宏未声明,但在标题中定义

这是map.h：

#define MAP_WIDTH   256
#define MAP_HEIGHT  256

typedef struct {
    char exit_n;
    char exit_s;
    char exit_w;
    char exit_e;
} room;

room map[MAP_WIDTH][MAP_HEIGHT];

void generate_map();

这个map.c：

#include "map.h"

void generate_map()
{
    char room_x,room_y;

    room_x = MAX_WIDTH/2;
    room_y = MAX_HEIGHT/2;

    // first room
    map[room_x][room_y].exit_n = 1;
}

所以,没什么异国情调.问题是编译器抱怨两个定义的常量MAX_WIDTH和MAX_HEIGHT：

map.c: In function ‘generate_map’:
map.c:18: error: ‘MAX_WIDTH’ undeclared (first use in this function)
map.c:18: error: (Each undeclared identifier is reported only once
map.c:18: error: for each function it appears in.)
map.c:19: error: ‘MAX_HEIGHT’ undeclared (first use in this function)

我究竟做错了什么？