C language structure is initialized as a function parameter

First : Remember that C Uninitialized variables in language cannot be used , Can't be used as a function parameter , It can be used as a function parameter in the way of address passing .
Preface :
In general , We will extract the code that initializes the structure , Package to init() Function , The initialization is completed by taking the structure variable as the function parameter , But sometimes, for example , Structure variables as parameters , Use... In the initialization function malloc Dynamic memory allocation , Such behavior cannot complete the initialization of external variables . This may lead to the use of uninitialized functions .

 Through various experiments , First, we should divide the initialization process in the function into two categories :1. Directly assign values to structural member variables 2. Use malloc Dynamically allocate memory and assign values 

1. Directly assign values to structural member variables ：

(1).o Is the structure variable , Direct assignment

typedef struct node
{
    
	int h[99];
	int m;
}k;
void init(int a,k * p) {
    
	a = 9;
	p->m = a;
}
int main() {
    
	int  a = 8;
	k o  ;
	init(a,&o);
	//c In language , Uninitialized variables , Cannot be used as a parameter 
}

here , You can see ,o Is the structure variable , But it's the address , It is feasible to assign a value directly to the address , and a It is value transfer , this , To the initialization function , Its a What you get is just a value . No matter how to modify , Can not affect the outside a value .
So if o Pointer variables for structures ?

(2).o Pointer variables for structs , Direct assignment

typedef struct node
{
    
	int h[99];
	int m;
}k;
void init(int a,k * p) {
    
	a = 9;
	p->m = a;
}
int main() {
    
	int  a = 8;
	k* o  ;
	init(a,o);
	//c In language , Uninitialized variables , Cannot be used as a parameter 
}

After the modification , The result of the operation is o For uninitialized variables , Cannot be used . So in other words : As a pointer variable, you need to complete initialization first (malloc() or Structure variable assignment ) By using , As a general variable , There is no need to first malloc(), And then assign a value .

of malloc() When is the function used , What are the advantages and disadvantages , It still needs to be understood .

2. Use malloc Dynamically allocate memory and assign values :

(1) o Is the structure variable , Assign value after allocating memory :

#include<stdlib.h> // Dynamic distribution function and random function 
typedef struct node
{
    
	int h[99];
	int m;
}k;
void init(int a,k * p) {
    
	p=(k*)malloc(sizeof(k));
	a = 9;
	p->m = a;
}
int main() {
    
	int  a = 8;
	k o  ;
	init(a,&o);
	//c In language , Uninitialized variables , Cannot be used as a parameter 
}

You can find , Its init() After the function ,o Nothing has changed , This is because in the init() Within the function , Original p The pointer stores variables o The address of , After dynamically allocating memory ,p The pointer points to the new ( Variable ) Memory address , This is the same as o Lost contact .

(1) o Pointer variables for structs , Assign value after allocating memory :

#include<stdlib.h> // Dynamic distribution function and random function 
typedef struct node
{
    
	int h[99];
	int m;
}k;
void init(int a,k ** p) {
    
	*p=(k*)malloc(sizeof(k));
	a = 9;
	(*p)->m = a;
	
}
int main() {
    
	int  a = 8;
	k* o  ;
	init(a,&o);
	//c In language , Uninitialized variables , Cannot be used as a parameter 
}

Because uninitialized pointers cannot be used directly as parameters , So we use the secondary pointer to receive , The address of the pointer variable to complete the initialization process .
Fundamentally , A pointer is a variable whose value is a memory address ( Or data objects ), So the secondary pointer is easy to understand .
In terms of the results , The initialization process was successful .
We use secondary pointer p To receive variable pointers o The address of , And then use *p Get its value, that is, the variable pointer o, At this time, we will directly pointer to variables o Dynamic memory allocation . That completes the initialization .

summary :

1.c In language , Whether it's a normal variable , Or pointer variables , It cannot be used directly without initialization , But it can be used by sending addresses .
In the use of malloc() On initialization , Only secondary pointers can be used to pass
Without using malloc() when , It is also divided into value passing and reference, that is, address passing .
2. There are two ways to initialize uninitialized variable functions :
1. For structural variables , Adopt direct assignment
2. Pointer variables to structures , Take secondary pointer parameters , Then dynamically allocate space , assignment
3.malloc() Function usage :
It dynamically allocates memory space , You need to forcibly convert the specified type during allocation , Then return to the first memory address , It is generally received with a pointer of the relevant data type .

Reference documents :
Reference structure