* How to pass C pointers to Caml @ 2010-03-01 3:55 Jianzhou Zhao 2010-03-01 6:22 ` [Caml-list] " Florent Monnier 0 siblings, 1 reply; 8+ messages in thread From: Jianzhou Zhao @ 2010-03-01 3:55 UTC (permalink / raw) To: caml-list I have been calling OCaml code from C in my project. The C code has some pointers to C structures. I got 'seg fault' when calling the OCaml function receiving C structure pointers. 18.7 at http://caml.inria.fr/pub/docs/manual-ocaml/manual032.html gives the examples that pass int into OCaml. These examples work for me. But, Does OCaml support to pass C structure pointers to OCaml? -- Jianzhou ^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [Caml-list] How to pass C pointers to Caml 2010-03-01 3:55 How to pass C pointers to Caml Jianzhou Zhao @ 2010-03-01 6:22 ` Florent Monnier 2010-03-01 13:23 ` Jianzhou Zhao 2010-03-01 13:24 ` Goswin von Brederlow 0 siblings, 2 replies; 8+ messages in thread From: Florent Monnier @ 2010-03-01 6:22 UTC (permalink / raw) To: caml-list Le lundi 1 mars 2010 04:55:00, Jianzhou Zhao a écrit : > I have been calling OCaml code from C in my project. > The C code has some pointers to C structures. > I got 'seg fault' when calling the OCaml function receiving > C structure pointers. > > 18.7 at http://caml.inria.fr/pub/docs/manual-ocaml/manual032.html > gives the examples that pass int into OCaml. These examples work for me. > But, Does OCaml support to pass C structure pointers to OCaml? Yes it does. Just cast your pointer to the type value. In this tutorial there is an example "Pointers to C structures": http://www.linux-nantes.org/~fmonnier/OCaml/ocaml-wrapping-c.php#ref_ptr the pointer to a C struct is wrapped on the ocaml side by an abstract type called "t" here, and it is provided back to C with print_t / dump_ptr. ^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [Caml-list] How to pass C pointers to Caml 2010-03-01 6:22 ` [Caml-list] " Florent Monnier @ 2010-03-01 13:23 ` Jianzhou Zhao 2010-03-01 13:24 ` Goswin von Brederlow 1 sibling, 0 replies; 8+ messages in thread From: Jianzhou Zhao @ 2010-03-01 13:23 UTC (permalink / raw) To: Florent Monnier; +Cc: caml-list On Mon, Mar 1, 2010 at 1:22 AM, Florent Monnier <monnier.florent@gmail.com> wrote: > Le lundi 1 mars 2010 04:55:00, Jianzhou Zhao a écrit : >> I have been calling OCaml code from C in my project. >> The C code has some pointers to C structures. >> I got 'seg fault' when calling the OCaml function receiving >> C structure pointers. >> >> 18.7 at http://caml.inria.fr/pub/docs/manual-ocaml/manual032.html >> gives the examples that pass int into OCaml. These examples work for me. >> But, Does OCaml support to pass C structure pointers to OCaml? > > Yes it does. Just cast your pointer to the type value. > > In this tutorial there is an example "Pointers to C structures": > http://www.linux-nantes.org/~fmonnier/OCaml/ocaml-wrapping-c.php#ref_ptr > > the pointer to a C struct is wrapped on the ocaml side by an abstract type > called "t" here, and it is provided back to C with print_t / dump_ptr. Thanks. This tutorial helps a lot when calling C from OCaml. What I was doing is more like calling OCaml from C, at section http://www.linux-nantes.org/~fmonnier/OCaml/ocaml-wrapping-c.php#ref_mlfromc. Because my main program is C. In this case, does OCaml have any restriction when passing a C pointer (which points to a struct) to OCaml runtme in this direction? Jianzhou > > _______________________________________________ > Caml-list mailing list. Subscription management: > http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list > Archives: http://caml.inria.fr > Beginner's list: http://groups.yahoo.com/group/ocaml_beginners > Bug reports: http://caml.inria.fr/bin/caml-bugs > -- Jianzhou ^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [Caml-list] How to pass C pointers to Caml 2010-03-01 6:22 ` [Caml-list] " Florent Monnier 2010-03-01 13:23 ` Jianzhou Zhao @ 2010-03-01 13:24 ` Goswin von Brederlow 2010-03-01 23:04 ` Florent Monnier 1 sibling, 1 reply; 8+ messages in thread From: Goswin von Brederlow @ 2010-03-01 13:24 UTC (permalink / raw) To: caml-list Florent Monnier <monnier.florent@gmail.com> writes: > Le lundi 1 mars 2010 04:55:00, Jianzhou Zhao a écrit : >> I have been calling OCaml code from C in my project. >> The C code has some pointers to C structures. >> I got 'seg fault' when calling the OCaml function receiving >> C structure pointers. >> >> 18.7 at http://caml.inria.fr/pub/docs/manual-ocaml/manual032.html >> gives the examples that pass int into OCaml. These examples work for me. >> But, Does OCaml support to pass C structure pointers to OCaml? > > Yes it does. Just cast your pointer to the type value. > > In this tutorial there is an example "Pointers to C structures": > http://www.linux-nantes.org/~fmonnier/OCaml/ocaml-wrapping-c.php#ref_ptr > > the pointer to a C struct is wrapped on the ocaml side by an abstract type > called "t" here, and it is provided back to C with print_t / dump_ptr. The problem with this trivial approach is that ocaml can store the pointer somewhere. When the C pointer is freeed then ocaml has a dangling pointer. Worse, if the GC allocates a new heap then the pointer might suddenly point into the heap and then BOOM. It is better to put the pointer into an abstract or custom block. MfG Goswin ^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [Caml-list] How to pass C pointers to Caml 2010-03-01 13:24 ` Goswin von Brederlow @ 2010-03-01 23:04 ` Florent Monnier 2010-03-02 10:19 ` Goswin von Brederlow 0 siblings, 1 reply; 8+ messages in thread From: Florent Monnier @ 2010-03-01 23:04 UTC (permalink / raw) To: caml-list Le lundi 1 mars 2010 14:24:45, Goswin von Brederlow a écrit : > Florent Monnier <monnier.florent@gmail.com> writes: > > Le lundi 1 mars 2010 04:55:00, Jianzhou Zhao a écrit : > >> I have been calling OCaml code from C in my project. > >> The C code has some pointers to C structures. > >> I got 'seg fault' when calling the OCaml function receiving > >> C structure pointers. > >> > >> 18.7 at http://caml.inria.fr/pub/docs/manual-ocaml/manual032.html > >> gives the examples that pass int into OCaml. These examples work for me. > >> But, Does OCaml support to pass C structure pointers to OCaml? > > > > Yes it does. Just cast your pointer to the type value. > > > > In this tutorial there is an example "Pointers to C structures": > > http://www.linux-nantes.org/~fmonnier/OCaml/ocaml-wrapping-c.php#ref_ptr > > > > the pointer to a C struct is wrapped on the ocaml side by an abstract > > type called "t" here, and it is provided back to C with print_t / > > dump_ptr. > > The problem with this trivial approach is that ocaml can store the > pointer somewhere. When the C pointer is freeed then ocaml has a > dangling pointer. Worse, if the GC allocates a new heap then the pointer > might suddenly point into the heap and then BOOM. A lot of bindings wrap C pointer, it is known to be a technic that does work. Dangerous that's true, be if you are very careful, it works. What you can do is set the pointer to NULL when the struct is freed, and then each function that uses this struct pointer can first check if the pointer is NULL or not before to use it, and if it's NULL raise an exception. > It is better to put the pointer into an abstract or custom block. You can do this too. -- ^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [Caml-list] How to pass C pointers to Caml 2010-03-01 23:04 ` Florent Monnier @ 2010-03-02 10:19 ` Goswin von Brederlow 2010-03-02 11:34 ` Florent Monnier 0 siblings, 1 reply; 8+ messages in thread From: Goswin von Brederlow @ 2010-03-02 10:19 UTC (permalink / raw) To: Florent Monnier; +Cc: caml-list Florent Monnier <monnier.florent@gmail.com> writes: > Le lundi 1 mars 2010 14:24:45, Goswin von Brederlow a écrit : >> Florent Monnier <monnier.florent@gmail.com> writes: >> > Le lundi 1 mars 2010 04:55:00, Jianzhou Zhao a écrit : >> >> I have been calling OCaml code from C in my project. >> >> The C code has some pointers to C structures. >> >> I got 'seg fault' when calling the OCaml function receiving >> >> C structure pointers. >> >> >> >> 18.7 at http://caml.inria.fr/pub/docs/manual-ocaml/manual032.html >> >> gives the examples that pass int into OCaml. These examples work for me. >> >> But, Does OCaml support to pass C structure pointers to OCaml? >> > >> > Yes it does. Just cast your pointer to the type value. >> > >> > In this tutorial there is an example "Pointers to C structures": >> > http://www.linux-nantes.org/~fmonnier/OCaml/ocaml-wrapping-c.php#ref_ptr >> > >> > the pointer to a C struct is wrapped on the ocaml side by an abstract >> > type called "t" here, and it is provided back to C with print_t / >> > dump_ptr. >> >> The problem with this trivial approach is that ocaml can store the >> pointer somewhere. When the C pointer is freeed then ocaml has a >> dangling pointer. Worse, if the GC allocates a new heap then the pointer >> might suddenly point into the heap and then BOOM. > > A lot of bindings wrap C pointer, it is known to be a technic that does work. > Dangerous that's true, be if you are very careful, it works. > What you can do is set the pointer to NULL when the struct is freed, and then > each function that uses this struct pointer can first check if the pointer is > NULL or not before to use it, and if it's NULL raise an exception. let x = ref None let called_function c_ptr = x := Some c_ptr How will you get x to be Some NULL? Your C code does not know about the copy. You need to wrap the C pointer into a custom or abstract block first to be able to NULL it. A finalizer in a custom block can also be helpfull here and free the pointer when ocaml no longer needs it. >> It is better to put the pointer into an abstract or custom block. > > You can do this too. Imho you must. Anything else is too dangerous. MfG Goswin ^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [Caml-list] How to pass C pointers to Caml 2010-03-02 10:19 ` Goswin von Brederlow @ 2010-03-02 11:34 ` Florent Monnier 2010-03-03 11:02 ` Goswin von Brederlow 0 siblings, 1 reply; 8+ messages in thread From: Florent Monnier @ 2010-03-02 11:34 UTC (permalink / raw) To: caml-list, Goswin von Brederlow Le mardi 2 mars 2010 11:19:58, vous avez écrit : > Florent Monnier <monnier.florent@gmail.com> writes: > > Le lundi 1 mars 2010 14:24:45, Goswin von Brederlow a écrit : > >> Florent Monnier <monnier.florent@gmail.com> writes: > >> > Le lundi 1 mars 2010 04:55:00, Jianzhou Zhao a écrit : > >> >> I have been calling OCaml code from C in my project. > >> >> The C code has some pointers to C structures. > >> >> I got 'seg fault' when calling the OCaml function receiving > >> >> C structure pointers. > >> >> > >> >> 18.7 at http://caml.inria.fr/pub/docs/manual-ocaml/manual032.html > >> >> gives the examples that pass int into OCaml. These examples work for > >> >> me. But, Does OCaml support to pass C structure pointers to OCaml? > >> > > >> > Yes it does. Just cast your pointer to the type value. > >> > > >> > In this tutorial there is an example "Pointers to C structures": > >> > http://www.linux-nantes.org/~fmonnier/OCaml/ocaml-wrapping-c.php#ref_p > >> >tr > >> > > >> > the pointer to a C struct is wrapped on the ocaml side by an abstract > >> > type called "t" here, and it is provided back to C with print_t / > >> > dump_ptr. > >> > >> The problem with this trivial approach is that ocaml can store the > >> pointer somewhere. When the C pointer is freeed then ocaml has a > >> dangling pointer. Worse, if the GC allocates a new heap then the pointer > >> might suddenly point into the heap and then BOOM. > > > > A lot of bindings wrap C pointer, it is known to be a technic that does > > work. Dangerous that's true, be if you are very careful, it works. > > What you can do is set the pointer to NULL when the struct is freed, and > > then each function that uses this struct pointer can first check if the > > pointer is NULL or not before to use it, and if it's NULL raise an > > exception. > > let x = ref None > > let called_function c_ptr = x := Some c_ptr > > How will you get x to be Some NULL? I mean often in a C library pointers to C struct are just pointers to something abstract because a lot of lib C API do hide the struct, and provide a function to free the things pointed. So in your wrapper after you call the destroy function you can set the pointer to NULL and in the other functions test if the pointer is NULL before to use it to prevent a user of the wrapper to call a function after having called the destroy function. > Your C code does not know about the > copy. You need to wrap the C pointer into a custom or abstract block > first to be able to NULL it. A finalizer in a custom block can also be > helpfull here and free the pointer when ocaml no longer needs it. > > >> It is better to put the pointer into an abstract or custom block. > > > > You can do this too. > > Imho you must. Anything else is too dangerous. Yes it is dangerous, but as I explained with some C libraries this is the only possible solution. If you can do it the other way, then do, while it's indeed safer. -- Cheers ^ permalink raw reply [flat|nested] 8+ messages in thread
* Re: [Caml-list] How to pass C pointers to Caml 2010-03-02 11:34 ` Florent Monnier @ 2010-03-03 11:02 ` Goswin von Brederlow 0 siblings, 0 replies; 8+ messages in thread From: Goswin von Brederlow @ 2010-03-03 11:02 UTC (permalink / raw) To: Florent Monnier; +Cc: caml-list, Goswin von Brederlow Florent Monnier <monnier.florent@gmail.com> writes: > Le mardi 2 mars 2010 11:19:58, vous avez écrit : >> Florent Monnier <monnier.florent@gmail.com> writes: >> > Le lundi 1 mars 2010 14:24:45, Goswin von Brederlow a écrit : >> >> Florent Monnier <monnier.florent@gmail.com> writes: >> >> > Le lundi 1 mars 2010 04:55:00, Jianzhou Zhao a écrit : >> >> >> I have been calling OCaml code from C in my project. >> >> >> The C code has some pointers to C structures. >> >> >> I got 'seg fault' when calling the OCaml function receiving >> >> >> C structure pointers. >> >> >> >> >> >> 18.7 at http://caml.inria.fr/pub/docs/manual-ocaml/manual032.html >> >> >> gives the examples that pass int into OCaml. These examples work for >> >> >> me. But, Does OCaml support to pass C structure pointers to OCaml? >> >> > >> >> > Yes it does. Just cast your pointer to the type value. >> >> > >> >> > In this tutorial there is an example "Pointers to C structures": >> >> > http://www.linux-nantes.org/~fmonnier/OCaml/ocaml-wrapping-c.php#ref_p >> >> >tr >> >> > >> >> > the pointer to a C struct is wrapped on the ocaml side by an abstract >> >> > type called "t" here, and it is provided back to C with print_t / >> >> > dump_ptr. >> >> >> >> The problem with this trivial approach is that ocaml can store the >> >> pointer somewhere. When the C pointer is freeed then ocaml has a >> >> dangling pointer. Worse, if the GC allocates a new heap then the pointer >> >> might suddenly point into the heap and then BOOM. >> > >> > A lot of bindings wrap C pointer, it is known to be a technic that does >> > work. Dangerous that's true, be if you are very careful, it works. >> > What you can do is set the pointer to NULL when the struct is freed, and >> > then each function that uses this struct pointer can first check if the >> > pointer is NULL or not before to use it, and if it's NULL raise an >> > exception. >> >> let x = ref None >> >> let called_function c_ptr = x := Some c_ptr >> >> How will you get x to be Some NULL? > > I mean often in a C library pointers to C struct are just pointers to > something abstract because a lot of lib C API do hide the struct, and provide > a function to free the things pointed. So in your wrapper after you call the > destroy function you can set the pointer to NULL and in the other functions > test if the pointer is NULL before to use it to prevent a user of the wrapper > to call a function after having called the destroy function. > >> Your C code does not know about the >> copy. You need to wrap the C pointer into a custom or abstract block >> first to be able to NULL it. A finalizer in a custom block can also be >> helpfull here and free the pointer when ocaml no longer needs it. >> >> >> It is better to put the pointer into an abstract or custom block. >> > >> > You can do this too. >> >> Imho you must. Anything else is too dangerous. > > Yes it is dangerous, but as I explained with some C libraries this is the only > possible solution. If you can do it the other way, then do, while it's indeed > safer. It is always possible to wrap the C pointer into an abstract or custom block. The pointer can still become dangling when the C side frees the target before ocaml forgets the block but, as the GC does not look into abstract/custom blocks, it will never result in the GC exploding because a dangling pointer points into the GC heap. MfG Goswin ^ permalink raw reply [flat|nested] 8+ messages in thread
end of thread, other threads:[~2010-03-03 11:02 UTC | newest] Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2010-03-01 3:55 How to pass C pointers to Caml Jianzhou Zhao 2010-03-01 6:22 ` [Caml-list] " Florent Monnier 2010-03-01 13:23 ` Jianzhou Zhao 2010-03-01 13:24 ` Goswin von Brederlow 2010-03-01 23:04 ` Florent Monnier 2010-03-02 10:19 ` Goswin von Brederlow 2010-03-02 11:34 ` Florent Monnier 2010-03-03 11:02 ` Goswin von Brederlow
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