[gambit-list] Segfault on OSX using Gambit's C Interface

Adam adam.mlmb at gmail.com
Wed Dec 16 18:27:56 EST 2015


Neat!

What was the problem with ___result ?

And, in what commit did you introduce ___return ?

2015-12-17 7:00 GMT+08:00 Marc Feeley <feeley at iro.umontreal.ca>:

> I have improved the FFI by introducing the ___return(val) macro which
> should be used from now on instead of ___result=val and
> ___result_voidstar=val.  So your *->My function can be coded like this:
>
> (define *->My (c-lambda (My*/nonnull) My "___return(*___arg1);"))
>
> This will hide the implementation of the FFI and the use of ___result and
> ___result_voidstar, which was confusing.
>
> Check the updated Gambit manual for details.
>
> Marc
>
> > On Dec 14, 2015, at 2:46 PM, Paolo Carlesi <pcarlesi at gmail.com> wrote:
> >
> > Thank you for the workaround!
> >
> > What is the best way to generate a foreign c-type object ?
> >
> > This seems to me the best approach (sorry if it's obvious to everybody
> but me):
> >
> > (define make-My (c-lambda () My "___result_voidstar =
> ___EXT(___alloc_rc)(sizeof(struct My));"))
> > (define My->* (c-lambda (My) My*/nonnull "___result_voidstar =
> &___arg1;"))
> >
> > has the advantage of not wasting time / space creating a new foreign
> object and a copy, like *->My below does.
> > It also doesn't trigger the c-lambda bug, not that I have verified
> carefully yet.
> > Members can easily be set up with a syntax like (My-x-set! (My->* xoxo)
> #xdeadbeef) .
> > Is this approach correct?
> >
> > Thanks Again
> >
> > Paolo
> >
> > Il giorno 14/dic/2015, alle ore 18:02, Marc Feeley <
> feeley at iro.umontreal.ca> ha scritto:
> >
> >> There appears to be a bug in the FFI when using a c-lambda with inline
> code that returns a foreign object.  So for now you will have to define a C
> function that does the work and use the name of that function in the
> c-lambda.  In other words:
> >>
> >> (c-declare #<<c-declare-end
> >>
> >> My indirect_My_star(My *ptr) { return *ptr; }
> >>
> >> c-declare-end
> >> )
> >>
> >> (define *->My (c-lambda (My*/nonnull) My "indirect_My_star"))
> >>
> >> Marc
> >>
> >>
> >>> On Dec 14, 2015, at 4:55 AM, Paolo Carlesi <pcarlesi at gmail.com> wrote:
> >>>
> >>> Hello,
> >>>
> >>> I'm experiencing regular segfaults on OSX 10.9.5, with the sample code
> shown below.
> >>> Tested with Gambit 4.8.1 + GCC 4.9  and with packaged Gambit 4.7.3 +
> LLVM.
> >>> No issue at all on Linux, GCC 4.8, with both Gambit 4.2.8 and Gambit
> 4.8.2.
> >>> Of course I may be doing something silly.
> >>>
> >>> A simple struct, "My" is generated inside a c procedure, then passed
> multiple times to another c-lambda procedure (*->My).
> >>> It's a simplified model of the code produced by a FFI SDL2 Library I'm
> using.
> >>> The doc says that the foreign object (xoxo in the example) should not
> be released while the scheme code is still referencing it.
> >>> I am seeing the refcount going wildly in the negative, one for each
> call to *->My
> >>> The only way -- clumsy -- I have found to avoid this is to increment
> the reference count with ___addref_rc ( not shown here ).
> >>>
> >>> What's wrong here?
> >>> Thanks!
> >>>
> >>>
> >>> LLDB output:
> >>>
> >>> Process 48149 launched: '/Users/paolo/scheme/spazz/fromlin' (x86_64)
> >>> Process 48149 stopped
> >>> * thread #1: tid = 0x40e3c9, 0x000000010000770c fromlin`___release_rc
> + 28, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS
> (code=EXC_I386_GPFLT)
> >>>  frame #0: 0x000000010000770c fromlin`___release_rc + 28
> >>> fromlin`___release_rc + 28:
> >>> -> 0x10000770c:  movq   %rax, (%rdx)
> >>> 0x10000770f:  movq   %rdx, 0x8(%rax)
> >>> 0x100007713:  jmp    0x100010240               ; ___free_mem
> >>> 0x100007718:  nopl   (%rax,%rax)
> >>>
> >>> stack trace:
> >>>
> >>> (lldb) bt
> >>> * thread #1: tid = 0x40e3c9, 0x000000010000770c fromlin`___release_rc
> + 28, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS
> (code=EXC_I386_GPFLT)
> >>> * frame #0: 0x000000010000770c fromlin`___release_rc + 28
> >>>  frame #1: 0x0000000100002579 fromlin`___release_fn0 + 9
> >>>  frame #2: 0x0000000100009009 fromlin`___garbage_collect + 3497
> >>>  frame #3: 0x00000001000091bf fromlin`___alloc_scmobj + 255
> >>>  frame #4: 0x000000010000d4eb fromlin`___POINTER_to_SCMOBJ + 43
> >>>  frame #5: 0x000000010000d5e4 fromlin`___STRUCT_to_SCMOBJ + 20
> >>>  frame #6: 0x0000000100002112 fromlin`___H__20_fromlin + 3378
> >>>  frame #7: 0x0000000100004aa4 fromlin`___call + 372
> >>>  frame #8: 0x0000000100004bc6 fromlin`___setup_pstate + 262
> >>>  frame #9: 0x00000001000063be fromlin`___setup + 3950
> >>> (lldb)
> >>>
> >>>
> >>> ;; SAMPLE CODE
> >>>
> >>> (c-declare #<<c-declare-end
> >>>
> >>> typedef struct My {
> >>> int r;
> >>> } My;
> >>>
> >>> c-declare-end
> >>> )
> >>>
> >>> (begin
> >>> (c-define-type My (struct "My"))
> >>> (c-define-type My* (pointer My (My*)))
> >>> (c-define-type My*/nonnull (nonnull-pointer My (My*)))
> >>> (c-define-type My*/release-rc (nonnull-pointer My (My*)
> "___release_My")))
> >>>
> >>>
> >>> (begin
> >>>  (c-declare #<<c-declare-end
> >>> static ___SCMOBJ ___release_My( void* ptr ) {
> >>>  ___EXT(___release_rc)( ptr );
> >>>  return ___FIX(___NO_ERR);
> >>> }
> >>> c-declare-end
> >>>  )
> >>>
> >>>  (define alloc-My (c-lambda () My*/release-rc "___result_voidstar =
> ___EXT(___alloc_rc)( sizeof( struct My ));"))
> >>>  (define *->My (c-lambda (My*/nonnull) My "___result_voidstar =
> (My*)___arg1;"))
> >>>  (define My-r (c-lambda (My*/nonnull) unsigned-int8 "___result =
> ___arg1->r;"))
> >>>  (define My-r-set! (c-lambda (My*/nonnull unsigned-int8) void
> "___arg1->r = ___arg2;")))
> >>>
> >>> (define (evil)
> >>> (let ((xoxo (alloc-My)))
> >>>    (My-r-set! xoxo #xff)
> >>>    (let loop ((c 1))
> >>>        (begin
> >>>            (if (= 0 (remainder c 10000))
> >>>                (begin (display c) (newline)))
> >>>            (*->My xoxo)
> >>>            (loop (+ 1 c))))))
> >>>
> >>> (evil)
> >>>
> >>> ;;END CODE
> >>>
> >>>
> >>>
> >>>
> >>> _______________________________________________
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> >>
> >
>
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