Memory alignment

Memory alignment restricts the memory addresses at which an object may begin. An alignment of $A$ bytes means that its starting byte address $a$ satisfies $a\mathrm{mod}A=0$. For example, address $4096$ satisfies eight-byte memory alignment, whereas address $4100$ does not. Alignment is a constraint on location, rather than another name for an object's size.

In the C programming language, object types have alignment requirements; _Alignof queries a type's requirement and _Alignas can request a supported stronger requirement. The actual values depend on the implementation. The C11 draft distinguishes fundamental from implementation-supported extended alignments and requires valid alignment values to be powers of two.^[ref]

An application binary interface makes such choices concrete. Under the ordinary AAPCS64 layout, a C structure containing a char tag followed by a double value has the character at offset $0$, seven unused padding bytes, and the double at offset $8$. Its total size is $16$ bytes and its alignment is $8$ bytes. The application binary interface aligns an aggregate as strictly as its most strictly aligned member and rounds the aggregate size up to a multiple of that alignment; this also keeps each element properly aligned in an array of these structures.^[ref]

This example is an explicit platform convention, rather than a universal promise that every C double needs eight-byte memory alignment. Likewise, a processor's ability to execute an unaligned load does not by itself relax the C programming language's object-access requirements. Keeping the language rule, the application binary interface layout, and the machine instruction separate explains why a byte buffer cannot automatically be treated as a properly located object of any chosen type.