[Openmcl-devel] Troubleshooting FFI calls on ARM

[Jeremy Shoemaker]

Okay, that was my thinking though I couldn't remember exactly how to get
function pointers in C. After some fun setting up a cross-compiler, I got
the test C program on the device, and then wrote the following Lisp code to
try to duplicate the code.

-----------------------------------------------------
(defun peek-getpid-memory ()
  (let* ((addr (%ptr-to-int (foreign-symbol-address "getpid"))))
    (loop for offset from 0 to 15
       do
         (format t "~X " (%get-unsigned-long (%int-to-ptr (+ addr (* 4
offset))))))))
(peek-getpid-memory)
-----------------------------------------------------

I hope I did that right. I'm not terribly familiar with CCL's FFI yet.
Given that I got the exact same output, I think it means I did.

-----------------------------------------------------
remarkable: ~/ ./test
ee1d2f70 e52d7004 e5123454 e3530000 da000002 e1a00003 e49d7004 e12fff1e
1a000003 e5121458 e3510000 11a03001 1afffff7 e3a07014 ef000000 e3530000
remarkable: ~/ ccl/armcl -l test.lisp
EE1D2F70 E52D7004 E5123454 E3530000 DA000002 E1A00003 E49D7004 E12FFF1E
1A000003 E5121458 E3510000 11A03001 1AFFFFF7 E3A07014 EF000000 E3530000
-----------------------------------------------------

That seems to indicate it's pointing at the same place. Assuming that the
address coming back from (foriegn-symbol-address "getpid") is the same
thing that is invoked by the reader macro in (#_getpid), then it's
confusing that I'm getting a segfault.

Any other ideas?

Jeremy

On Wed, Jun 19, 2019 at 4:26 PM Ron Garret <ron at flownet.com> wrote:

>
> On Jun 19, 2019, at 2:04 PM, Jeremy Shoemaker <jeremy at codingkoi.com>
> wrote:
>
> > One suggestion: use %get-signed-long to read out a few bytes of memory
> at the location reported by (external "getpid”) and then write a little C
> program that does the same thing and see if you get the same result.
>
> I'm not sure I entirely follow, at least the C program part. Are you
> suggesting peeking at the memory pointed to by the `getpid` function in C?
>
>
> Yes.  Exactly.
>
> I'm not quite sure how to do that, but my C is pretty rusty.
>
>
> #include <unistd.h>
> #include <stdio.h>
>
> int main() {
>   unsigned int *p = (unsigned int *)getpid;
>   for (int i=0; i<16; i++) printf("%x ", *(p+i));
> }
>
>
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