=pod

=head1 NAME

OSSL_CALLBACK, OSSL_PASSPHRASE_CALLBACK - OpenSSL Core type to define callbacks

=head1 SYNOPSIS

 #include <openssl/core.h>
 typedef int (OSSL_CALLBACK)(const OSSL_PARAM params[], void *arg);
 typedef int (OSSL_PASSPHRASE_CALLBACK)(char *pass, size_t pass_size,
                                        size_t *pass_len,
                                        const OSSL_PARAM params[],
                                        void *arg);

=head1 DESCRIPTION

For certain events or activities, provider functionality may need help from
the application or the calling OpenSSL libraries themselves.  For example,
user input or direct (possibly optional) user output could be implemented
this way.

Callback functions themselves are always provided by or through the calling
OpenSSL libraries, along with a generic pointer to data I<arg>.  As far as
the function receiving the pointer to the function pointer and I<arg> is
concerned, the data that I<arg> points at is opaque, and the pointer should
simply be passed back to the callback function when it's called.

=over 4

=item B<OSSL_CALLBACK>

This is a generic callback function.  When calling this callback function,
the caller is expected to build an L<OSSL_PARAM(3)> array of data it wants or
is expected to pass back, and pass that as I<params>, as well as the opaque
data pointer it received, as I<arg>.

=item B<OSSL_PASSPHRASE_CALLBACK>

This is a specialised callback function, used specifically to prompt the
user for a passphrase.  When calling this callback function, a buffer to
store the pass phrase needs to be given with I<pass>, and its size with
I<pass_size>.  The length of the prompted pass phrase will be given back in
I<*pass_len>.

Additional parameters can be passed with the L<OSSL_PARAM(3)> array I<params>,

=back

=head1 RETURN VALUES

Functions of type B<OSSL_CALLBACK> and B<OSSL_PASSPHRASE_CALLBACK>
must return 1 on success and 0 on failure.

=head1 SEE ALSO

L<openssl-core.h(7)>

=head1 HISTORY

The types described here were added in OpenSSL 3.0.

=head1 COPYRIGHT

Copyright 2022-2025 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
