为什么总是提示未声明标识符？我用的VS。。

#include "stdafx.h"
#include <stdio.h>
#include<stdlib.h>
#include <iostream>
#define BUFF_SIZE 256
void display_usage (void);
int line;
int main(int argc, char *argv[ ])
{
char buffer[BUFF_SIZE];
FILE *fp;
if (argc < 2)
{
display_usage();
return 1;
}
if (fopen_s(&fp, argv[1], "rt"))
{
fprintf(stderr, "Error opening file, %s", argv[1]);
return (1);
}
line = (1);
while (fgets(buffer, BUFF_SIZE, fp) != NULL)
fprintf(stdout, "%4d:\t%s", line++, buffer);
fclose(fp);
return 0;
}
void display_usage(void)
{
fprintf(stderr, "\nProper Usage is:");
fprintf(stderr, "\n\nlist_it filename.ext\n");
}