HELP！！！！ME -Function winapi::um::wincrypt::CertEnumSystemStore

[Yihsiwei]

How to fill in the fourth parameter of this function? Please help me

pub unsafe extern "system" fn CertEnumSystemStore(
    dwFlags: DWORD, 
    pvSystemStoreLocationPara: *mut c_void, 
    pvArg: *mut c_void, 
    pfnEnum: PFN_CERT_ENUM_SYSTEM_STORE
) -> BOOL

    wincrypt::CertEnumSystemStore(
        wincrypt::CERT_SYSTEM_STORE_CURRENT_USER,
        ptr::null_mut(),
        ptr::null_mut(),
        (Here I need to fill in a function address PTR. How can I convert it? Please help me)
    );