Memory management in Forth

Question

So I\'m just learning Forth and was curious if anyone could help me understand how memory management generally works. At the moment I only have (some) experience with the C stac

Answer 1

With Forth you enter a different world.

In a typical Forth like ciforth on linux (and assuming 64 bits) you can configure your Forth to have a linear memory space that is as large as your swap space (e.g. 128 Gbyte). That is yours to fill in with arrays, linked lists, pictures whatever. There are no restrictions. Forth only provides you with a HERE pointer to help you keep track of memory you have used up. Even that you can ignore, and there is even a word in the 1994 standard that provides scratch space that floats in the free memory (PAD).

Is there something like malloc() free() ? Not necessarily in a small kernel of a couple of dozen kilobytes. But you can just include a file with an allocate utility and set aside a couple of Gbyte to use for dynamic memory.

As an example I'm currently working with tiff files. A typical 140 Mbyte picture takes a small chunk out of the dictionary advancing HERE. Rows of pixels are transformed, decompressed etc. For that I use dynamic memory, so I ALLOCATE space for the decompression result of a row. I've to manually FREE them again when the results have been used up for another transformation. It feels totally different from c. There is more control and more danger.

Your question about scopes etc. In Forth if you know the address, you can access the data structure. Even if you jotted F7FFA1003 on a piece of paper. Trying to make programs safer by separate name spaces is not prominent in Forth style. So called wordlist (see also VOCABULARY) provide facilities in that direction.