malloc is guaranteed to return memory that's suitably aligned for storing any object that has a fundamental alignment and that could fit in the amount of memory that's allocated. A fundamental alignment is an alignment that's less than or equal to the largest alignment that's supported by the implementation without an alignment specification. (In Visual C++, this is the alignment that's required for a double, or 8 bytes. In code that targets 64-bit platforms, it’s 16 bytes.) For example, a four-byte allocation would be aligned on a boundary that supports any four-byte or smaller object.

Visual C++ permits types that have extended alignment, which are also known as over-aligned types. For example, the SSE types __m128 and __m256, and types that are declared by using __declspec(align( n )) where n is greater than 8, have extended alignment. Memory alignment on a boundary that's suitable for an object that requires extended alignment is not guaranteed by malloc. To allocate memory for over-aligned types, use _aligned_malloc and related functions.

Stack Usage
align
__declspec