quirc image memory page alignment

[graindcafe]

This PR make quirc use mmap instead of calloc to allocate memory of quirc->image.
The use case I have for this is to be able to use mmap() on the application side with the flag MAP_FIXED and memory address given by quirc. The image I capture is accessible in a dmabuf, I access it by using mmap() with the dmabuf’s fd. With quirc using calloc(), my call to mmap() fails because of non-aligned memory page.
Instead of calloc/free, this PR proposes to use mmap/munmap.
I believe this does not alter the behaviour of quirc.

[dlbeer]

This is a good idea. However:

• mmap() returns MAP_FAILED on error, not NULL

• Windows is a supported platform for quirc, so you'll need to either use something platform-independent or introduce a pair of portable wrappers that use mmap/munmap on Unix and whatever's appropriate on Windows.

Another alternative may be to continue using malloc, but to over-allocate by PAGE_SIZE, so that you guarantee yourself room to shift to a page-aligned address within the allocated block.

[graindcafe]

Thanks for the review, gonna fix this.

Do you think it's okay to revert to previous calloc/free for windows with some preprocessor checks? Not sure if it's relevant to have page alignment for Windows, but in such a case maybe I should use _aligned_malloc.

[dlbeer]

On Mon, Dec 27, 2021 at 02:50:54AM -0800, Quentin GODRON wrote: Thanks for the review, gonna fix this. Do you think it's okay to revert to previous calloc/free for windows with some preprocessor checks?

Have you considered just over-allocating with malloc? That way you don't need to depend on anything platform-specific. You'd probably need to track an extra pointer though, or have a macro to skip from the pointer returned by malloc to the start of the page-aligned image area. Cheers, Daniel

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