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Just make a fast allocator that uses heap instead of the stack. You only need to malloc once and it can be used for any type since like you pointed out, it's effective type can be changed.
Sometimes, especially in embedded systems, it is useful to have a bunch of statically allocated heaps. You can see them in a memory map, and the linker will tell you if they don't fit in memory.
There is also the case where you have some raw data from a file or network, that you want to re-interpret as a struct. That is always dangerous with endianness and struct padding, but it is a very common practice. You could always memcpy from a char array to a struct, but that can waste memory.
> Yes, alloca. (Don't use it.)
> Just make a fast allocator that uses heap instead of the stack. You only need to malloc once and it can be used for any type since like you pointed out, it's effective type can be changed.