Working By Example

Now that we've learned how to use pointers.py, lets build some small scripts to try out some of its features.

Making one equal two

Note: This may not work depending on your build of CPython.

Lets start out with creating a pointer to 1, and then moving 2 to it:

from pointers import _

ptr = _&1
ptr <<= 2

assert 1 == 2

Running this will work just fine, and no AssertionError will be raised.

But how do we revert our changes now? 1 has been overwritten, so we can't just move a 1 back into the pointer.

If you want, you can take a second to think about how to do it.

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We can cache our 1 by using memory allocation. Since the 1 will be copied to its own memory space, it won't get affected by overwriting 1.

You can try this out yourself.

from pointers import malloc, _

one = malloc(28)
one <<= 1

ptr = _&1
ptr <<= 2

print(1, ~one)

Running this will output 2 1!

Ok, lets allocate a 1 before we overwrite it:

from pointers import malloc, free, _

cache = malloc(28)
cache <<= 1

ptr = _&1

Then, lets move the allocated 1 back into our pointer at the end of the program:

ptr = _&1
ptr <<= 2

assert 1 == 2

ptr <<= ~cache
assert 1 != 2

Don't forget to free the memory as well:

free(cache)

Here's the final result:

from pointers import malloc, free, _

cache = malloc(28)
cache <<= 1

ptr = _&1
ptr <<= 2

assert 1 == 2

ptr <<= ~cache
assert 1 != 2
free(cache)