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

Todd Blanchard tblanchard at mac.com
Wed Nov 22 01:23:08 EST 2017


Playground code:

LibCClang startFromRange:(LibCClang defaultRange)

it worked if the resulting struct is not all zeroes.

A tiny C program is the equivalent.

int main()
{
  if(clang_getRangeStart(clang_getArbitraryRange()).begin_int_data == 0) printf("That failed\n"); 
  else printf("That worked\n")
  return 0;
}


> On Nov 21, 2017, at 11:18 PM, Ben Coman <btc at openinworld.com> wrote:
> 
> Hi Todd,
> 
> Could you advise the the Playground code to use to test this?
> And maybe a C program that links to the microclang shared library that does the equivalent test?
> 
> cheers -ben
> 
> On 22 November 2017 at 13:38, Todd Blanchard <tblanchard at mac.com <mailto:tblanchard at 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
> 
> I'm stuck.  The FFI64 plugin is way beyond my comprehension.
> 
> // microclang.c
> 
> typedef struct {
>   const void *ptr_data[2];
>   unsigned int_data;
> } CXSourceLocation;
> 
> /**
>  * \brief Identifies a half-open character range in the source code.
>  *
>  * Use clang_getRangeStart() and clang_getRangeEnd() to retrieve the
>  * starting and end locations from a source range, respectively.
>  */
> typedef struct {
>   const void *ptr_data[2];
>   unsigned begin_int_data;
>   unsigned end_int_data;
> } CXSourceRange;
> 
> const char* first = "first_pointer";
> const char* second = "second_pointer";
> 
> // return a fake range with non zero data
> CXSourceRange clang_getArbitraryRange()
> {
>   CXSourceRange range = {0};
>   range.ptr_data[0] = (void*)first;
>   range.ptr_data[1] = (void*)second;
>   range.begin_int_data = 17;
>   range.end_int_data = 24;
>   return range;
> }
> 
> // Actual clang function - range is always zero'd here despite having values in the image
> CXSourceLocation clang_getRangeStart(CXSourceRange range) {
>   // Special decoding for CXSourceLocations for CXLoadedDiagnostics.
>   if ((uintptr_t)range.ptr_data[0] & 0x1) {
>     CXSourceLocation Result = { { range.ptr_data[0], nullptr }, 0 };
>     return Result;    
>   }
>   
>   CXSourceLocation Result = { { range.ptr_data[0], range.ptr_data[1] },
>     range.begin_int_data };
>   return Result;
> }
> 
> 
> 
> 
> 

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