OSSL_CALLBACK(3SSL) OpenSSL OSSL_CALLBACK(3SSL)
NAME
OSSL_CALLBACK, OSSL_PASSPHRASE_CALLBACK - OpenSSL Core type to define
callbacks
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);
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 arg. As
far as the function receiving the pointer to the function pointer and
arg is concerned, the data that arg points at is opaque, and the pointer
should simply be passed back to the callback function when it's called.
OSSL_CALLBACK
This is a generic callback function. When calling this callback
function, the caller is expected to build an OSSL_PARAM(3) array of
data it wants or is expected to pass back, and pass that as params,
as well as the opaque data pointer it received, as arg.
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 pass, and its
size with pass_size. The length of the prompted pass phrase will be
given back in *pass_len.
Additional parameters can be passed with the OSSL_PARAM(3) array
params,
RETURN VALUES
Functions of type OSSL_CALLBACK and OSSL_PASSPHRASE_CALLBACK must return
1 on success and 0 on failure.
SEE ALSO
openssl-core.h(7)
HISTORY
The types described here were added in OpenSSL 3.0.
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
<https://www.openssl.org/source/license.html>.
3.5.7 2026-06-09 OSSL_CALLBACK(3SSL)
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