A set of C interview questions about memory alignment. Many people make mistakes!

This is a set of interview questions from a friend's company , It's interesting to share .

subject ： Judge the following Location1 and Location2 What is the size of each structure ？

public struct Location1
    {
        public int X;
        public int Y;
        public long Z;
    }

    public struct Location2
    {
        public int X;
        public long Y;
        public int Z;
    }

According to feedback 90% People say it's the same size , After all, from the code point of view, it was only done once Y and Z Sequential exchange , Is that really the case ？ It can be used windbg Just debug .

The complete code is as follows ：

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
            Location1 location1;
            Location2 location2;

            location1.X = 10;
            location1.Y = 11;
            location1.Z = 12;

            location2.X = 10;
            location2.Y = 11;
            location2.Z = 12;

            Debugger.Break();
        }
    }

    public struct Location1
    {
        public int X;
        public int Y;
        public long Z;
    }

    public struct Location2
    {
        public int X;
        public long Z;
        public int Y;
    }
}

windbg The output is as follows ：

0:000> !clrstack -a
OS Thread Id: 0x1750 (0)
        Child SP               IP Call Site
00000000007fef68 00007ff9a84b9ad2 [HelperMethodFrame: 00000000007fef68] System.Diagnostics.Debugger.BreakInternal()
00000000007ff050 00007ff989c0f5ee System.Diagnostics.Debugger.Break()

00000000007ff0a0 00007ff92db5090b ConsoleApp2.Program.Main(System.String[]) [D:\net5\ConsoleApp4\ConsoleApp2\Program.cs @ 26]
    PARAMETERS:
        args (0x00000000007ff110) = 0x0000000003492cf0
    LOCALS:
        0x00000000007ff0e0 = 0x0000000b0000000a
        0x00000000007ff0c8 = 0x000000000000000a

00000000007ff2f8 00007ff98d086913 [GCFrame: 00000000007ff2f8] 
0:000> dp 0x00000000007ff0c8
00000000`007ff0c8  00000000`0000000a 00000000`0000000c
00000000`007ff0d8  00000000`0000000b 0000000b`0000000a
00000000`007ff0e8  00000000`0000000c 00000000`007ff1f8

because Stack It grows from big to small , So use dp When ordered , location2 It's in line location1 In front of , You can see it clearly

1. location2: 000000000000000a 000000000000000c 000000000000000b It's obvious size=3*8=24byte.

2. location1: 0000000b0000000a 000000000000000c its size=2*8=16byte

Then why is it bad 8byte Well ？ If you are familiar with C/C++ My friends should know at this time , In fact, that is Memory alignment , Why does it show up Memory alignment ？ We know , Memory is based on byte Addressable , That is, one address stores one byte, but cpu It's not like that , Its number of reads at a time is based on Address bus To the , At present, we cpu Basic it is 64 Root data bus , That is, it can be read at one time 8 individual byte.

In order to be able to make cpu Read more efficiently , The compiler will do it properly padding operation , The purpose is to Press 8 alignment , If not aligned ,cpu There may be incomplete reading , This leads to the situation that it must be read at least twice , It will definitely affect cpu Efficient , Another reason is ： Some machines must be aligned to access , Otherwise, it will be abnormal , So the compiler for better platform portability , Only alignment ！