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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | NOTES | SEE ALSO | COLOPHON |
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io_uring_prep_accept(3) liburing Manual io_uring_prep_accept(3)
io_uring_prep_accept - prepare an accept request
#include <sys/socket.h>
#include <liburing.h>
void io_uring_prep_accept(struct io_uring_sqe *sqe,
int sockfd,
struct sockaddr *addr,
socklen_t *addrlen,
int flags);
void io_uring_prep_accept_direct(struct io_uring_sqe *sqe,
int sockfd,
struct sockaddr *addr,
socklen_t *addrlen,
int flags,
unsigned int file_index);
void io_uring_prep_multishot_accept(struct io_uring_sqe *sqe,
int sockfd,
struct sockaddr *addr,
socklen_t *addrlen,
int flags);
void io_uring_prep_multishot_accept_direct(struct io_uring_sqe *sqe,
int sockfd,
struct sockaddr *addr,
socklen_t *addrlen,
int flags);
The io_uring_prep_accept(3) function and its three variants
prepare an accept request similar to accept4(2). The submission
queue entry sqe is setup to use the file descriptor sockfd to
start accepting a connection request described by the socket
address at addr and of structure length addrlen and using modifier
flags in flags.
The three variants allow combining the direct file table and
multishot features.
Direct descriptors are io_uring private file descriptors. They
avoid some of the overhead associated with thread shared file
tables and can be used in any io_uring request that takes a file
descriptor. The two direct variants here create such direct
descriptors. Subsequent to their creation, they can be used by
setting IOSQE_FIXED_FILE in the SQE flags member, and setting the
SQE fd field to the direct descriptor value rather than the
regular file descriptor. Direct descriptors are managed like
registered files.
To use an accept direct variant, the application must first have
registered a file table of a desired size using
io_uring_register_files(3) or io_uring_register_files_sparse(3).
Once registered, io_uring_prep_accept_direct(3) allows an entry in
that table to be specifically selected through the file_index
argument. If the specified entry already contains a file, the
file will first be removed from the table and closed, consistent
with the behavior of updating an existing file with
io_uring_register_files_update(3). file_index can also be set to
IORING_FILE_INDEX_ALLOC for this variant and an unused table index
will be dynamically chosen and returned. Likewise,
io_uring_prep_multishot_accept_direct will have an unused table
index dynamically chosen and returned for each connection
accepted. If both forms of direct selection will be employed,
specific and dynamic, see io_uring_register_file_alloc_range(3)
for setting up the table so dynamically chosen entries are made
against a different range than that targeted by specific requests.
Note that old kernels don't check the SQE file_index field meaning
applications cannot rely on a -EINVAL CQE res being returned when
the kernel is too old because older kernels may not recognize they
are being asked to use a direct table slot.
When a direct descriptor accept request asks for a table slot to
be dynamically chosen but there are no free entries, -ENFILE is
returned as the CQE res.
The multishot variants allow an application to issue a single
accept request, which will repeatedly trigger a CQE when a
connection request comes in. Like other multishot type requests,
the application should look at the CQE flags and see if
IORING_CQE_F_MORE is set on completion as an indication of whether
or not the accept request will generate further CQEs. Note that
for the multishot variants, setting addr and addrlen may not make
a lot of sense, as the same value would be used for every accepted
connection. This means that the data written to addr may be
overwritten by a new connection before the application has had
time to process a past connection. If the application knows that a
new connection cannot come in before a previous one has been
processed, it may be used as expected. The multishot variants are
available since 5.19.
See the man page accept4(2) for details of the accept function
itself.
None
The CQE res field will contain the result of the operation.
io_uring_prep_accept(3) generates the installed file descriptor as
its result.
io_uring_prep_accept_direct(3) and file_index set to a specific
direct descriptor generates 0 on success. The caller must
remember which direct descriptor was picked for this request.
io_uring_prep_accept_direct(3) and file_index set to
IORING_FILE_INDEX_ALLOC generates the dynamically chosen direct
descriptor.
io_uring_prep_multishot_accept(3) generates the installed file
descriptor in each result.
io_uring_prep_multishot_accept_direct(3), generates the
dynamically chosen direct descriptor in each result.
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 generates 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).
Note that the direct versions of accept do not accept SOCK_CLOEXEC
, and setting that flag will result in an -EINVAL error in the
CQE.
io_uring_get_sqe(3), io_uring_submit(3),
io_uring_register_files(3), io_uring_register_files_sparse(3),
io_uring_register_file_alloc_range(3), io_uring_register(2),
accept4(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 13, 2022 io_uring_prep_accept(3)
Pages that refer to this page: io_uring_prep_accept(3), io_uring_prep_accept_direct(3), io_uring_prep_multishot_accept(3), io_uring_prep_multishot_accept_direct(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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