Re: [Pharo-dev] [Vm-dev] FFI Struct Argument Pass By Value Fails on Mac 64 bit

Nov. 24, 2017

      On 22 November 2017 at 21:59, Ben Coman <btc@openinworld.com> wrote:
...
On 22 November 2017 at 13:38, Todd Blanchard <tblanchard@mac.com> wrote:
...
I've been trying to track this down for a couple weeks now.
I have concluded that structs passed by value to functions on the 64 bit
VM are not properly populated.  The struct's memory is all zero'd.
I found this while trying to work with LibClang and found that functions
that fetched code locations from code ranges always returned invalid zero'd
locations.  After spending some time with lldb I have traced the problem
into the native code and found that the argument is not correct.
I've carved out the wee bit of clang to reproduce this in a tiny library.
The gist of it is below and the entire file is included.  Basically the
struct passed to the function clang_getRangeStart is zero'd memory
regardless of the data I send from the image side.
The build command I used on sierra is clang -shared -undefined
dynamic_lookup -o microclang.dylib microclang.c
On Ubuntu 16.04 I used...
$ clang -shared -fPIC -o libmicroclang.so microclang.c
$ clang test.c -L. -l microclang
   test.c:6:53: error: no member named 'begin_int_data' in
'CXSourceLocation'
   if(clang_getRangeStart(clang_getArbitraryRange()).begin_int_data == 0)
I presume you meant...
    if(clang_getRangeStart(clang_getArbitraryRange()).int_data == 0)
so correcting and continuing...
$ clang test.c -L. -l microclang
$ LD_LIBRARY_PATH=. ./a.out
That failed
So I'm not sure how to proceed.
I was expecting that would work while Pharo failed.
Now interestingly...
$ clang test.c microlang.c
$ ./a.out
That worked
So it seems a similar problem exists outside our FFI.
cheers -ben
P.S. I refactored you code to extract a header file (attached)
The issue is still beyond my ken, but I've made some progress towards
isolating/understanding the issue.
Attached zip exploded here for easy reference...

___microlang.h___
typedef unsigned  uintptr_t;

typedef struct {
  const void *ptr_data[2];
} CXSourceRange_;

CXSourceRange_ clang_getArbitraryRange_();
int clang_getRangeEnd_(CXSourceRange_ range);

___microclang.c___
#include "microclang.h"
const char* libraryString = "library_pointer";

CXSourceRange_ clang_getArbitraryRange_()
{       CXSourceRange_ range = {0};
        range.ptr_data[0] = (void*)libraryString;
        return range;
}

int clang_getRangeEnd_(CXSourceRange_ range)
{       // Special decoding for CXSourceLocations for CXLoadedDiagnostics.
        if ((uintptr_t)range.ptr_data[0] & 0x1)
        {       return 0;       }
        else
        {       return 1;       }
}

___test.c___
#include <stdio.h>
#include "microclang.h"
const char* localString =  "local_pointer";

void test( CXSourceRange_ range, char *note )
{       int result = clang_getRangeEnd_(range);
        if(result == 0)
        {       printf("That failed (%s)\n", note); }
        else
        {       printf("That worked (%s)\n", note); }
}

int main()
{       CXSourceRange_ range1 = clang_getArbitraryRange_();
        test(range1, "library string");

        CXSourceRange_ range2 = {0};
        range2.ptr_data[0] = (void*)localString;
        test(range2, "local string");
}

___Makefile___
default: clean static shared

clean:
        rm -f *so *App
        @echo

shared:
        clang -g -o libmicroclang.so -shared -fPIC microclang.c
        clang -g -o sharedApp test.c -L. -lmicroclang
        LD_LIBRARY_PATH=. ./sharedApp
        @echo

static:
        clang -g -o staticApp test.c microclang.c
        ./staticApp
        @echo

Now running...
$ make > report

gives...
___report___
rm -f *so *App

clang -g -o staticApp test.c microclang.c
./staticApp
That worked (library string)
That worked (local string)

clang -g -o libmicroclang.so -shared -fPIC microclang.c
clang -g -o sharedApp test.c -L. -lmicroclang
LD_LIBRARY_PATH=. ./sharedApp
That failed (library string)
That worked (local string)

cheers -ben