[Rd] Object are not destroy while using error (Rf_error)

[Tomas Kalibera]

On 1/21/23 16:55, Antoine Lucas wrote:
> Dear all,
>
> I try to understand why on my computer I do not clear all data with this
> code:
>
>
> #include <R.h>
> static int count = 0;
>
> class A {
> public:
>    A(){  printf("c %d\n", count);
>      count++;  }
>
>    ~A(){count--;
>      printf("d %d\n", count);  }
> };
>
> extern "C" {
>    void testAL(){
>      A a;
>      {
>        A b;
>      }
>      error("does not write [d 0]");
>    }
> }
>
> To run with R: I build  gcc -shared -I/opt/R-202301/lib/R/include/
> myError.cpp -o myError.so
>
> then in R: dyn.load("myError.so")
> .C("testAL")
>
> This writes c0, c1, d1 but not d0.
> If I comment line "error", I does write latest d0.
>
> How could I get all my objects destroy while sending en error message to R ?

The problem is that Rf_error() uses a long jump, which unwinds the C++ 
frames without calling destructors. Using the C API of R from C++ code 
is tricky because of this incompatibility of C long jumps with C++ 
destructors.

Technically, you need to protect all calls from C++ to R against long 
jumps. There is an API for that, see R_UnwindProtect in 6.12 of "Writing 
R Extensions". So, your wrapper will get called in case of a long jump 
and will handle it in a way compatible with your C++ frames (possibly by 
throwing a C++ exception).

Similarly, you should not throw C++ exceptions over R frames on the 
stack, because R's meta-data about long jump targets on its stacks would 
go out of sync, causing a crash later. You again would have to convert 
C++ exceptions to long jumps.

So, in your example where you use the R API to only trigger an R error, 
you could instead throw a C++ exception, and catch it later, below all 
your C++ code but before unwinding could potentially reach R frames. And 
there you could call Rf_error().

Best
Tomas

(this is related: 
https://blog.r-project.org/2019/03/28/use-of-c-in-packages )

>
> Regards,
>
> Antoine Lucas.
>
> 	[[alternative HTML version deleted]]
>
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