io_uring_prep_accept(3)

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).

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.

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)