Roommate, a king of time, I took care of the C language structure memory alignment

C Memory alignment rules for language structures ：

1. The first member is offset from the structure variable by 0 The address of .
2. Other member variables are aligned to a number （ Align numbers ） An integral multiple of the address of .
3. The total size of the structure is the maximum number of alignments （ Each member variable has an alignment number ） Integer multiple .
4. If the structure is nested , The nested structure is aligned to an integral multiple of its maximum alignment , The overall size of the structure is the maximum number of alignments （ The number of alignments with nested structures ） Integer multiple .

attach 1：

Align numbers = Compiler default alignment number And The smaller value of the member size .
VS The default value in is 8
Next, the landlord uses some examples to help you understand the structural alignment rules （ notes ： The building owner uses VS2013 compiler ）

attach 2： Common types are VS The amount of space occupied in the compiler

C Language standards ：
sizeof(long long)>＝sizeof(long)>＝sizeof(int)

Example 1：

1、 Because the first type is int type （ Occupy 4 Bytes ）, The compiler will default to an offset of 0 The address of .

2、 From the second structure member and other subsequent member variables to be aligned to a number （ Align numbers ） An integral multiple of the address of .double Type account 8 Bytes ,VS2013 The default number of alignments is 8, The smaller of the two is 8, So the alignment number is 8, The address to align to is 8 Integer multiple , So when you store it down , It can only be put in 8 The address of , skip 4、5、6、7 address , Because they're not 8 Integer multiple , Cannot be used to store , So we can only waste this space , Because the structure should be aligned .

3、 The third structure member variable is char type , The space occupied is 1, Follow VS The default number of alignments 8 Compare , smaller , Therefore, the default alignment number of the third structure member is 1, And the next address is 16,16 yes 1 Integer multiple , therefore char The type is put directly into 16 Location at address

4、 The space occupied by such a structure is 17 Bytes , But you think the memory space of this structure takes up 17 A byte ？ No , There are also three rules for structure alignment ： The total size of the structure is the maximum number of alignments （ Each member variable has an alignment number ） Integer multiple . The maximum number of alignments in this structure is double The corresponding 8, So , The final memory space occupied by the structure must be 8 Integer multiple , It will automatically complete .

5、 The end result is that this structure accounts for 24 Bytes

Example 2 ：

1、 According to the first rule of structure memory alignment , Will now char The type is offset by 0 The address of .

2、 According to our second rule and 4 A rule ： If the structure is nested , The nested structure is aligned to an integral multiple of its maximum alignment , The overall size of the structure is the maximum number of alignments （ The number of alignments with nested structures ） Integer multiple . The maximum number of alignments inside the structure is double Type corresponding to 8, So the structure should be from the address to 8 Start storing at an integer multiple of .

3、int The space occupied by type is 4,VS The compiler default alignment number is 8, The smaller value is 4, So the alignment number is 4,32 yes 4 Integer multiple , So from 32 Start storing... At the address int type .

4、 The size of the structure calculated at this time is 36, But the maximum number of alignments is student Inside the structure double Type corresponding to 8, The size of the structure must be an integral multiple of the maximum number of alignments , So we need to make up

5、 So the final result of the calculated structure size is 40！

attach 2：

But everything I can't explain , I can say C The language standard prescribes , That's it , No why? ,C The structure memory alignment rule in language is like this , No why? , Everything is regulated by the standard ！

remarks ：

The landlord is not talented , No joy, no spray. , If there are mistakes or areas that need to be improved , Thank you very much for pointing out , I will actively learn to adopt . Thank you for your support and encouragement , I will continue to work hard and create more high-quality articles to report back and forth to my family . Programming hobby xdm, If you have any questions about programming learning, you can discuss them with me （ I try my best to help ）, After all “ People gather firewood and the fire is high ”, Let's exchange and study together , Common progress ！