NAME

       io_uring_prep_recvmsg - prepare a recvmsg request

SYNOPSIS

       #include <sys/types.h>
       #include <sys/socket.h>
       #include <liburing.h>

       void io_uring_prep_recvmsg(struct io_uring_sqe *sqe,
                                  int fd,
                                  struct msghdr *msg,
                                  unsigned flags);

       void io_uring_prep_recvmsg_multishot(struct io_uring_sqe *sqe,
                                            int fd,
                                            struct msghdr *msg,
                                            unsigned flags);

DESCRIPTION

       The  io_uring_prep_recvmsg(3)  function  prepares  a recvmsg request. The submission queue
       entry sqe is setup to use the file descriptor fd to start receiving the data indicated  by
       msg with the recvmsg(2) defined flags in the flags argument.

       This  function  prepares an async recvmsg(2) request. See that man page for details on the
       arguments specified to this prep helper.

       The multishot version allows the application to issue  a  single  receive  request,  which
       repeatedly posts a CQE when data is available. It requires the IOSQE_BUFFER_SELECT flag to
       be set and no MSG_WAITALL flag to be set.  Therefore each CQE will take a buffer out of  a
       provided  buffer  pool for receiving.  The application should check the flags of each CQE,
       regardless of its result.  If a posted CQE does not have the  IORING_CQE_F_MORE  flag  set
       then the multishot receive will be done and the application should issue a new request.

       Unlike  recvmsg(2),  multishot  recvmsg  will  prepend a struct io_uring_recvmsg_out which
       describes the layout of the rest of the buffer in  combination  with  the  initial  struct
       msghdr  submitted  with  the  request. See io_uring_recvmsg_out(3) for more information on
       accessing the data.

       Multishot variants are available since kernel 6.0.

       After calling this function, additional io_uring internal modifier flags may be set in the
       SQE ioprio field. The following flags are supported:

       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.

              Can  be  used with the CQE IORING_CQE_F_SOCK_NONEMPTY flag, which io_uring will set
              on CQEs after a recv(2) or recvmsg(2) operation. If set, the socket still had  data
              to  be  read  after  the  operation completed. Both these flags are available since
              5.19.

RETURN VALUE

       None

ERRORS

       The CQE res field will contain the result of the operation. See the related man  page  for
       details  on  possible  values.  Note that where synchronous system calls will return -1 on
       failure and set errno to the actual error value, io_uring never uses  errno.   Instead  it
       returns the negated errno directly in the CQE res field.

NOTES

       As with any request that passes in data in a struct, that data must remain valid until the
       request has been successfully submitted. It need not remain valid until completion. Once a
       request  has  been  submitted,  the in-kernel state is stable. Very early kernels (5.4 and
       earlier) required state to be stable until the completion occurred. Applications can  test
       for  this  behavior  by  inspecting  the  IORING_FEAT_SUBMIT_STABLE  flag passed back from
       io_uring_queue_init_params(3).

SEE ALSO

       io_uring_get_sqe(3), io_uring_submit(3), recvmsg(2)