# io_uring_prep_accept

## 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.
    


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