I've been struggling with accessing C structs from ruby. The problem is: I expose a struct from my framework (definition below). However when accessing/allocating it from ruby it seems the offsets are off by 4 bytes (as if macruby think it's supposed to be a double)
Definition:
#ifndef __SCRIPTINGBRIDGE__
union _vec3_t {
float f[3];
struct { float x; float y; float z; };
struct { float r; float g; float b; };
struct { vec2_t xy; float andY; };
};
typedef union _vec3_t vec3_t;
#else
typedef struct _vec3_t { float x; float y; float z; } vec3_t; // Bridgesupport
freaks out when given the above union so I show it a struct with the same layout
#endif
Example output:
irb(main):001:0> framework "./GLMath.framework"
=> true
irb(main):002:0> v1 = vec3_create(1,2,3)
=> #<vec3_t x=1.0 y=3.0 z=NaN>
irb(main):003:0> v2 = v1.class.new(4,5,6)
=> #<vec3_t x=4.0 y=5.0 z=6.0>
irb(main):004:0> printVec3(v1)
0x676f1310 - 0x676f1314 - 0x676f1318
Vec3: [1.00, 0.00, 3.00]
=> nil
irb(main):005:0> printVec3(v2)
0x676f1310 - 0x676f1314 - 0x676f1318
Vec3: [4.00, 0.00, 5.00]
=> nil
in the above example, vec3_create & printVec3 are C functions, shared using the
scripting bridge. So when the struct is allocated from the c side, ruby only
sees the first & last items, and the last item is on the offset of the second.
when I then pass it to printVec3, it gets the first and last items only (last
item taken from the place of the second item, => offsets are double what they
should be). then when I allocate it using FFI in ruby, it's correcly read from
the ruby side, but when I pass it to a c function, it gets the first item in
the correct place, then the second item in the place of the last.
The type definitions in my bridge support file are:
<struct name='vec3_t'
type='{_vec3_t="x"f"y"f"z"f}'/>
<function name='vec3_create' inline='true'>
<arg type='f'/>
<arg type='f'/>
<arg type='f'/>
<retval type='{_vec3_t=fff}'/>
</function>
<function name='printVec3'>
<arg type='{_vec3_t=fff}'/>
</function>
Right now I'm working around this by wrapping everything in an object on the
objective-c side, but that is very slow and bloated.
Has anyone ever gotten structs to work properly or know what is causing this?
Note: if I return a pointer to the struct, and then assign to it's address
using a boxed struct created from ruby, that works fine it seems. It's only if
I return a struct directly.
(However, myPointer[0].x = foo; would not work. I'd have to do
a=myPointer[0];a.x=foo;myPointer[0]=a)
– Fjölnir
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