Matlab’s internal memory representation (2012)

http://undocumentedmatlab.com/blog/matlabs-internal-memory-representation

by PascLeRasc • 11 years ago

72 9 11 years ago

chrisBob• 11 years ago

I recently learned about copy on write the hard way: We had a buggy dll from a hardware vendor that was editing our input, so the obvious solution was to keep a copy of the data we sent it. To our surprise, even the copy we made changed unless we did it in an assignment like A = 1.0*X;

On a related note the tech support at MATLAB is very responsive. Every time I have had an issue they get back to me quickly, and I feel like the follow up and really fix bugs.

dr_zoidberg• 11 years ago

This reminds me of the "copy list" expression in Python:

    In [1]: a = [1, 2, 3]
    In [2]: b = a
    In [3]: a is b
    Out[3]: True
    In [4]: b = a[:]
    In [5]: a is b
    Out[5]: False

Usually this kind of "optimization" is done to avoid copying large ammounts of data in memory, but as you said, unless it's widesepread knowledge or explicitly stated, it can lead to some confusing situations/bugs.

maxerickson• 11 years ago

That's just how Python works though, not an optimization. Python has names for objects more than it has variables, = binds a name to an object.

animefan• 11 years ago

In almost all cases knowledge of copy-on-write doesn't have to be widespread because it is equivalent to value semantics.

It is only because they were using a buggy dll (which for non-matlab users, is custom native code that matlab calls against using a FFI), that the expected semantics of matlab were violated.

imurray3• 11 years ago

A different (probably slightly cheaper) hack to get a safe copy of an array is:

    A = X; A(1) = A(1);

Then one can evilly edit A in-place in a mex file without accidentally editing other arrays like X. However, the docs tell us never to edit arrays from input arguments in place, so a future version of Matlab could be cleverer and still break things. I wish their mex API would provide a supported way to edit things in-place. Native Matlab code does allow inplace editing:

   A = fun(A);

If function fun is defined that way too, A can be altered in-place by native matlab code, but not (officially) by mex functions.

nimrody2• 11 years ago

What Matlab function edits its inputs in-place? I cannot think of any.

Actually Matlab's pass by value is one of its best properties (as a language). This is how functions work in math and is much easier to reason about.

I really dislike Julia's pass by reference semantics. Even if it enables better performance.

Matlab’s internal memory representation (2012)

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