Hacker News new | ask | show | jobs
by Animats 7 days ago
Trying to figure out Fil-C's "inviscaps".[1]

When you allocate space with Fil-C's "malloc", some additional bounds checking data precedes the space the program gets to use. Pointers are "fat pointers", with a pointer to the beginning of the buffer and a pointer to someplace within the buffer, allowing ordinary C pointer manipulation.

There's much new verbiage around this. But it's roughly the same idea as GCC "fat pointers".[2][3] So it's not a new idea. It's one that's been tried several times, but never caught on.

There's a performance penalty. Especially if the compiler can't hoist the checks out of loops.

[1] https://fil-c.org/invisicaps

[2] https://williambader.com/bounds/example.html

[3] https://www.doc.ic.ac.uk/~awl03/projects/miro/MIRO.pdf
1 comments
pizlonator 7 days ago
Invisicaps are not exactly fat pointers.

Fat pointers show up inline in memory, which has a bunch of problems:

- sizeof(void*) changes

- either you let the bounds get corrupted by bad casts, unions, and other issues, or you impose restrictions that prevent unions from working compatibly, or you end up supporting unions by having issues with races, or you need special hardware. Invisicaps sidestep all of those issues

link
Animats 7 days ago
The "flight pointer" looks an awful lot like a fat pointer. See the diagram at "The intuition of inviscaps" in [1]. A "flight pointer" has a pointer to the beginning of the buffer, which they call the "lower bound ptr", and a pointer to someplace within the buffer, which they call the "integer ptr". The pointer to the beginning of the buffer lets the checker find the size of the buffer, which is stored preceding the buffer. The compiler has to arrange things so that the lower bound ptr is carried around wherever a "flight pointer" goes. This is supposed to be invisible to the programmer.

[1] https://fil-c.org/invisicaps

link
pizlonator 7 days ago
other implementations of fat pointers (that I’m aware of) have no distinction between flight and rest; they store what I call flight pointers in memory literally.

The closest technique to invisicaps is softbound, but that has issues that invisicaps resolve (better story for races, more comprehensive safety for all of the C and C++ languages, no need for large virtual memory reservations, and lock freedom)

link