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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | NOTES | SEE ALSO | COLOPHON |
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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 is done and the application must
issue a new request if it still wishes to receive data from the
socket.
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.
None
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),
io_uring_buf_ring_init(3), io_uring_buf_ring_add(3), recvmsg(2)
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 2025-08-11. (At that time,
the date of the most recent commit that was found in the
repository was 2025-08-02.) 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
liburing-2.2 March 12, 2022 io_uring_prep_recvmsg(3)
Pages that refer to this page: io_uring_recvmsg_cmsg_firsthdr(3), io_uring_recvmsg_cmsg_nexthdr(3), io_uring_recvmsg_name(3), io_uring_recvmsg_out(3), io_uring_recvmsg_payload(3), io_uring_recvmsg_payload_length(3), io_uring_recvmsg_validate(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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