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.
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