关于c程序中Access violation错误问题

#include
#include

#define MAX 10

void trans(int d, char b[])
{
char st[MAX];
char ch;
int i = 0;
int top = -1;
while (d != 0)
{
ch ='0'+d % 2;
top++;
st[top] = ch;

    d / 2;
}

while (top != -1)
{
    b[i] = st[top]; 
    top--;

    i++;
}
b[i] = '\0';

}

void main()
{
int n;
char str[MAX];
//scanf("%d", &n);
do
{
scanf("%d", &n);
} while (n < 0);

trans(n, str);
printf("对应的二进制数:%s\n", str);

system("pause");

}

这是10转2进制的程序，为什么出现Access violation的错误啊，还有ch ='0'+d % 2;这语句中为什么要加一个'0'啊，求大神解释？