#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define WIDTH 70
#define HEIGHT 40
#define BUFSIZE 1000
#define COM_OTHER 0
#define COM_QUIT 1
#define HISTORY_SIZE 100

char canvas[WIDTH][HEIGHT];
char *history[HISTORY_SIZE];

void print_canvas(FILE *fp)
{
  int x, y;

  fprintf(fp, "----------\n");

  for (y = 0; y < HEIGHT; y++) {
    for (x = 0; x < WIDTH; x++) {
      char c = canvas[x][y];
      fputc(c, fp);
    }
    fputc('\n', fp);
  }
  fflush(fp);
}

int init_canvas()
{
  memset(canvas, ' ', sizeof(canvas));
}

void draw_line(int x0, int y0, int x1, int y1)
{
  int i;
  for (i = 0; i <= 10; i++) {
    int x = x0 + 0.1 * i * (x1 - x0);
    int y = y0 + 0.1 * i * (y1 - y0);
    canvas[x][y] = '#';
  }
}

void line_command()
{
  int x0, y0, x1, y1;
  x0 = atoi(strtok(NULL, " "));
  y0 = atoi(strtok(NULL, " "));
  x1 = atoi(strtok(NULL, " "));
  y1 = atoi(strtok(NULL, " "));

  draw_line(x0, y0, x1, y1);
}

int interpret_command(const char *command)
{
  char buf[BUFSIZE];
  strcpy(buf, command);

  char *s = strtok(buf, " ");
  char c = tolower(s[0]);

  switch (c) {
  case 'l':
    line_command();
    break;
  case 'q':
    return COM_QUIT;
  default:
    break;
  }
  return COM_OTHER;
}

int main()
{
  int com, n, i;
  const char *canvas_file = "canvas.txt";
  FILE *fp;
  char buf[BUFSIZE];

  if ((fp = fopen(canvas_file, "w")) == NULL) {
    fprintf(stderr, "error: cannot open %s.\n", canvas_file);
    return 1;
  }

  init_canvas();
  print_canvas(fp);

  n = 0;
  while (1) {
    printf("%d > ", n);
    fgets(buf, BUFSIZE, stdin);

    history[n] = (char*)malloc(sizeof(char) * strlen(buf));
    strcpy(history[n], buf);
    if (++n >= HISTORY_SIZE) break;
			       
    com = interpret_command(buf);
    if (com == COM_QUIT) break;
    print_canvas(fp);
  }

  fclose(fp);

  if ((fp = fopen("history.txt", "w")) != NULL) {
    for (i = 0; i < n; i++) 
      fprintf(fp, "%s", history[i]);
    fclose(fp);
  }
}

error: cannot open canvas.txt.