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2012-03-21T06:22:58Z

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= DNAStructs =

{{Warning/Working/Development| Needs to be merged with [[Dev:Source/Architecture/SDNA_Notes]] }}

The following excerpt functions as basis for DNA Struct documentation.

from <code>source/blender/makesdna/intern/dna_genfile.c</code>:

(<code>source/blender/blenloader/intern/genfile.c</code> in pre-blender2.5)

* please note: no builtin security to detect input of double structs
* if you want a struct not to be in DNA file: add two hash marks above it (#[enter]#[enter]) ''FIXME''

Structure DNA data is added to each blender file and to each executable, this to detect
in <code>.blend</code> files new variables in structs, changed array sizes, etc. It's also used for
converting endian and pointer size (32-64 bits)
As an extra, Python uses a call to detect run-time the contents of a blender struct.

Create a structDNA: only needed when one of the input include (<code>.h</code>) files change.<BR/>
File Syntax:
<pre>
SDNA (4 bytes) (magic number)
NAME (4 bytes)
[nr] (4 bytes) amount of names (int)
[string]
[string]
...
...
TYPE (4 bytes)
[nr] amount of types (int)
[string]
[string]
...
...
TLEN (4 bytes)
[len] (short) the lengths of types
[len]
...
...
STRC (4 bytes)
[nr] amount of structs (int)
[typenr][nr_of_elems] [typenr][namenr] [typenr][namenr] ...
</pre>

'''Remember to read/write integer and short aligned'''

While writing a file, the names of a struct is indicated with a type number, to be found with:
: <code>type= findstruct_nr(SDNA ''', char ''')</code>
The value of <code>type</code> corresponds with the the index within the structs array.

For the moment: the complete DNA file is included in a <code>.blend</code> file. For the future we can think of smarter methods, like only included the used structs. Only needed to keep a file short though...

'''ALLOWED AND TESTED CHANGES IN STRUCTS:'''
* Type change (a char to float will be divided by 255).
* Location within a struct (everthing can be randomly mixed up).
* Struct within struct (within struct etc), this is recursive.
* Adding new elements, will be default initialized zero.
* Removing elements.
* Change of array sizes.
* Change of a pointer type: When the name doesn't change the contents is copied.

'''NOT YET:'''
* array (<code>vec[3]</code>) to <code>float</code> struct (<code>vec3f</code>)

'''DONE:'''
* Endian compatibility
* Pointer conversion (32-64 bits)

'''IMPORTANT:'''
* Do not use <code>#define</code>s in structs for array lengths, this cannot be read by the dna functions.
* Do not use <code>#if 0 ... #endif</code> to remove elements from structs, this cannot be read by the dna functions.
* Other preprocessor directives may also be unsupported.
* Typedefs in struct declarations appear to be unsupported.
* Do not use <code>uint</code>, but <code>unsigned int</code> instead, <code>ushort</code> and <code>ulong</code> are allowed.
* Only use a <code>long</code> in Blender if you want this to be the size of a pointer. So it is 32 bits or 64 bits, dependant at the CPU architecture.
* <code>chars</code> are always unsigned.
* When storing pointers, always put the * next to the variable and not the type (i.e. void *x, not void* x). If you do, you will get a message that makesdna couldn't find whatever structure you did that in.
* Aligment of variables has to be done in such a way, that any system does not create 'padding' (gaps) in structures. So make sure that:
** short: 2 aligned
** int: 4 aligned
** float: 4 aligned
** double: 8 aligned
** long: 8 aligned
** struct: 8 aligned
* The <code>sdna</code> functions have several error prints builtin, always check Blender running from a console.

-- [[User:JesterKing|NathanLetwory]] - 21 May 2004

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