用C语言写一个贪吃蛇游戏，只能使用标准库函数

好的，下面是一个简单的贪吃蛇游戏代码，仅供参考：

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <conio.h>
#include <windows.h>

#define BOARD_WIDTH  20
#define BOARD_HEIGHT 20

typedef struct _Point {
    int x;
    int y;
} Point;

typedef struct _Snake {
    Point head;
    Point tail;
    int size;
    int direction;
} Snake;

void display(char board[][BOARD_WIDTH + 1], Snake* snake) {
    system("cls");
    for (int i = 0; i < BOARD_HEIGHT; i++) {
        printf("%s\n", board[i]);
    }
    printf("Score: %d\n", snake->size - 1);
}

void init(char board[][BOARD_WIDTH + 1], Snake* snake) {
    for (int i = 0; i < BOARD_HEIGHT; i++) {
        for (int j = 0; j < BOARD_WIDTH; j++) {
            board[i][j] = ' ';
        }
        board[i][BOARD_WIDTH] = '\0';
    }
    snake->head.x = BOARD_WIDTH / 2;
    snake->head.y = BOARD_HEIGHT / 2;
    snake->tail.x = snake->head.x;
    snake->tail.y = snake->head.y;
    snake->size = 1;
    snake->direction = 0;
    board[snake->head.y][snake->head.x] = 'O';
}

Point generate_food(char board[][BOARD_WIDTH + 1]) {
    Point food;
    do {
        food.x = rand() % BOARD_WIDTH;
        food.y = rand() % BOARD_HEIGHT;
    } while (board[food.y][food.x] != ' ');
    board[food.y][food.x] = '@';
    return food;
}

int move(Snake* snake, char board[][BOARD_WIDTH + 1]) {
    Point new_head;
    switch (snake->direction) {
        case 0: new_head.x = snake->head.x + 1; new_head.y = snake->head.y; break;
        case 1: new_head.x = snake->head.x - 1; new_head.y = snake->head.y; break;
        case 2: new_head.x = snake->head.x; new_head.y = snake->head.y - 1; break;
        case 3: new_head.x = snake->head.x; new_head.y = snake->head.y + 1; break;
    }
    if (new_head.x < 0 || new_head.x >= BOARD_WIDTH || new_head.y < 0 || new_head.y >= BOARD_HEIGHT || board[new_head.y][new_head.x] == 'O') {
        return 0;
    }
    if (board[new_head.y][new_head.x] == '@') {
        snake->size++;
        board[new_head.y][new_head.x] = 'O';
        return 1;
    }
    board[snake->tail.y][snake->tail.x] = ' ';
    Point old_tail = snake->tail;
    snake->tail = *(Point*)(&board[snake->tail.y][snake->tail.x - 1]);
    board[snake->tail.y][snake->tail.x] = 'O';
    snake->head = new_head;
    *(Point*)(&board[snake->head.y][snake->head.x]) = snake->head;
    board[old_tail.y][old_tail.x] = ' ';
    return 1;
}

void play() {
    char board[BOARD_HEIGHT][BOARD_WIDTH + 1];
    Snake snake;
    init(board, &snake);
    Point food = generate_food(board);
    display(board, &snake);
    while (1) {
        if (_kbhit()) {
            char ch = _getch();
            if (ch == 'w' && snake.direction != 3) {
                snake.direction = 2;
            } else if (ch == 's' && snake.direction != 2) {
                snake.direction = 3;
            } else if (ch == 'a' && snake.direction != 0) {
                snake.direction = 1;
            } else if (ch == 'd' && snake.direction != 1) {
                snake.direction = 0;
            }
        }
        if (!move(&snake, board)) {
            printf("Game over!\n");
            break;
        }
        if (snake.size >= BOARD_WIDTH * BOARD_HEIGHT) {
            printf("You win!\n");
            break;
        }
        if (snake.size == food.x) {
            food = generate_food(board);
        }
        display(board, &snake);
        Sleep(100);
    }
}

int main() {
    srand((unsigned int)time(NULL));
    play();
    return 0;
}

在Windows平台下，可以使用`conio.h`库中的`_kbhit()`和`_getch()`函数来获取键盘输入。`Sleep()`函数可以用来让程序暂停一段时间，实现动画效果。

该代码中使用了一个字符数组`board`来表示游戏画面，其中空格表示空白区域，`O`表示蛇身，`@`表示食物。`Snake`结构体表示蛇，其中`head`和`tail`表示蛇头和蛇尾的坐标，`size`表示蛇的长度，`direction`表示蛇的移动方向。

`init()`函数用来初始化游戏，将蛇放在画面中央，初始化长度为1，方向向右。`generate_food()`函数用来随机生成食物，并返回其坐标。`move()`函数用来移动蛇，判断蛇是否碰到边界、自身或食物，如果碰到食物，则长度加1，生成新的食物。`play()`函数是游戏主循环，不断监听键盘输入并移动蛇，直到游戏结束。

注意，本代码仅供参考，可能存在不足之处，需要根据实际情况进行修改和完善。