C language character pointer and string initialization

Before , When I was working on the storage structure of the graph , Use adjacency matrix to represent a graph , Encountered a segment error problem ：

Just input one edge above , There will be an error , And segment error , It is often the memory address access error , For example, the array is out of bounds , Accessed unassigned address , Access the address assigned by the system , They all make mistakes , So I located the error at the input side

That's what I did before the loop , Defines two character pointers , Ready to store strings , But just one scanf after , As long as one of these variables is accessed, a segment error will occur , This address cannot be accessed ？

So I did an experiment , Do the following ：

Go straight to the code ：

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>



void test(char* str)
{
	printf("str=%s\n", str);
	if (strcmp("love", str) == 0) {
		printf(" ha-ha , String equality \n");
	}
	else 
	{
		printf(" Strings are not equal \n");
	}
}

int main()
{
	char* v1;
	scanf("%s", &v1);
	test(v1);
	system("pause");
	return 0;
}

    Then memory address access error will occur

    Then I changed it to the following code ：

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>



void test(char* str)
{
	printf("str=%s\n", str);
	if (strcmp("love", str) == 0) {
		printf(" ha-ha , String equality \n");
	}
	else 
	{
		printf(" Strings are not equal \n");
	}
}

int main()
{
	
	test("love");

	system("pause");
	return 0;
}

Running normally ：

        

        

let me put it another way , The problem is ：

         

  in other words , Out-of-service sacnf To initialize the char*, Let's analyze ：

  

So that's to say ,char* p The address of the constant string is stored , Then we can't use scanf To initialize the  , Because if you use scanf To initialize the , We don't have a spatial point at all in the constant region . And even if there is a space pointing to the constant region , You can't use scanf To initialize , Such as the following ：

        

Because the data read / write of the constant area is not allowed to be changed once it is defined  

So if we have to use scanf To initialize a string , The first thing we should consider here is an array of characters , The code is as follows ：

        

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>



void test(char* str)
{
	printf("str=%s\n", str);
	if (strcmp("love", str) == 0) {
		printf(" ha-ha , String equality \n");
	}
	else 
	{
		printf(" Strings are not equal \n");
	}
}

int main()
{
	char str[10] = "pxx";//OK, And with '\0'
	scanf("%s", str);
	test(str);//char* Points to an array space 
	system("pause");
	return 0;
}

  Running results ：

        

about test Come on , Equivalent to using char* str Yes char str[10] The array space of , This is the time char*  str There's a point , And can be changed , We can use scanf To assign values directly .

        The code changes as follows ：

                 

        Running results ：