io_uring_prep_recv(3)

NAME
io_uring_prep_recv - prepare a recv request

SYNOPSIS
#include <liburing.h>

void io_uring_prep_recv(struct io_uring_sqe *sqe,
                        int sockfd,
                        void *buf,
                        size_t len,
                        int flags);

void io_uring_prep_recv_multishot(struct io_uring_sqe *sqe,
                                  int sockfd,
                                  void *buf,
                                  size_t len,
                                  int flags);

DESCRIPTION

The io_uring_prep_recv(3) function prepares a recv request. The submission queue entry sqe is setup to use the file descriptor sockfd to start receiving the data into the destination buffer buf of size len and with modifier flags flags.

This function prepares an async recv(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. Length can either be set to 0, in which case there are no limits on how much data a single invocation of the receive will transfer, or it can be set to a positive value. If the latter is the case, then each trigger invocation of the receive multishot will transfer at most length bytes. This can be useful if the ring is handling many receive multishot operations on different sockets, to ensure fairness between them, particularly when used with bundles. 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 is done and the application must issue a new request if it still wishes to receive data from the socket. 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.

IORING_RECVSEND_BUNDLE
    If set and provided buffers are used with IOSQE_BUFFER_SELECT , the receive operation will attempt to fill multiple buffers with rather than just pick a single buffer to fill. To receive multiple buffers in a single receive, the buffer group ID set in the SQE must be of the ring provided type. If set, the CQE res field indicates the total number of bytes received, and the buffer ID returned in the CQE flags field indicates the first buffer in the receive operation. The application must process the indicated initial buffer ID and until all res bytes have been seen to know which is the last buffer in the receive operation. The buffers consumed will be contiguous from the initial buffer, in the order in which they appear in the buffer ring. The CQE struct does not contain the position of the buffer in the buffer ring, therefore in order to identify buffers contained by the bundle, it is advised to maintain the cached head index per buffer ring. This uint16_t index represents the position of the next buffer to be consumed within the ring. Upon completion of a receive operation, the cached head index should be incremented accordingly. Receiving in bundles can improve performance when more than one chunk of data is available to receive, by eliminating redundant round trips through the networking stack. Receive bundles may be used by both single shot and multishot receive operations. Note that, internally, bundles rely on the networking stack passing back how much data is left in the socket after the initial receive. This means that the initial receive may contain less buffers than what is available, with the followup receive(s) containing more buffers. Available since 6.10.

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
Despite accepting a size_t number of bytes, these functions can transfer at most INT_MAX bytes per call (the maximum for the underlying syscall interface).

SEE ALSO
io_uring_get_sqe(3), io_uring_submit(3), io_uring_buf_ring_init(3), io_uring_buf_ring_add(3), recv(2)