is a <b>void *</b>. &quot;Old&quot; style callbacks are the same except they are initialised with a call to BN_GENCB_set_old() and <b>callback</b> is of type <b>void (*callback)(int, int, void *)</b>.</p>

<p>A callback is invoked through a call to <b>BN_GENCB_call</b>. This will check the type of the callback and will invoke <b>callback(a, b, gencb)</b> for new style callbacks or <b>callback(a, b, cb_arg)</b> for old style.</p>

<p>It is possible to obtain the argument associated with a BN_GENCB structure 