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

Paolo Carlesi pcarlesi at gmail.com
Tue Dec 15 14:15:23 EST 2015


"My" is the struct I've been showing in my other email, defined inside a C-declare and wrapped by c-define-type. 
From gambit's point of viewI it is a scheme-object like everything else, I suppose.

Here is the code. For whatever reason "new-My" doesn't trigger the bug in my simple use case, even if it returns an object.
I was wrong about a detail: that bug is also present in linux. Doesn't crash, but corrupts the contents of the structure (!).

;; START

(c-declare #<<c-declare-end
typedef struct My {
      int r;
} My;

static ___SCMOBJ ___release_My( void* ptr ) {
    ___EXT(___release_rc)( ptr );
    return ___FIX(___NO_ERR);
}

// workaround to c-lambda bug (gambit mailing list 2015-12-14
My indirect_star_to_My(My *ptr) {
        return *ptr;
}
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")))

(define alloc-My (c-lambda () My*/release-rc "___result_voidstar = ___EXT(___alloc_rc)( sizeof( struct My ));"))
(define *->My (c-lambda (My*/nonnull) My "indirect_star_to_My"))
(define My-r (c-lambda (My*/nonnull) int "___result = ___arg1->r;"))
(define My-r-set! (c-lambda (My*/nonnull int) void "___arg1->r = ___arg2;"))
(define My->* (c-lambda (My) My*/nonnull "___result_voidstar = &___arg1;"))

;; this one returns My, as a foreign object (struct), not as a foreign object (pointer)
;; doesn't segfault, at least in my use case. why?
(define new-My (c-lambda () My "___result_voidstar = ___EXT(___alloc_rc)(sizeof(struct My));"))

(define (evil)
   (let* ((xoxo (new-My))
          (xoxop (My->* xoxo)))
      (My-r-set! xoxop #xff)
      (let loop ((c 1))
          (begin
              (if (= 0 (remainder c 1000000))
                  (begin (display c)
			             (display "-")
			             (display (My-r xoxop))
			             (newline)))
              (loop (+ 1 c))))))
(evil)

;; END






Il giorno 15/dic/2015, alle ore 19:25, Adam <adam.mlmb at gmail.com> ha scritto:

> Paolo, wait, what is your definition of your My type here - "scheme-object"?
> 
> 
> 2015-12-15 3:46 GMT+08:00 Paolo Carlesi <pcarlesi at gmail.com>:
> 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
> >>
> >>
> >>
> >>
> >> _______________________________________________
> >> Gambit-list mailing list
> >> Gambit-list at iro.umontreal.ca
> >> https://webmail.iro.umontreal.ca/mailman/listinfo/gambit-list
> >
> 
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