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Currently four different levels of routines for .blend file reading exist;

/* interface level */
1) BIF_init() -> calls 3
2) BIF_read_file() -> calls 11, optional 4
3) BIF_read_homefile() -> calls 11 or 12, and then 4
4) init_userdef_file()

/* kernel level */
11) BKE_read_file() -> calls 21, and then 14
12) BKE_read_file_from_memory() -> calls 22, and then 14
13) BKE_read_file_from_memfile() -> calls 23, and then 14
14) setup_app_data()

/* loader module level */
21) BLO_read_from_file() -> calls 24
22) BLO_read_from_memory() -> calls 24
23) BLO_read_from_memfile() -> calls 24

/* loader module, internal */
24) blo_read_file_internal()

Note: - BIF_read_homefile() has additional UI initialize calls, like windows fullscreen and executing commandline options

- Reading from memory (12) only happens for the compiled-in .B.blend

- The "memfile" here is a name I gave to the undo "file" structure. Which is constructed out of memory chunks with basic compression features.

- the kernel function setup_app_data() sets globals like "current screen" and "current scene".

So far, so good. The levels as mentioned here clearly distinguish UI from kernel, and should enable for example game loading (runtime) or background (no UI) loading. In the past years however, 'bad level' dependencies were added, and especially the patches for 'file versions' were added in too many places. The latter is evidently a result of the problem that the "UserDef" struct cannot be initialized/patched if there's not a need for a UI.

Here's how the flow goes in four different cases:


Starting up Blender, in foreground with UI --------------------

- creator/creator.c, main() -> calls 1 - If the commandline contains a filename, it calls 11


Starting up Blender, in background without UI --------------------

- creator/creator.c, main() -> calls 11 if the commandline has a filename

Note: no Userdef is read, nor initialized. Please note that this was already an existing problem for using Yafray, not setting proper file paths in background mode. The Yafray paths don't belong in the User menu.


Starting up Blender as a runtime executable --------------------

This only has calls to 22


Loading a file from within the UI (with F1, CTRL+O, using pulldowns) -----

Only calls allowed to 2. It detects if a UserDef has been read too, and in that case the init_userdef_file() will be executed.

Hope this is understandable :)