alloc

    fn alloc(
        _: *anyopaque,
        len: usize,
        alignment: Alignment,
        return_address: usize,
    ) ?[*]u8 {
        _ = return_address;
        assert(len > 0);
        switch (allocStrat(alignment)) {
            .raw => {
                // `std.c.max_align_t` isn't the whole story, because if `len` is smaller than
                // every C type with alignment `max_align_t`, the allocation can be less-aligned.
                // The implementation need only guarantee that any type of length `len` would be
                // suitably aligned.
                //
                // For instance, if `len == 8` and `alignment == .@"16"`, then `malloc` may not
                // fulfil this request, because there is necessarily no C type with 8-byte size
                // but 16-byte alignment.
                //
                // In theory, the resulting rule here would be target-specific, but in practice,
                // the smallest type with an alignment of `max_align_t` has the same size (it's
                // usually `c_longdouble`), so we can just extend the allocation size up to the
                // alignment of `max_align_t` if necessary.
                const actual_len = @max(len, @alignOf(std.c.max_align_t));
                const ptr = c.malloc(actual_len) orelse return null;
                assert(alignment.check(@intFromPtr(ptr)));
                return @ptrCast(ptr);
            },
            .posix_memalign => {
                // The posix_memalign only accepts alignment values that are a
                // multiple of the pointer size
                const effective_alignment = @max(alignment.toByteUnits(), @sizeOf(usize));
                var aligned_ptr: ?*anyopaque = undefined;
                if (c.posix_memalign(&aligned_ptr, effective_alignment, len) != 0) {
                    return null;
                }
                assert(alignment.check(@intFromPtr(aligned_ptr)));
                return @ptrCast(aligned_ptr);
            },
            .manual_align => {
                // Overallocate to account for alignment padding and store the original pointer
                // returned by `malloc` before the aligned address.
                const padded_len = len + @sizeOf(usize) + alignment.toByteUnits() - 1;
                const unaligned_ptr: [*]u8 = @ptrCast(c.malloc(padded_len) orelse return null);
                const unaligned_addr = @intFromPtr(unaligned_ptr);
                const aligned_addr = alignment.forward(unaligned_addr + @sizeOf(usize));
                const aligned_ptr = unaligned_ptr + (aligned_addr - unaligned_addr);
                manualAlignHeader(aligned_ptr).* = unaligned_ptr;
                return aligned_ptr;
            },
        }
    }