用c语言写一个贪吃蛇游戏”的说明如下：（参照演示程序和源程序框架）启动程序时，输入的用户名如果以前没有，则系统自动完成注册，如果有，则进行密码比较以判断是否允许登录。用户名将用于英雄榜中。

好的，这是一个比较复杂的问题。首先，你需要了解贪吃蛇游戏的规则和逻辑，然后你需要学习如何使用C语言进行编程。

以下是一个简单的贪吃蛇游戏的源程序框架，可以帮助你开始编写你自己的贪吃蛇游戏：

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

#define MAX_X 20
#define MAX_Y 20

enum direction {UP, DOWN, LEFT, RIGHT};

struct snake {
  int x;
  int y;
  struct snake *next;
};

struct snake *head, *tail;
int food_x, food_y;
enum direction dir = RIGHT;
int score = 0;
char username[20];

void init_game();
void init_snake();
void init_food();
void display();
void update();
void game_over();
void save_score();
int check_collision(int x, int y);
void register_user();
void login();

int main() {
  int choice;
  printf("Welcome to Snake Game!\n");
  printf("1. Register\n");
  printf("2. Login\n");
  printf("Enter your choice: ");
  scanf("%d", &choice);
  if (choice == 1) {
    register_user();
  } else if (choice == 2) {
    login();
  } else {
    printf("Invalid choice!\n");
    exit(0);
  }
  init_game();
  while (1) {
    display();
    update();
    Sleep(100);
  }
  return 0;
}

void init_game() {
  srand(time(NULL));
  init_snake();
  init_food();
}

void init_snake() {
  head = (struct snake*) malloc(sizeof(struct snake));
  tail = (struct snake*) malloc(sizeof(struct snake));
  head->x = 3;
  head->y = 1;
  head->next = tail;
  tail->x = 1;
  tail->y = 1;
  tail->next = NULL;
}

void init_food() {
  food_x = rand() % MAX_X + 1;
  food_y = rand() % MAX_Y + 1;
}

void display() {
  system("cls");
  printf("Score: %d\n", score);
  for (int i = 0; i < MAX_X; i++) {
    for (int j = 0; j < MAX_Y; j++) {
      if (i == head->x && j == head->y) {
        printf("O");
      } else if (i == food_x && j == food_y) {
        printf("*");
      } else {
        int found = 0;
        struct snake *ptr = head->next;
        while (ptr != tail) {
          if (ptr->x == i && ptr->y == j) {
            printf("o");
            found = 1;
            break;
          }
          ptr = ptr->next;
        }
        if (!found) {
          printf(" ");
        }
      }
    }
    printf("\n");
  }
}

void update() {
  int new_x = head->x, new_y = head->y;
  switch (dir) {
    case UP:
      new_x--;
      break;
    case DOWN:
      new_x++;
      break;
    case LEFT:
      new_y--;
      break;
    case RIGHT:
      new_y++;
      break;
  }
  if (check_collision(new_x, new_y)) {
    game_over();
    return;
  }
  struct snake *new_head = (struct snake*) malloc(sizeof(struct snake));
  new_head->x = new_x;
  new_head->y = new_y;
  new_head->next = head;
  head = new_head;
  if (new_x == food_x && new_y == food_y) {
    score++;
    init_food();
  } else {
    struct snake *ptr = head;
    while (ptr->next != tail) {
      ptr = ptr->next;
    }
    struct snake *old_tail = tail;
    tail = ptr;
    free(old_tail);
  }
}

void game_over() {
  printf("Game over! Your score is %d.\n", score);
  save_score();
  exit(0);
}

void save_score() {
  FILE *fp;
  fp = fopen("high_scores.txt", "a");
  fprintf(fp, "%s %d\n", username, score);
  fclose(fp);
}

int check_collision(int x, int y) {
  if (x < 0 || x >= MAX_X || y < 0 || y >= MAX_Y) {
    return 1;
  }
  struct snake *ptr = head->next;
  while (ptr != tail) {
    if (ptr->x == x && ptr->y == y) {
      return 1;
    }
    ptr = ptr->next;
  }
  return 0;
}

void register_user() {
  char password[20], confirm[20];
  FILE *fp;
  fp = fopen("users.txt", "a+");
  printf("Enter your username: ");
  scanf("%s", username);
  while (!feof(fp)) {
    char name[20], pass[20];
    fscanf(fp, "%s %s", name, pass);
    if (strcmp(username, name) == 0) {
      printf("Username already exists!\n");
      fclose(fp);
      exit(0);
    }
  }
  printf("Enter your password: ");
  scanf("%s", password);
  printf("Confirm your password: ");
  scanf("%s", confirm);
  if (strcmp(password, confirm) != 0) {
    printf("Passwords do not match!\n");
    fclose(fp);
    exit(0);
  }
  fprintf(fp, "%s %s\n", username, password);
  fclose(fp);
}

void login() {
  char password[20];
  FILE *fp;
  fp = fopen("users.txt", "r");
  printf("Enter your username: ");
  scanf("%s", username);
  while (!feof(fp)) {
    char name[20], pass[20];
    fscanf(fp, "%s %s", name, pass);
    if (strcmp(username, name) == 0) {
      printf("Enter your password: ");
      scanf("%s", password);
      if (strcmp(password, pass) != 0) {
        printf("Incorrect password!\n");
        fclose(fp);
        exit(0);
      }
      fclose(fp);
      return;
    }
  }
  printf("Username does not exist!\n");
  fclose(fp);
  exit(0);
}

这个程序包含了很多功能，包括注册、登录、保存分数、检查碰撞等等。你可以按照程序框架，编写自己的贪吃蛇游戏程序。