This is in general impossible on a garden-variety multi-tasking
operating system. You can't predict how much system memory is available
at a particular time. The millisecond after you get some number, some
other process outside of your control might start and take away memory.

If you want to do things like this - you unfortunately didn't tell us
why you want to do this -, you need an operating system which provides
features for resource reservation in advance. I only know proprietary
OS' for telecom systems who do this.

You can't in a reliable way.

/Thomas