新手问一段代码

#include "stdafx.h"
#include <string.h>
#include <stdlib.h>
#include <iostream>

using namespace std;
int main(int argc, char* argv[])
{
    char a[]  = "abc";
    int n = strcmp(a,"abc");
    cout<<n<<endl;
    strcpy(a,"def");
    cout<<a<<endl;

    char* b = (char*)malloc(strlen(a)+strlen("abc")+1);
    strcpy(b,a);
    strcat(b,"abc");
    cout<<b<<endl;
    return 0;
}


请问为什么将里面的char a[] = "abc"换成char* a = "abc"，运行时会发生内存不能为written的错误？