BIO_get_rpoll_descriptor, BIO_get_wpoll_descriptor - obtain a structure which can be used to determine

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3.5.0                                              2025-06-04                     BIO_GET_RPOLL_DESCRIPTOR(3SSL)

BIO_get_rpoll_descriptor() and BIO_get_wpoll_descriptor(), on success, fill *desc with a poll descriptor.
       A poll descriptor is a tagged union structure which represents some kind of OS or non-OS resource which
       can be used to synchronise on I/O availability events.

       BIO_get_rpoll_descriptor() outputs a descriptor which can be used to determine when the BIO can
       (potentially) next be read, and BIO_get_wpoll_descriptor() outputs a descriptor which can be used to
       determine when the BIO can (potentially) next be written.

       It is permissible for BIO_get_rpoll_descriptor() and BIO_get_wpoll_descriptor() to output the same
       descriptor.

       Poll descriptors can represent different kinds of information. A typical kind of resource which might be
       represented by a poll descriptor is an OS file descriptor which can be used with APIs such as select().

       The kinds of poll descriptor defined by OpenSSL are:

       BIO_POLL_DESCRIPTOR_TYPE_NONE
           Represents  the  absence  of a valid poll descriptor. It may be used by BIO_get_rpoll_descriptor() or
           BIO_get_wpoll_descriptor() to indicate that the BIO is not pollable for readability  or  writeability
           respectively.

           For this type, no field within the value field of the BIO_POLL_DESCRIPTOR is valid.

       BIO_POLL_DESCRIPTOR_TYPE_SOCK_FD
           The  poll  descriptor represents an OS socket resource. The field value.fd in the BIO_POLL_DESCRIPTOR
           is valid if it is not set to -1.

           The resource is whatever kind of handle is used by a given OS to represent sockets, which may vary by
           OS. For example, on Windows, the value is a SOCKET for  use  with  the  Winsock  API.  On  POSIX-like
           platforms, it is a file descriptor.

           Where a poll descriptor of this type is output by BIO_get_rpoll_descriptor(), it should be polled for
           readability  to  determine  when  the  BIO  might  next be able to successfully complete a BIO_read()
           operation; likewise, where a poll descriptor of this type is output by BIO_get_wpoll_descriptor(), it
           should be polled for writeability to determine when the  BIO  might  next  be  able  to  successfully
           complete a BIO_write() operation.

       BIO_POLL_DESCRIPTOR_CUSTOM_START
           Type  values beginning with this value (inclusive) are reserved for application allocation for custom
           poll descriptor types. Any of the definitions in the union field value can be used by the application
           arbitrarily as opaque values.

       Because poll descriptors are a tagged union structure, they can represent different kinds of information.
       New types of poll descriptor may be defined, including by applications, according to their needs.

       The BIO_get_rpoll_descriptor() and BIO_get_wpoll_descriptor() functions were added in OpenSSL 3.2.

       BIO_get_rpoll_descriptor, BIO_get_wpoll_descriptor - obtain a structure which can be used to determine
       when a BIO object can next be read or written

       The functions BIO_get_rpoll_descriptor() and BIO_get_wpoll_descriptor() return 1  on  success  and  0  on
       failure.

       These  functions  are  permitted  to  succeed  and  initialise  *desc  with  a  poll  descriptor  of type
       BIO_POLL_DESCRIPTOR_TYPE_NONE to indicate that the BIO is not pollable for  readability  or  writeability
       respectively.

SSL_handle_events(3), SSL_get_event_timeout(3), SSL_get_rpoll_descriptor(3), SSL_get_wpoll_descriptor(3),
       bio(7)

        #include <openssl/bio.h>

        typedef struct bio_poll_descriptor_st {
            uint32_t type;
            union {
                int        fd;
                void       *custom;
                uintptr_t  custom_ui;
            } value;
        } BIO_POLL_DESCRIPTOR;

        int BIO_get_rpoll_descriptor(BIO *b, BIO_POLL_DESCRIPTOR *desc);
        int BIO_get_wpoll_descriptor(BIO *b, BIO_POLL_DESCRIPTOR *desc);