如何检查malloc地址是否与QWord对齐？

Question

I want to test create NVMe I/O Submission Queue Invalid Queue Address Offset

I use C posix_memalign to alloc a memory range

But how to check that the address i get is QWord aligned?

I know if i want the address is word aligned then address end is 00h and double word aligned address end is 000h

So is qword aligned address end is 0000h?

Can i check by below?

if( address & 0xF != 0 )
    printf("Not qword aligned");

Thanks

Answer 1

A simple way is to test the low bits with a bitwise and. There isn't a perfectly-portable way of doing that with no implementation-defined behavior, but that doesn't matter because you're targeting one specific architecture. So, you can test that p is aligned on a 16-byte boundary, on any C compiler for the x86, with (uintptr_t)(void*)p & 0x0f == 0 .

However, this is an XY problem: posix_memalign() is defined to do what you want. Indeed, malloc() is guaranteed to return memory that meets the strictest alignment requirements of any type, and you can check alignof(max_align_t) .

In this case, you might want to use aligned_alloc() , which exists in the standard library to allocate memory with a stricter alignment requirement, and has a simpler interface, or calloc() , which allows you to specify the element size and also zeroes out the memory.