Provided by: liburing-dev_2.4-1_amd64 
NAME
io_uring_prep_accept - prepare an accept request
SYNOPSIS
#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);
DESCRIPTION
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.
RETURN VALUE
None
ERRORS
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.
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), io_uring_register_files(3),
io_uring_register_files_sparse(3), io_uring_register_file_alloc_range(3),
io_uring_register(2), accept4(2)