io_uring_enter(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CQE ERRORS | COLOPHON

io_uring_enter(2)       Linux Programmer's Manual      io_uring_enter(2)

NAME         top

       io_uring_enter - initiate and/or complete asynchronous I/O

SYNOPSIS         top

       #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         top

       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.

              This command works like an async poll(2) and the
              completion event result is the returned mask of events.

       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, 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 __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
              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 __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 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.

       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.

       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         top

       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         top

       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.

       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         top

       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.

COLOPHON         top

       This page is part of the liburing (A library for io_uring)
       project.  Information about the project can be found at 
       ⟨https://github.com/axboe/liburing⟩.  If you have a bug report for
       this manual page, send it to io-uring@vger.kernel.org.  This page
       was obtained from the project's upstream Git repository
       ⟨https://github.com/axboe/liburing⟩ on 2024-06-14.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2024-06-03.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to
       man-pages@man7.org

Linux                          2019-01-22              io_uring_enter(2)

Pages that refer to this page: io_uring_enter2(2)io_uring_enter(2)io_uring_register(2)io_uring_setup(2)syscalls(2)io_uring_prep_send_zc(3)io_uring_prep_send_zc_fixed(3)io_uring_register_ring_fd(3)io_uring(7)