internal variables in macros should use private names

this sets the volume to 0.0:

float f = 1.0;
const struct spa_pod *param = spa_pod_builder_add_object (&b,
            SPA_TYPE_OBJECT_Props, SPA_PARAM_Props,
            SPA_PROP_volume, SPA_POD_Float(f));

this sets the volume to 1.0:

float vol = 1.0;
const struct spa_pod *param = spa_pod_builder_add_object (&b,
            SPA_TYPE_OBJECT_Props, SPA_PARAM_Props,
            SPA_PROP_volume, SPA_POD_Float(vol));

guess why ...

#define spa_pod_builder_add_object(b,type,id,...)				\
({										\
	struct spa_pod_frame f;							\
	spa_pod_builder_push_object(b, &f, type, id);				\
	spa_pod_builder_add(b, ##__VA_ARGS__, 0);				\
	spa_pod_builder_pop(b, &f);						\
})

it took me a while :/