Prevent Garbage collection from removing objects created by C-Libraries

[birdistheword96]

I am running into a Memory fault issue which appears to be caused by garbage collection. In the compiled C library, I have a definition for a Micropython object that contains a buffer, which can be written into by the uPy application, and accessed by another task external to Micropython:

mp_obj_t mp_obj_new_msg_channel(mp_obj_t _name)
{
    /* Create the uPy object. */

    mp_obj_upy_channel_t *o = m_new_obj(mp_obj_msg_channel_t);

    o->base.type = &mp_type_msg_channel;
    o->name = _name;
    memset(o->buffer, 0x00, sizeof(float32_t) * 64);

    return MP_OBJ_FROM_PTR(o);
}

However, garbage collection appears to be sometimes removing these items (despite them being declared globally in the uPy application) as the memory always fault happens roughly a second after garbage collection.

It seems like I can set objects as MICROPY_PORT_ROOT_POINTERS in the micropython port config file, but I can only see examples of that where the number of objects is known at compile time. I need to be able to create and remove any number of these object, without fear that they will be removed by the GC, and am happy to handle removing that memory in the C level.

Can I define an object type that the GC will not remove? or do I need to pre-allocate those objects at compile type to flag them as not used by the GC?

[karfas]

Maybe you find some tips in https://forum.micropython.org/viewtopic.php?f=2&t=10765&p=59215&hilit=garbage#p59215 ?

[jimmo]

It seems like I can set objects as MICROPY_PORT_ROOT_POINTERS in the micropython port config file, but I can only see examples of that where the number of objects is known at compile time. I need to be able to create and remove any number of these object, without fear that they will be removed by the GC, and am happy to handle removing that memory in the C level.

MICROPY_PORT_ROOT_POINTERS has been removed in favour of MP_REGISTER_ROOT_POINTER. It's much simpler because you no longer need to have mpconfigport.h know about your root pointers.

See for example https://github.com/micropython/micropython/blob/master/ports/stm32/pin.c#L679

In your case where you have a list of objects, you should make your registered root pointer be a list (i.e. mp_obj_list_t) and then you can add/remove pointers to it as required. (Remember your objects don't need to be roots themselves, they just need to be reachable from a root, e.g. this list).

You an also use m_tracked_calloc and m_tracked_free if you need to provide a malloc/free like interface to some underlying API.