Saturday, May 5, 2012

return a matrix - imitate pthread_create()

As explained in other blog posts, a language-neutral dll (dynamic link library) should not receive or return an instance of C++ matrix type, since another language may not know its memory layout. So how does a function return a matrix (or 2 matrices?)

I feel we can take a page from pthread_create(), where the parameter representing the new thread is a pre-allocated but empty buffer. The pthread_create() function-call populates that buffer.

Well, some people don't like pre-allocation. They want to return a pointer. Where is the pointee physically?
- stack? pointee will be bulldozed before use
- global area like a global variable? next call to the same function will overwrite it. Concurrent call to the function will lead to "overwrite each other"
- heap? who will free the memory? Has to be caller. Caller would need to copy the data somewhere if it is not in a position to "free it after use"

In the pre-allocate scenario, the caller would set up an empty matrix in the format of the library, pass its address to the function, and later read the populated matrix. Matrix can be on heap, stack or global.

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http://www.linkedin.com/in/tanbin