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

Adam adam.mlmb at gmail.com
Mon Dec 14 12:22:07 EST 2015


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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