=pod

=head1 NAME

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

=head1 SYNOPSIS

 #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);

=head1 DESCRIPTION

BIO_get_rpoll_descriptor() and BIO_get_wpoll_descriptor(), on success, fill
I<*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:

=over 4

=item 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 I<value> field of the B<BIO_POLL_DESCRIPTOR>
is valid.

=item BIO_POLL_DESCRIPTOR_TYPE_SOCK_FD

The poll descriptor represents an OS socket resource. The field I<value.fd>
in the B<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 B<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.

=item 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 I<value> can be used by the application arbitrarily as opaque values.

=back

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.

=head1 RETURN VALUES

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 I<*desc> with a poll
descriptor of type B<BIO_POLL_DESCRIPTOR_TYPE_NONE> to indicate that the BIO is
not pollable for readability or writeability respectively.

=head1 SEE ALSO

L<SSL_handle_events(3)>, L<SSL_get_event_timeout(3)>, L<SSL_get_rpoll_descriptor(3)>,
L<SSL_get_wpoll_descriptor(3)>, L<bio(7)>

=head1 HISTORY

The SSL_get_rpoll_descriptor() and SSL_get_wpoll_descriptor() functions were
added in OpenSSL 3.2.

=head1 COPYRIGHT

Copyright 2022-2023 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

=cut
