sd_bus_get_property_string equivalent for gdbus?

Question

I try to read out the ActiveState property of a systemd unit with gdbus/glib-2.0. For sd-bus there exists the convenient function sd_bus_get_property_string. What would the equivalent call if gdbus is used. I am ware of the gdbus introspect command, but I need to implement that in C/C++.

I managed to start and stop units already. Now I need to verify that a unit has been successful started/stopped. I am new to dbus and have been searching the internet for some hours for an example, without finding something helpful.

score 0 · Answer 1 · answered Mar 16 '20 at 08:02

I also implemented some systemd stuff in C++. Here was my solution:

std::string Unit::GetPropertyString(const std::string& property) const
{
    sd_bus_error err = SD_BUS_ERROR_NULL;
    char* msg = nullptr;
    int r;

    r = sd_bus_get_property_string(m_bus,
        "org.freedesktop.systemd1",
        ("/org/freedesktop/systemd1/unit/" + m_unit).c_str(),
        "org.freedesktop.systemd1.Unit",
        property.c_str(),
        &err,
        &msg);

    if (r < 0)
    {
        std::string err_msg(err.message);
        sd_bus_error_free(&err);

        std::string err_str("Failed to get " + property + " for service "
                            + m_name + ". Error: " + err_msg);

        throw slib_exception(err_str);
    }

    sd_bus_error_free(&err);

    // Free memory (avoid leaking)
    std::string ret(msg);
    free (msg);

    return ret;
}

From this, you can call

activestate = GetPropertyString("ActiveState");
substate = GetPropertyString("SubState");

I found that a lot of the <systemd/sd-bus.h> wasn't well documented. There is a fantastic explanation by the author here: http://0pointer.net/blog/the-new-sd-bus-api-of-systemd.html

But outside of the few examples he gives, I found it was easier to inspect the source code. Specifically, I found it nice looking into the source-code of the systemctl and journalctl applications to see how sd-bus was used in those contexts.

sd_bus_get_property_string equivalent for gdbus?

1 Answers1