When you pass a value to a function, you create a pointer to
that value in the OCaml runtime – the GC can’t collect the old value until
List.iter completes because the value is still live (internally, it’s part of a
local root but, in practice, as List.iter is implemented in OCaml directly it’s
because an OCaml function parameter references the value). Note that in this
example:
let a = [1; 2; 3]
and b = [4; 5; 6]
and c = [7; 8; 9] in
let myref = ref a in
(* No allocations are done after here *)
myref := a;
myref := b;
myref := c;;
the assignments to [myref] do not result in any memory being
allocated at all (my point is that action of assigning to a reference does not
implicitly result in an allocation).
David
From: caml-list-bounces@yquem.inria.fr
[mailto:caml-list-bounces@yquem.inria.fr] On Behalf Of ivan chollet
Sent: 08 August 2009 18:10
To: ivan.chollet@free.fr
Cc: caml-list@yquem.inria.fr
Subject: [Caml-list] Re: ocaml sefault in bytecode: unanswered questions
Yes
it was a freebsd 6.4 with ocaml 3.10.2
I’ll
run the program on linux later and see how it goes.
Thanks
for your advices regarding debugging. I pretty much tried all of these though…
the thing is my error is not an ocaml error at runtime but an error of the
ocaml runtime. And to analyze a core dump of ocamlrun, I just thought my best
bet was gdb. Whatever.
OK
I’ll try to provide you with a minimal ocaml code that produce an ocamlrun
error. Might take a little while as I’m not free.
In
the meantime, I’ve got a newbie question regarding ocaml garbage collector and
the same List.iter stuff:
Say
you do a “List.iter myfun !myref”, where !myref is a list (hehe…), and where
myfun is a function that does reallocations of myref (that is affectations like
myref := [some new or old objects]). The pointers myref that are generated
through this process are destroyed each time a new reallocation of myref is
done. Of course the underlying linked lists that are not referenced anymore
shouldn’t be collected by the GC before the end of the main “List.iter”,
otherwise it’s iterating through a linked list that has been garbage collected.
My
question is: does the GC know that it cannot collect the unreferenced myref
pointers before the end of the List.iter?
Sorry,
I just wanted to ask this question to rule it out.
Thanks
again.
On
07-08-2009, ivan chollet <ivan.chollet@free.fr> wrote:
>
>
This GDB was configured as "i386-marcel-freebsd"...(no debugging
symbols
>
found)...
>
>
Not very informative. So here are my questions:
I
suppose you are running freebsd ? Which version of freebsd, of ocaml ?
>
>
>
>
- What is the best way to
produce and analyze core dumps in ocaml?
>
Should I compile in bytecode or native? Is there any special gdb
"trick"
>
that gives you more information? Is there any special "trick" while
>
compiling the ocaml runtime to make it throw more information?
>
gdb
is not the perfect tool to debug ocaml program. You should give a
try
to ocamldebug which is a better option for bytecode (see below for
options).
Bytecode is more informative when coming to reporting
backtrace
(at least with old version of ocaml).
Compile
every program with "-g" option (just like gcc).
If
you have compiled everything with "-g" option, you can also use the
environment
variable OCAMLRUNPARAM="b" to get a backtrace for your
exception,
at runtime.
>
- Then, my main question
is actually: in bytecode, what can produce
>
segfaults? My ocaml code is completely standard, excepted that I use the
>
Marshal module. So my question is rather: outside the Marshal module, what
>
can cause segfault?
Some
part of the bytecode are just standard C, everything can cause a
segfault
just as C. These errors are not very common but it is possible
that
some case are not well handled on freebsd. Most probably a porting
issue.
Marshal
module can easily trigger a segfault when you map the loaded data
to
a type which doesn't match the dumped data.
Example:
List.length
(Marshal.from_string (Marshal.to_string 1234 []) 0);;
Here
the integer value is marshalled and then unmarshalled as a list ->
segfault.
>
>
- Slightly unrelated
question: I have been able to generate
>
segfaults by running ocaml code that: 1) iterates recursively through a list
>
reference 2) changes the reference while still iterating on it. For example,
>
you just do a "List.iter myfun !myref", and within the function
myfun, you
>
do stuff like "myref := List.filter somefilterfunction !myref". It is
not
>
good to program like this, but for some reason I thought ocaml would not
>
segfault on that. Is this expected behavior? If it's not, I'll be happy to
>
provide some simple source code that illustrates it. (nevermind I have
>
actually cleaned all my code base from these dirty uses of references)
>
Could
you provide a minimal example code for this error ? I don't think
this
should generate a segfault.
Regards
Sylvain
Le Gall