C语言 --free()释放内存是出错

Question

#include<stdio.h>
#include<string.h>
#include <stdlib.h> 
char* mymalloc();
int main()
{
	char* str = NULL;
	str = mymalloc();
	printf("%s", str);
	free(str);//这里释放内存出错
	//system("pause");
	return 0;
}

char* mymalloc()
{
	char* a = (char*)malloc(sizeof(char) * 20);
	char* b = "hello world";
	memset(a, '0', strlen(a));
	strcpy_s(a, 12, b);
	return a;
}

Answer 1

memset(a, '0', strlen(a));----这里用strlen(a)是不对的

memset(a, 0, sizeof(char) * 20);