instrumenting free system memory on Pico?

[phoddie]

I looked into getting the instrumentation display in xsbug of free system memory working for Pico. Currently it is just a stub that returns a constant:

moddable/xs/platforms/pico/xsPlatform.c

Lines 678 to 680 in 43bcf83

	uint32_t pico_memory_remaining() {
	return (1024);
	}

The Pico runtime uses newlib for malloc in stdlib. That defines a mallinfo function which returns statistics including what appears to be free space in fordblks:

#include "malloc.h"
....
struct mallinfo info = mallinfo();
return (int32_t)info.fordblks;

Running this in the helloworld example shows that there are 256 bytes free. That doesn't seem correct...

I'm wondering if malloc grows the heap dynamically as needed, in which case some additional calculations are needed. Or, perhaps I've misunderstood mallinfo and fordblks. Perhaps there's a different call that would work here? Anyone with experience on the internals of newlib have a hint?