Type Alias ConversationCallback 

pub type ConversationCallback = unsafe extern "C" fn(num_msg: c_int, msg: *const *const pam_message, resp: *mut *mut pam_response, appdata: *mut c_void) -> c_int;

The type of pam_conv::conv.

The exact format of messages varies between Linux-PAM and other implementations. See libpam_sys_helpers::PtrPtrVec for details (and a workaround).

use libpam_sys::aliases::ConversationCallback;
use libpam_sys::pam_conv;
fn convo() -> ConversationCallback {
    // ...
}
let conv = pam_conv {
    conv: convo(),
    appdata_ptr: std::ptr::null_mut(),
};