R help - Jul 2024 - C API - no NULL pointer guarantee?

Hello,

I'm working on bindings for the API (for zig), and was wondering if the
R's C API guarantees it won't return null pointers?
The only reference I found in the "Writing R Extensions" manual where
this not the case is `R_tryEval` and `R_tryEvalSilent`.
Otherwise it's unclear.

The reason I care about this is syntax. Because I don't know whether SEXPs
are NULL or not, then I wrap the SEXP in an optional type, and the burden is on
the user to either check or assert every time you want to handle an optional
SEXP.
It's not too bad and provides null safety, but can get excessive.

See an example from one of my test functions (question mark unwraps the
optional. asserting it is not null):
```
export fn testAsScalarVector() Robject {
    const results = rzig.vec.allocVector(.List, 23).?.protect();
    defer rzig.gc.protect_stack.unprotectAll();

    results.?.setListObj(0, rzig.vec.asScalarVector(@as(f32, 1.32456e+32)));
    results.?.setListObj(1, rzig.vec.asScalarVector(@as(f32, -9.87123e-32)));
    results.?.setListObj(2, rzig.vec.asScalarVector(math.inf(f32)));
    results.?.setListObj(3, rzig.vec.asScalarVector(-math.inf(f32)));
    results.?.setListObj(4, rzig.vec.asScalarVector(math.nan(f32)));

    results.?.setListObj(5, rzig.vec.asScalarVector(@as(f64, -9.1e+300)));
    results.?.setListObj(6, rzig.vec.asScalarVector(@as(f64, 1.2e-300)));
    results.?.setListObj(7, rzig.vec.asScalarVector(math.inf(f64)));
    results.?.setListObj(8, rzig.vec.asScalarVector(-math.inf(f64)));
    results.?.setListObj(9, rzig.vec.asScalarVector(math.nan(f64)));

    results.?.setListObj(10, rzig.vec.asScalarVector(-9.1e+307));
    results.?.setListObj(11, rzig.vec.asScalarVector(1.2e-307));
    results.?.setListObj(12, rzig.vec.asScalarVector(1.0e+500)); // Inf
    results.?.setListObj(13, rzig.vec.asScalarVector(-1.0e+500)); // -Inf

    results.?.setListObj(14, rzig.vec.asScalarVector(5));
    results.?.setListObj(15, rzig.vec.asScalarVector(-5));
    results.?.setListObj(16, rzig.vec.asScalarVector(@as(u32, 4)));
    results.?.setListObj(17, rzig.vec.asScalarVector(@as(i32, -4)));
    results.?.setListObj(18, rzig.vec.asScalarVector(@as(u0, 0)));
    results.?.setListObj(19, rzig.vec.asScalarVector(@as(u150, 2_000_000_000)));
    results.?.setListObj(20, rzig.vec.asScalarVector(@as(i150,
-2_000_000_000)));
    results.?.setListObj(21, rzig.vec.asScalarVector(true));
    results.?.setListObj(22, rzig.vec.asScalarVector(false));

    return results;
}
```

It would be nice to be able to drop the question mark, but only if R guarantees
null safety at the API level. Then I would declare optional return types only
for documented cases like `R_tryEval`

Appreciate your time an help!
Thanks,
- Erez

? Sat, 27 Jul 2024 14:36:20 +0300
"Erez Shomron" <r-mails at erezsh.org> ?????:
> I'm working on bindings for the API (for zig), and was wondering if
> the R's C API guarantees it won't return null pointers? The only
> reference I found in the "Writing R Extensions" manual where this
not
> the case is `R_tryEval` and `R_tryEvalSilent`.
Based on what I've been reading while working on (still very much
incomplete) <https://aitap.codeberg.page/R-api>, I think that these are
the only two cases where a SEXP can be null, precisely because a null
pointer is distinguished from every possible value that could be
returned by eval().

Some APIs may accept a null SEXP (with comments in the source code that
this is to support some legacy code, not the intended use), but when R
wants to return a value, it is always allocated through the memory
manager. A failing allocation will longjmp() away from the frame, not
return a null pointer.

It's hard to prove a negative, though.

-- 
Best regards,
Ivan