[Openmcl-devel] Troubleshooting FFI calls on ARM

[Ron Garret]

Try:

? (macroexpand '(#_getpid))
(%FF-CALL (%REFERENCE-EXTERNAL-ENTRY-POINT (LOAD-TIME-VALUE (EXTERNAL "getpid"))) :SIGNED-FULLWORD)

and then dig into that.

On Jun 21, 2019, at 9:09 AM, Jeremy Shoemaker <jeremy at codingkoi.com> wrote:

> Okay, I thought I had tried this already, but it definitely works with "printf" using both methods. I was using "getpid" because it's the first example in the documentation. I had already tried the underlying EXTERNAL-CALL approach with "getpid", it segfaults both ways.
> 
> I went back and tried the other examples there. "getpid" and "log2" both segfault, but the example with "write" works. I was thinking maybe there might be some connection between which library they're part of, but the first and last are both posix functions and "log2" is from libc.
> 
> Any ideas on how I can go about figuring out why some of them work and the others don't?
> 
> Thanks,
> Jeremy
> 
> On Fri, Jun 21, 2019 at 10:03 AM lsxvdqe <lsxvdqe at gmail.com> wrote:
> Do other functions (like printf) segfault as well?
> What happens if you call some foreign function with EXTERNAL-CALL?
> #_getpid comes from the interface database, so try to FFI directly.
> You will have to do this anyway, if you want to make some bindings.
> 
> On Thu, Jun 20, 2019 at 09:31:33AM -0500, Jeremy Shoemaker wrote:
> > I decided this morning to make sure everything on the device is sane, at
> > least as far as this particular function is concerned. Writing a C program
> > that just calls getpid and prints the result works fine with no segfaults.
> > It seems like it's definitely something on the CCL side of things.
> > 
> > Jeremy
> > 
> > On Wed, Jun 19, 2019 at 9:36 PM Jeremy Shoemaker <jeremy at codingkoi.com>
> > wrote:
> > 
> > > 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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