What does __attribute__ ((aligned(x))) do?

Question

NONE of the existing answer to that question made anything clearer for me. I don't know what it's supposed to mean when a variable is declared "within the boundary of x bytes", which is the answer given in every question regarding this. I need an actual explanation with example of what it is, and what it does compared to variables declared without that attribute, and why it should be used.

Bonus question: When used in front of an array like here __attribute__ ((aligned (32))) double output[4]; , does that mean every single element of the array is aligned, or just the array as a whole?

Thanks in advance.

Answer 1

There is no such thing as __attribute__ in the C++ language. __attribute__ language extension of GCC and compatible compilers.

__attribute__ ((aligned (X))) is essentially same as the standard alignas specifier:

// non-standard
__attribute__ ((aligned (32))) double output[4];

// standard
alignas(32) double output[4];

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Every object has a memory address that designates the location of the byte in memory where the object is stored. The underlying representation of the address is an integer number. A memory address is aligned to X bytes when the number of the address is divisible by X.

Every type has a minimum requirement for the address where the object can be stored. alignas can be used to increase the requirement.