38. 内存分区代码分析

1. 返回栈区地址

#include <stdio.h>
int *fun()
{
    int a = 10;
    return &a;
}
int main(int argc, char *argv[])
{
    int *p = NULL;
    p = fun();
    *p = 100; // 操作野指针指向的内存，err
    
    return 0;
}

2. 返回data区地址

#include <stdio.h>

int *fun()
{
	static int a = 10;
    
	return &a; // 函数调用完毕,a不释放
}

int main(int argc, char *argv[])
{
    int *p = NULL;
    p = func();
    *p = 100; // ok
    printf("*p = %d\n", *p);
    
    return 0;
}

3. 值传递1

#include <stdio.h>
#include <stdlib.h>

void fun(int *tmp)
{
	tmp = (int *)malloc(sizeof(int));
	*tmp = 100;
}
int main(int argc, char *argc[])
{
	int *p = NULL;
	fun(p); // 值传递，形参修改不会影响实参
	printf("*p = %d\n", *p);
	
	return 0;
}

4. 值传递2

#include <stdio.h>
#include <stdlib.h>

void fun(int *tmp)
{
	*tmp = 100;
}

int main(int argc, char *argv[])
{
	int *p = NULL;
	p = (int *)malloc(sizeof(int));

	fun(p); //值传递
	printf("*p = %d\n", *p); //ok，*p为100

	return 0;
}

5. 返回堆区地址

#include <stdio.h>
#include <stdlib.h>

int *fun()
{
	int *tmp = NULL;
	tmp = (int *)malloc(sizeof(int));
	*tmp = 100;
	return tmp; // 返回堆区地址，函数调用完毕，不释放
}

int main(int argc, char *argv[])
{
	int *p = NULL;
	p = fun();
    printf("*p = %d\n", *p);//ok

    //堆区空间，使用完毕，手动释放
    if( p != NULL)
    {
        free(p);
        p = NULL;
    }
    
    return 0;
}