core.stdc.stdlib.malloc & alignment

[Hiemlick Hiemlicker via Digitalmars-d-learn]

On Wednesday, 29 June 2016 at 02:24:55 UTC, captaindet wrote:
> is there an alignment guarantee for core.stdc.stdlib.malloc?
>
> more specifically, using DMD and compiling for 32bit on 
> windows, can i assume proper alignment for int or uint 
> variables?
>
> background: i like to re-use a (ubyte) buffer, sometimes it 
> will store only bytes, sometimes it shall store uints.
>
> thanks

Yes, the C standard requires malloc to be aligned to the platform 
size(4 for 32bit, 8 for 64-bit).

Alternatively, you can use code like

		size_t capacity = capacity;
		size_t size = T.sizeof*capacity;
		enforce(size > 0, "Cannot allocate fixed array of size 0.");
		if (mem is null)
			mem = cast(T*)malloc(size+Alignment);
		else
			mem = cast(T*)realloc(mem, size+Alignment);

		enforce(cast(int)mem > 0, "Could not malloc memory block.");
		
		// Adjust memory pointer to be aligned
		size_t tmp = (cast(size_t)mem) & (Alignment - 1);
		size_t ofs = (Alignment - tmp) & (Alignment - 1);
		mem = cast(T*)((cast(size_t)mem) + ofs);
		
		// Add extra space to be additional end capacity.
		capacity = cast(size_t)((size + Alignment - ofs)/T.sizeof);

which malloc's a block of memory and aligns it.