如何根据要处理的文件更改void *的大小

Question

I am currently trying to make a program that can read a .blend file. Well trying is the important part, since I am already stuck on reading the file block info.

Im gonna quickly explain my problem, please refer this page for context

So in the .blend header there is a char that determines wheter or not the pointer size, later used in the file info block (Or just fileBlock on the linked webpage) among other things, is 4 or 8 bytes long. From what I have read, in c++ the void pointer only changes size according to the target platform it was compiled for ( 8 bytes for 64 bit and 4 bytes for 32 bits ). However .blend files can have either one, regardless of the platform I presume.

Now since blender itself does also read its own files using c, there must be a way to change the pointer to match the required pointer size, according to the info in the header. However my best guess would be to dynamically allocate a void pointer array to either one or two pointers, which then makes actually using the data even more complicated.

Please help me find the intended way of handling the different pointer sizes!

Answer 1

Yup, that's painful. The solution is not to treat them as C++ at all. Instead, create your own class BlendPointer to abstract this away. Those would be read from a BlendFile , and that BlendFile would store whether its BlendPointer s are 4 or 8 bytes on disk.

Answer 2

Go back to the top of the wiki page and you will find the File Header structure. The header of a blend file starts with "BLENDER" which is followed by the pointer size for the file -

Size of a pointer
All pointers in the file are stored in this format
'_' (underscore) means 4 bytes or 32 bit
'-' (minus) means 8 bytes or 64 bits.

So by reading the eighth byte of the file you know the size of the pointers in the file.

if (file_bytes[7] == "_")
    ptr_size = 4;
else if (file_bytes[7] == "-")
    ptr_size = 8;

The copy of blender creating the file determines the sizes used for the file, so a 32bit build will save 32bit pointers in the file while a 64 bit build will save 64bit pointers.

You should also read the next byte, it tells you whether the file was saved as big or little endian, to see if you need to do any byte swapping. The use of blender on big endian machines might be getting smaller, but you may still come across big endian files.

Another important thing that doesn't seem to be mentioned, is that blend files can be compressed and often are. Reading a compressed blend file will mean using gzread() to read the file. A compressed file has the first two bytes set to 0x1f 0x8b

You will find the code that blender uses to read blend files in source/blender/blenloader .