| .\" Copyright (C) 2022 Jens Axboe <axboe@kernel.dk> |
| .\" |
| .\" SPDX-License-Identifier: LGPL-2.0-or-later |
| .\" |
| .TH io_uring_prep_accept 3 "March 13, 2022" "liburing-2.2" "liburing Manual" |
| .SH NAME |
| io_uring_prep_accept \- prepare an accept request |
| .SH SYNOPSIS |
| .nf |
| .B #include <sys/socket.h> |
| .B #include <liburing.h> |
| .PP |
| .BI "void io_uring_prep_accept(struct io_uring_sqe *" sqe "," |
| .BI " int " sockfd "," |
| .BI " struct sockaddr *" addr "," |
| .BI " socklen_t *" addrlen "," |
| .BI " int " flags ");" |
| .PP |
| .BI "void io_uring_prep_accept_direct(struct io_uring_sqe *" sqe "," |
| .BI " int " sockfd "," |
| .BI " struct sockaddr *" addr "," |
| .BI " socklen_t *" addrlen "," |
| .BI " int " flags "," |
| .BI " unsigned int " file_index ");" |
| .PP |
| .BI "void io_uring_prep_multishot_accept(struct io_uring_sqe *" sqe "," |
| .BI " int " sockfd "," |
| .BI " struct sockaddr *" addr "," |
| .BI " socklen_t *" addrlen "," |
| .BI " int " flags ");" |
| .PP |
| .BI "void io_uring_prep_multishot_accept_direct(struct io_uring_sqe *" sqe "," |
| .BI " int " sockfd "," |
| .BI " struct sockaddr *" addr "," |
| .BI " socklen_t *" addrlen "," |
| .BI " int " flags ");" |
| .fi |
| .SH DESCRIPTION |
| .PP |
| The |
| .BR io_uring_prep_accept (3) |
| function prepares an accept request. The submission queue entry |
| .I sqe |
| is setup to use the file descriptor |
| .I sockfd |
| to start accepting a connection request described by the socket address at |
| .I addr |
| and of structure length |
| .I addrlen |
| and using modifier flags in |
| .IR flags . |
| |
| For a direct descriptor accept request, the offset is specified by the |
| .I file_index |
| argument. 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. To do so, |
| .B IOSQE_FIXED_FILE |
| must be set in the SQE |
| .I flags |
| member, and the SQE |
| .I fd |
| field should use the direct descriptor value rather than the regular file |
| descriptor. Direct descriptors are managed like registered files. |
| |
| If the direct variant is used, the application must first have registered |
| a file table using |
| .BR io_uring_register_files (3) |
| of the appropriate size. Once registered, a direct accept request may use any |
| entry in that table, as long as it is within the size of the registered table. |
| If a specified entry already contains a file, the file will first be removed |
| from the table and closed. It's consistent with the behavior of updating an |
| existing file with |
| .BR io_uring_register_files_update (3). |
| Note that old kernels don't check the SQE |
| .I file_index |
| field, which is not a problem for liburing helpers, but users of the raw |
| io_uring interface need to zero SQEs to avoid unexpected behavior. This also |
| means that applications should check for availability of |
| .B IORING_OP_ACCEPT_DIRECT |
| before using it, they cannot rely on a |
| .B -EINVAL |
| CQE |
| .I res |
| return. |
| |
| For a direct descriptor accept request, the |
| .I file_index |
| argument can be set to |
| .BR IORING_FILE_INDEX_ALLOC , |
| In this case a free entry in io_uring file table will |
| be used automatically and the file index will be returned as CQE |
| .IR res . |
| .B -ENFILE |
| is otherwise returned if there is no free entries in the io_uring file table. |
| |
| The multishot version accept and accept_direct 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 |
| .I flags |
| and see if |
| .B IORING_CQE_F_MORE |
| is set on completion as an indication of whether or not the accept request |
| will generate further CQEs. The multishot variants are available since 5.19. |
| |
| For multishot with direct descriptors, |
| .B IORING_FILE_INDEX_ALLOC |
| must be used as the file descriptor. This tells io_uring to allocate a free |
| direct descriptor from our table, rather than the application passing one in. |
| Failure to do so will result in the accept request being terminated with |
| .BR -EINVAL . |
| The allocated descriptor will be returned in the CQE |
| .I res |
| field, like a non-direct accept request. |
| |
| These functions prepare an async |
| .BR accept4 (2) |
| request. See that man page for details. |
| |
| .SH RETURN VALUE |
| None |
| .SH ERRORS |
| The CQE |
| .I res |
| field will contain the result of the operation. For singleshot accept, the |
| non-direct accept returns the installed file descriptor as its value, the |
| direct accept returns |
| .B 0 |
| on success. The caller must know which direct descriptor was picked for this |
| request. For multishot accept, the non-direct accept returns the installed |
| file descriptor as its value, the direct accept returns the file index used on |
| success. See the related man page for details on possible values for the |
| non-direct accept. Note that where synchronous system calls will return |
| .B -1 |
| on failure and set |
| .I errno |
| to the actual error value, io_uring never uses |
| .IR errno . |
| Instead it returns the negated |
| .I errno |
| directly in the CQE |
| .I res |
| field. |
| .SH 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 |
| .B IORING_FEAT_SUBMIT_STABLE |
| flag passed back from |
| .BR io_uring_queue_init_params (3). |
| .SH SEE ALSO |
| .BR io_uring_get_sqe (3), |
| .BR io_uring_submit (3), |
| .BR io_uring_register (2), |
| .BR accept4 (2) |