On 4/29/23 19:26, Michael Milton wrote:
I'm trying to learn about R's PROTECT system. To that end I've tried to
create an example of C code that doesn't protect anything. I was hoping it
would either segfault from trying to access deallocated memory, or maybe
print out nonsense results because the unprotected memory got overwritten,
but I can't make either happen.
Here's my current code (all in R, using the inline package for simplicity):
gctorture(TRUE)
z = inline::cfunction(body="
SEXP vec_1 = Rf_ScalarInteger(99);
SEXP vec_2 = Rf_allocVector(VECSXP, 10);
SET_VECTOR_ELT(vec_2, 1, vec_1);
Rf_PrintValue(vec_2);
")
My thinking was that, with torture mode enabled, the allocation of vec_2
should ensure that vec_1 is collected, and then trying to put it into vec_2
and then print it would then fail. But it consistently prints 99 in the
list's second element.
Why does this code not have issues? Alternatively, is there a simpler
example of C code that demonstrates missing PROTECT calls?
It is not guaranteed that a PROTECT error will always lead to memory
corruption or a crash in all executions of a program, not even with
gctorture.
To increase the chances, you can instruct gctorture to run the gc more
often (but the execution would be slower). You can build R with strict
write barrier checking (see e.g. Writing R Extensions, look for
gctorture) to make it more likely errors will be detected. There is
always a tradeoff between this probability and slowdown of the
execution. In theory we could invalidate all unreachable objects at
every allocation, but that would be so slow that we could not test any
programs in practice.
Then, not every piece of memory can be used by any allocation, due to
how the memory allocator and gc works. If you need to provoke an error
e.g. for educational purposes, perhaps chances would be higher if you
allocate an object of the same type and size as the one not protected in
error.
You can also play with R sources - e.g. introduce a PROTECT error into
some key part of the R interpreter, it should not be too hard to trigger
that via make check-devel.
Best
Tomas
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