Provided by: liburing-dev_2.4-1_amd64 bug

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,
                           sigset_t *sig, 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.

       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_enter2(unsigned int fd, unsigned int to_submit,
                                  unsigned int min_complete, unsigned int flags,
                                  const void *arg, size_t argsz);

              which  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_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.

       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;
               };
               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 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).  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.

              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 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_REALTIME
                   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.

              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 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.

       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_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.

       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(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_setup(2) will not have  this  error.  This  error
              should  be  extremely  rare  and indicates the machine is running critically low on
              memory and. 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.

       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.