[gnuastro-devel] [task #14572] Protect gal_data_t's array and block pointers

[Mohammad Akhlaghi]

Follow-up Comment #4, task #14572 (project gnuastro):

Thank you for the extra explanation Antonio. Unfortunately I am still a little
confused. I completely agree with defining `gal_data_t' as an opaque structure
and only access it through functions. I just don't understand the significance
of the "subtile" (or "subarray") concept. Is it a new concept, or just a
re-naming of the existing ones? Let me explain:

Currently, the larger entity is called a "Block" (because it is contiguous in
memory) and the "sub"-entity is called a "tile", as the ASCII diagram in the
book
<https://www.gnu.org/software/gnuastro/manual/html_node/Tessellation-library.html>
shows.

For example, let's assume we have the following two `gal_data_t's: `block' and
`tile'. The first is a contiguous patch of memory and `block->block=NULL'.
This shows that `block->array' is the start of this contiguous patch of
memory. However, `tile->block=block' and `tile->array' points to somewhere
within `block's allocated space.

The main advantage (purpose) of this system is that no extra allocation is
necessary: two tiles may overlap, they may be read/written on separately
(e.g., in parallel), and so on. Many Gnuastro programs currently rely on this
feature to work in parallel. It is really up to the library programmer to be
sure that they aren't writing to a tile before reading it in another tile that
(might) overlap with the first one. 

In this context, from what I understood, the "subtile" you mention are very
similar to the already defined "block" (separately allocated patches of
memory), am I correct?

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