Understand the structure in C language in one article

1、 What is a structure

The structure is C Language An important data type in , This data type consists of a group called members （ Or domain , Or element ） Different data components , Each of these members can have a different type . Structures are usually used to represent data of different types but related . The keyword is struct.

2、 Declaration of a structure

We use keywords struct Declare structure , The specific format is as follows ：

struct book
{
    
	char title[100];
	char author[100];
	char date[100];
}

We have defined three attributes of a book through the above statement , Namely titleauthordate, And the format is char type （ Character ）

Be careful ： The declaration can also be incomplete , For example, omit the structure label .

3、 Reference to structure

After defining the structure variable, you can reference it in the program , But the reference of structural variables is different from that of general variables , Because there are many different types of variables in structural variables , Therefore, structural variables cannot be referenced as a whole , Only one member can be referenced .

1、 You can only quote his internal members separately .

// Structure variable name . Member name 
book.title = " The romance of The Three Kingdoms ";
book.author = " Luo Guanzhong ";
book.date = "2006";

2、 If the member in the structure type is also a structure type , You need to use several ".", Go down level by level . Because only the lowest level members can be operated . In fact, a structure is a kind of encapsulation of data , When the structure member is also a structure , Structure members can be released completely .

Before packaging ：

struct age
{
    
	int year;
	int month;
	int day;
}

struct student
{
    
	char name[20];
	int num;
	struct age birthday;
	float score;
}

After encapsulation ：

struct student
{
    
	char name[20];
	int num;
	int year;
	int month;
	int day;
	float score;
}

3、 You can reference Structural variable Members of the Address , You can also quote Structural variable The address of . Such as &student1.num and &student1, Former mark student1.num The first address of this member in memory , The latter represents structural variables student1 First address in memory .

stay C In language , The first address of the structure variable is the first address of the first member of the structure . therefore &student1 It is equivalent to the first member name The first address , and name Is an array , The array name represents the first address of the array . But here's the thing , They are equivalent to They represent the address of the same memory space , But their types are different .

4、 Structure action

1、 Structs are the same as other types of underlying data types , for example int type 、char type , It's just that the structure can be made into the data type you want , For future use .
2、 Structure pointer is used as function entry parameter , Improve the scalability of the program .

5、 The size of the structure is aligned with the memory

The size of the structure is not simply the sum of the structure elements , Because our mainstream computers use 32bit Word length CPU, For this type of CPU take 4 The number of bytes is more efficient than taking one byte , And more convenient .

So the first address of each member in the structure is 4 An integer multiple of , When fetching data elements, it will be relatively more efficient , This is the origin of memory alignment .

Compilers on each specific platform have their own default “ Alignment factor ”( Also called alignment modulus ). Programmers can use precompile commands #pragma pack(n),n=1,2,4,8,16 To change this coefficient , Among them n That's what you're going to specify “ Alignment factor ”.

6、 Structure alignment rules

1、 Data member alignment rules ： structure (struct)( Or in association with (union)) Data members of , The first data member is placed in offset by 0 The place of , In the future, the alignment of each data member is in accordance with #pragma pack The specified value and the length of the data member itself , The smaller one is going to .

2、 structure ( Or in association with ) The overall alignment rule of ： After the data members are aligned , structure ( Or in association with ) Alignment itself , The alignment will follow #pragma pack Specified values and structures ( Or in association with ) In the maximum data member length , The smaller one is going to .

3、 combination 1、2 It can be inferred that ： When #pragma pack Of n When the value equals or exceeds the length of all data members , This n The size of the value will have no effect .

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