C language address assignment problem

#include "stdio.h"
#include "string.h"
void GetMemory1(char* p)
{
    
    
	p = (char*)malloc(100);  //指针变量存放于栈中，被自动回收
}
void GetMemorySol1(char** p)//运用二级指针（类似于间址寻址）
{
    
    
	*p = (char*)malloc(100);
}
char* GetMemorySol2(char* p)
{
    
    
	p = (char*)malloc(100);
	return p;
}
char* GetMemory2()
{
    
    //char p[]存放于栈空间，函数执行完后被释放，p的地址不再指向p
	char p[] = "hello world"; 
	return p;
}
char* GetMemory3()
{
    
    
	char* p = "hello world";
	return p;
}
char* GetMemory4()
{
    
    
	static char p[] = "hello world";
	return p;
}


void test1(void)
{
    
    
	char* str = NULL;
	GetMemory1(str);
	strcpy(str, "hello world");
	printf(str);
}
void test1Sol(void)
{
    
    
	char* str = NULL;
	GetMemory1(&str);
	strcpy(str, "hello world");
	printf(str);
}
void test2(void)
{
    
    
	char* str = NULL;
	str = GetMemory2();
	printf(str);
}
void test3() {
    
    
	char* str = NULL;
	str = GetMemory3();
	printf(str);
}
void test4()
{
    
    
	char* str = NULL;
	str = GetMemory4();
	printf(str);
}
void test5(void)
{
    
    
	char* str = (char*)malloc(100);
	strcpy(str, "hello");
	free(str);
	//str = NULL;
	if (str != NULL)
	{
    
    
		strcpy(str, "world");
		printf(str);
	}
}
void test6()
{
    
    
	char* str = (char*)malloc(100);
	strcpy(str, "hello");
	str += 6;
	free(str);
	if (str != NULL)
	{
    
    
		strcpy(str, "world");
		printf(str);
	}
}
int main()
{
    
    
	test6();
	return 0;
}