING_POLL_ADD_MULTI** and the initial poll
                  operation with **IORING_POLL_ADD_MULTI** the operation is level
                  triggered, i.e. if there is data ready or events pending
                  etc. at the time of submission a corresponding CQE will be
                  posted.  Potential further completions beyond the first
                  caused by a **IORING_POLL_ADD_MULTI** are edge triggered.
    
           **IORING_OP_POLL_REMOVE**
                  Remove or update 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.
    
                  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.  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. Updating an existing poll is available since 5.13.
    
           **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, _off_ 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, _addr_ holds a
                  pointer to the _epoll_event_ 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.
    
                  This command also supports the following modifiers in
                  _ioprio:_
    
                       **IORING_RECVSEND_POLL_FIRST** If set, io_uring will
                       assume the socket is currently full and attempting to
                       send data will be unsuccessful. For this case,
                       io_uring will arm internal poll and trigger a send of
                       the data when there is enough space available.  This
                       initial send attempt can be wasteful for the case
                       where the socket is expected to be full, setting this
                       flag will bypass the initial send attempt and go
                       straight to arming poll. If poll does indicate that
                       data can be sent, the operation will proceed.
    
           **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.
    
                  This command also supports the following modifiers in
                  _ioprio:_
    
                       **IORING_RECVSEND_POLL_FIRST** If set, io_uring will
                       assume the socket is currently empty and attempting to
                       receive data will be unsuccessful. For this case,
                       io_uring will arm internal poll and trigger a receive
                       of the data when the socket has data to be read.  This
                       initial receive attempt can be wasteful for the case
                       where the socket is expected to be empty, setting this
                       flag will bypass the initial receive attempt and go
                       straight to arming poll. If poll does indicate that
                       data is ready to be received, the operation will
                       proceed.
    
           **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.
    
                  This command also supports the following modifiers in
                  _ioprio:_
    
                       **IORING_RECVSEND_POLL_FIRST** If set, io_uring will
                       assume the socket is currently full and attempting to
                       send data will be unsuccessful. For this case,
                       io_uring will arm internal poll and trigger a send of
                       the data when there is enough space available.  This
                       initial send attempt can be wasteful for the case
                       where the socket is expected to be full, setting this
                       flag will bypass the initial send attempt and go
                       straight to arming poll. If poll does indicate that
                       data can be sent, the operation will proceed.
    
           **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.
    
                  This command also supports the following modifiers in
                  _ioprio:_
    
                       **IORING_RECVSEND_POLL_FIRST** If set, io_uring will
                       assume the socket is currently empty and attempting to
                       receive data will be unsuccessful. For this case,
                       io_uring will arm internal poll and trigger a receive
                       of the data when the socket has data to be read.  This
                       initial receive attempt can be wasteful for the case
                       where the socket is expected to be empty, setting this
                       flag will bypass the initial receive attempt and go
                       straight to arming poll. If poll does indicate that
                       data is ready to be received, the operation will
                       proceed.
    
           **IORING_OP_TIMEOUT**
                  This command will register a timeout operation. The _addr_
                  field must contain a pointer to a struct __kernel_timespec
                  structure, _len_ must contain 1 to signify one
                  __kernel_timespec 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** as the
                  default 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_REALTIME** instead of **CLOCK_MONOTONIC**.
    
                  Since 5.16, **IORING_TIMEOUT_ETIME_SUCCESS** can be set in
                  _timeout_flags_ , which will result in the expiration of the
                  timer and subsequent completion with **-ETIME** not being
                  interpreted as an error. This is mostly relevant for linked
                  SQEs, as subsequent requests in the chain would not get
                  canceled by the timeout, if this flag is set. See
                  **IOSQE_IO_LINK** for more details on linked SQEs.
    
                  Since 6.4, **IORING_TIMEOUT_MULTISHOT** can be set in
                  _timeout_flags_ , which will result in the timer producing
                  multiple consecutive completions like other multi shot
                  operations e.g.  **IORING_OP_READ_MULTISHOT** or
                  **IORING_POLL_ADD_MULTI**.  _off_ must be set to the amount of
                  desired completions.  **IORING_TIMEOUT_MULTISHOT** must not be
                  used with **IORING_TIMEOUT_ABS**.
    
           **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 __kernel_timespec 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.
    
                  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 5.15.
    
           **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.
    
                  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 5.15.
    
           **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 or -1. 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 unlinkat(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 mkdirat(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 symlinkat(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 linkat(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.
    
           **IORING_OP_SOCKET**
                  Issue the equivalent of a socket(2) system call.  _fd_ must
                  contain the communication domain, _off_ must contain the
                  communication type, _len_ must contain the protocol, and
                  _rw_flags_ is currently unused and must be set to zero. See
                  also socket(2) for the general description of the related
                  system call. Available since 5.19.
    
                  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 5.19.
    
           **IORING_OP_URING_CMD**
                  Issues an asynchronous, per-file private operation, similar
                  to ioctl(2).  Further information may be found in the
                  dedicated man page of **IORING_OP_URING_CMD**.
    
                  Available since 5.19.
    
           **IORING_OP_SEND_ZC**
                  Issue the zerocopy equivalent of a send(2) system call.
                  Similar to **IORING_OP_SEND** , but tries to avoid making
                  intermediate copies of data. Zerocopy execution is not
                  guaranteed and may fall back to copying. The request may
                  also fail with **-EOPNOTSUPP** , when a protocol doesn't support
                  zerocopy, in which case users are recommended to use
                  copying sends instead.
    
                  The _flags_ field of the first _struct io_uring_cqe_ may likely
                  contain **IORING_CQE_F_MORE** , which means that there will be a
                  second completion event / notification for the request,
                  with the _user_data_ field set to the same value. The user
                  must not modify the data buffer until the notification is
                  posted. The first cqe follows the usual rules and so its
                  _res_ field will contain the number of bytes sent or a
                  negative error code. The notification's _res_ field will be
                  set to zero and the _flags_ field will contain
                  **IORING_CQE_F_NOTIF**.  The two step model is needed because
                  the kernel may hold on to buffers for a long time, e.g.
                  waiting for a TCP ACK, and having a separate cqe for
                  request completions allows userspace to push more data
                  without extra delays. Note, notifications are only
                  responsible for controlling the lifetime of the buffers,
                  and as such don't mean anything about whether the data has
                  atually been sent out or received by the other end. Even
                  errored requests may generate a notification, and the user
                  must check for **IORING_CQE_F_MORE** rather than relying on the
                  result.
    
                  _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. When _addr2_ is non-zero it
                  points to the address of the target with _addr_len_
                  specifying its size, turning the request into a sendto(2)
                  system call equivalent.
    
                  Available since 6.0.
    
                  This command also supports the following modifiers in
                  _ioprio:_
    
                       **IORING_RECVSEND_POLL_FIRST** If set, io_uring will
                       assume the socket is currently full and attempting to
                       send data will be unsuccessful. For this case,
                       io_uring will arm internal poll and trigger a send of
                       the data when there is enough space available.  This
                       initial send attempt can be wasteful for the case
                       where the socket is expected to be full, setting this
                       flag will bypass the initial send attempt and go
                       straight to arming poll. If poll does indicate that
                       data can be sent, the operation will proceed.
    
                       **IORING_RECVSEND_FIXED_BUF** If set, instructs io_uring
                       to use a pre-mapped buffer. The _buf_index_ field should
                       contain an index into an array of fixed buffers. See
                       io_uring_register(2) for details on how to setup a
                       context for fixed buffer I/O.
    
           **IORING_OP_SENDMSG_ZC**
                  Issue the zerocopy equivalent of a sendmsg(2) system call.
                  Works just like **IORING_OP_SENDMSG** , but like
                  **IORING_OP_SEND_ZC** supports **IORING_RECVSEND_FIXED_BUF**.  For
                  additional notes regarding zero copy see **IORING_OP_SEND_ZC**.
    
                  Available since 6.1
    
           **IORING_OP_WAITID**
                  Issue the equivalent of a waitid(2) system call.  _len_ must
                  contain the idtype being queried/waited for and _fd_ must
                  contain the 'pid' (or id) being waited for.  _file_index_ is
                  the 'options' being set (the child state changes to wait
                  for).  _addr2_ is a pointer to siginfo_t, if any, being
                  filled in. See also waitid(2) for the general description
                  of the related system call. Available since 6.5.
    
           **IORING_OP_SETXATTR**
    
           **IORING_OP_GETXATTR**
    
           **IORING_OP_FSETXATTR**
    
           **IORING_OP_FGETXATTR**
                  Issue the equivalent of a setxattr(2) or getxattr(2) or
                  fsetxattr(2) or fgetxattr(2) system call.  _addr_ must
                  contain a pointer to a buffer containing the name of the
                  extended attribute.  _addr2_ must contain a pointer to a
                  buffer of maximum length _len_ , in which the value of the
                  extended attribute is to be placed or is read from.
                  Additional flags maybe provided in _xattr_flags_.  For
                  setxattr(2) or getxattr(2) _addr3_ must contain a pointer to
                  the path of the file.  For fsetxattr(2) or fgetxattr(2) _fd_
                  must contain the file descriptor of the file.
    
                  Available since 5.19.
    
           **IORING_OP_BIND**
                  Issues the equivalent of the bind(2) system call.  _fd_ must
                  contain the file descriptor of the socket, _addr_ must
                  contain a pointer to the sockaddr struct containing the
                  address to assign and _addr2_ must contain the length of the
                  address.
    
                  Available since 6.11.
    
           **IORING_OP_LISTEN**
                  Issues the equivalent of the listen(2) system call.  _fd_
                  must contain the file descriptor of the socket and _addr_
                  must contain the backlog parameter, i.e. the maximum amount
                  of pending queued connections.
    
                  Available since 6.11.
    
           **IORING_OP_FTRUNCATE**
                  Issues the equivalent of the ftruncate(2) system call.  _fd_
                  must contain the file descriptor of the file to truncate
                  and _off_ must contain the length to which the file will be
                  truncated.
    
                  Available since 6.9.
    
           **IORING_OP_READ_MULTISHOT**
                  Like **IORING_OP_READ** , but similar to requests prepared with
                  io_uring_prep_multishot_accept(3) additional reads and thus
                  CQEs will be performed based on this single SQE once there
                  is more data available.  Is restricted to pollable files
                  and will fall back to single shot if the file does not
                  support **NOWAIT**.  Like other multishot type requests, the
                  application should look at the CQE flags and see if
                  **IORING_CQE_F_MORE** is set on completion as an indication of
                  whether or not the read request will generate further CQEs.
                  Available since 6.7.
    
           **IORING_OP_FUTEX_WAIT**
                  Issues the equivalent of the **futex_wait**(2) system call.
                  _addr_ must hold a pointer to the futex, _addr2_ must hold the
                  value to which the futex has to be changed so this caller
                  to **futex_wait**(2) can be woken by a call to **futex_wake**(2),
                  _addr3_ must hold the bitmask of this **futex_wait**(2) caller.
                  For a caller of **futex_wake**(2) to wake a waiter additionally
                  the bitmask of the waiter and waker must have at least one
                  set bit in common.  _fd_ must contain additional flags passed
                  in.
    
                  Available since 6.7.
    
           **IORING_OP_FUTEX_WAKE**
                  Issues the equivalent of the **futex_wake**(2) system call.
                  _addr_ must hold a pointer to the futex, _addr2_ must hold the
                  maximum number of waiters waiting on this futex to wake,
                  _addr3_ must hold the bitmask of this **futex_wake**(2) call.  To
                  wake a waiter additionally the bitmask of the waiter and
                  waker must have at least one set bit in common.  _fd_ must
                  contain additional flags passed in.
    
                  Available since 6.7.
    
           **IORING_OP_FUTEX_WAITV**
                  Issues the equivalent of the **futex_waitv**(2) system call.
                  _addr_ must hold a pointer to the futexv struct, _len_ must
                  hold the length of the futexv struct, which may not be 0
                  and must be smaller than **FUTEX_WAITV_MAX**(as of 6.11 ==
                  128).
    
                  Available since 6.7.
    
           **IORING_OP_FIXED_FD_INSTALL**
                  This operation is used to insert a registered file into the
                  regular process file table.  Consequently _fd_ must contain
                  the file index and **IOSQE_FIXED_FILE** must be set.  The
                  resulting regular fd is returned via cqe->res.  Additional
                  flags may be passed in via _install_fd_flags_.  Currently
                  supported flags are: **IORING_FIXED_FD_NO_CLOEXEC** , which
                  overrides a potentially set **O_CLOEXEC** flag set on the
                  initial file.
    
                  Available since 6.8.
    
           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 link 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(2) 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, a
           negative error code is returned. The caller should not rely on
           _errno_ variable.
    


## ERRORS



           These are the errors returned by io_uring_enter(2) 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.
    
           **EBADR** At least one CQE was dropped even with the
                  **IORING_FEAT_NODROP** feature, and there are no otherwise
                  available CQEs. This clears the error state and so with no
                  other changes the next call to io_uring_enter(2) will not
                  have this error. This error should be extremely rare and
                  indicates the machine is running critically low on memory.
                  It may be reasonable for the application to terminate
                  running unless it is able to safely handle any CQE being
                  lost.
    
           **EBUSY** If the **IORING_FEAT_NODROP** feature flag is set, then **EBUSY**
                  will be returned if there were overflow entries,
                  **IORING_ENTER_GETEVENTS** flag is set and not all of the
                  overflow entries were able to be flushed to the CQ ring.
    
                  Without **IORING_FEAT_NODROP** 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.
    
           **EEXIST** The thread submitting the work is invalid. This may occur
                  if **IORING_ENTER_GETEVENTS** and **IORING_SETUP_DEFER_TASKRUN** is
                  set, but the submitting thread is not the thread that
                  initially created or enabled the io_uring associated with
                  _fd._
    
           **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.**
    
           **EOWNERDEAD**
                  The ring has been setup with **IORING_SETUP_SQPOLL** and the sq
                  poll kernel thread has been killed.
    


## 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.
    
           **EINVAL IORING_OP_TIMEOUT** was specified, but _timeout_flags_
                  specified more than one clock source or
                  **IORING_TIMEOUT_MULTISHOT** was set alongside
                  **IORING_TIMEOUT_ABS**.
    


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