Problems with getelementptr

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1

Hello!

I'm having trouble with pointer traversing. I have a design as follows: class -> map -> classFunctions

Starting with a pointer to the class, I want to get a pointer to a classFunction via a pointer to the map.

I can't get that function pointer!

How shall I think to get the traversing right (see code below)? Is it something with the fact that I am using only pointers in my structs?

Best regards
Anders

; My user defined function
int %puts_kernel(sbyte* %string)
{
        %tmp.0 = call int (sbyte*, ...)* %printf( sbyte* %string )
        ret int 0

}

;the map and the class. The class is called "Kernel".
"myKernelMap" = type {int (sbyte*)*}
"Kernel" = type {"myKernelMap"*}

;Allocating...OK
"myKernelMapInstance" = malloc "myKernelMap"
%myKernel = malloc "Kernel"

;Assigning the map to the class...OK
"myKernelMapPTR" = getelementptr "Kernel"* %myKernel, long 0, ubyte 0
store "myKernelMap"* "myKernelMapInstance", "myKernelMap"** "myKernelMapPTR"

; Try to get pointer to the functionPointer in the map, NOT OK! :frowning:
%puts_kernelPTR = getelementptr "Kernel"* %myKernel, long 0, ubyte 0, long 0, ubyte 0
store int (sbyte*)* %puts_kernel, int (sbyte*)** %puts_kernelPTR

;Want to call the function with a string
%tmp.11 = load int (sbyte*)** %puts_kernelPTR
%result = call int %tmp.11(sbyte* %string)

2

Hello!

I'm having trouble with pointer traversing. I have a design as follows:
class -> map -> classFunctions

Starting with a pointer to the class, I want to get a pointer to a
classFunction via a pointer to the map.

Okay...

I can't get that function pointer!

How shall I think to get the traversing right (see code below)? Is it
something with the fact that I am using only pointers in my structs?

Best regards
Anders

; My user defined function
declare int %puts_kernel(sbyte* %string)

;the map and the class. The class is called "Kernel".
"myKernelMap" = type {int (sbyte*)*}

Okay, this declares a structure that has a single pointer-to-function
element in it.

"Kernel" = type {"myKernelMap"*}

This is a single element structure with a pointer to the map.

;Allocating...OK
"myKernelMapInstance" = malloc "myKernelMap"
%myKernel = malloc "Kernel"

Okay, these are of type %myKernelMap and %Kernel, respectively.

;Assigning the map to the class...OK
"myKernelMapPTR" = getelementptr "Kernel"* %myKernel, long 0, ubyte 0

This gives you a pointer to the '%myKernelMap*' in the Kernel type.

store "myKernelMap"* "myKernelMapInstance", "myKernelMap"** "myKernelMapPTR"

And this stores the pointer as you would expect.

; Try to get pointer to the functionPointer in the map, NOT OK! :frowning:
%puts_kernelPTR = getelementptr "Kernel"* %myKernel, long 0, ubyte 0, long 0, ubyte 0

Okay, here you're getting into trouble. myKernel is of type Kernel, which
doesn't contain a pointer to a function. In your case, you need to
actually emit a load to get to the structure in question, something like
this:

%t1 = getelementptr "Kernel"* %myKernel, long 0, ubyte 0
%t2 = load "myKernelMap"**
%puts_kernelPTR = getelementptr "myKernelMap"*, long 0, ubyte 0

store int (sbyte*)* %puts_kernel, int (sbyte*)** %puts_kernelPTR

This should now work.

;Want to call the function with a string
%tmp.11 = load int (sbyte*)** %puts_kernelPTR
%result = call int %tmp.11(sbyte* %string)

This should also work.

-Chris

3

Thanks for yet another patient and professional answer! :slight_smile:

Actually, I solved it some time after I posted the message, just the way you
suggested. Good to know that it is your recommendation as well.

Thanks again!

Anders