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

Adam adam.mlmb at gmail.com
Mon Dec 14 13:05:24 EST 2015


Marc,

Also, do you have any idea when the bug was introduced, do you think it was
after or before 4.7.0?

Thanks


2015-12-15 1:22 GMT+08:00 Adam <adam.mlmb at gmail.com>:

> Marc,
>
> Can you please give us a more exact definition of this bug - does it only
> apply when returning a pointered c-define-type type from inlined C code to
> Scheme, or does it affect inlined code returning a ___SCMOBJ too?
>
> Also if you could provide a patch (as this is the only severe Gambit bug
> that has been for like 2-3 years) would be great.
>
> Thanks!
>
>
> 2015-12-15 1:02 GMT+08:00 Marc Feeley <feeley at iro.umontreal.ca>:
>
>> 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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