Q:

Linux Function Interception for OpenCL

I'm fairly new to C so be gentle.
I want to use the library interception method for Linux to replace calls to the OpenCL library with my own library. I understand that this can be done using LD_PRELOAD. So I can just re-implement the OpenCL functions as defined in the OpenCL header file within my own library which can then be linked against. 
The problem is that this OpenCL header also contains some extern struct definitions, e.g.
typedef struct _cl_mem * cl_mem;
which are not defined within the OpenCL header. Is it possible these structs are defined within the OpenCL shared lib? If not, where might they be defined?
Cheers
Chris

A:

That typedef declares a type pointing to a struct, the contents of which are undeclared. This means code using it can't do things like checking its size, copying the struct, or inspecting its contents - it simply has no idea what size it is.
This is a traditional technique in C to create an opaque, or private, type. You can declare the struct inside your OpenCL library, and the official header puts no restrictions on what that struct contains. It could even be empty, if all you need is an ID you can store in the pointer itself, though this is rarely done. 
An example of the same technique used in the standard C library is the FILE type. It might be as simple as an integer file descriptor, or as complex as a struct containing the entire filesystem state; standard C code won't know. The particulars are known to the library only.
In short, you can declare that struct however you like - as long as you implement every function that handles that struct. The program that links to your library never handles the struct, only pointers to it.

