io_uring_prep_recvmsg(3) liburing Manual io_uring_prep_recvmsg(3)

io_uring_prep_recvmsg - prepare a recvmsg request

#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);

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:

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.


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.

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).

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

March 12, 2022 liburing-2.2