在定义FILE * fp 之后，fopen的用法是: fp = fopen(filename,"w")。

而对于fopen_s来说，还得定义另外一个变量errno_t err

然后err = fopen_s(&fp,filename,"w")。

返回值的话，对于fopen来说，打开文件成功的话返回文件指针（赋值给fp），

打开失败则返回NULL值；对于fopen_s来说，打开文件成功返回0，失败返回非0。

在vs编程中，经常会有这样的警告：

warning C4996: 'fopen': This function or variable may be unsafe. Consider using fopen_s instead. To disable deprecation, use_CRT_SECURE_NO_WARNINGS. See online help for details.  是因为  fopen_s比fopen多了溢出检测，更安全一些。

（在以后的文章里还有get与get_s的比较,strcpy strcpy_s的比较，他们的共同点都是用来一些不可预料的行为，以后将进行详尽解释）

实例1：

//环境VS2012

#include "stdafx.h"
#include "stdlib.h"
#include "string.h"
int _tmain(int argc, _TCHAR* argv[])
{
 FILE* fp;
 int i;
 char ch,filename[100];
 char houzhui[5]={".txt"};
 printf_s("请输入所用的文件名:");
 gets_s(filename);
 strcat_s(filename,houzhui);
 printf_s("您输入的文件名是：%s\n",filename);

 if((i = fopen_s(&fp,filename,"w")) != 0)
 {
  printf_s("无法打开此文件\n");
  exit(0);
 }
 printf_s("请输入一个准备存储到磁盘的字符串（以&号结束）:\n");
 ch = getchar();
 while (ch!='&')
 {
  fputc(ch,fp);
  putchar(ch);
  ch=getchar();
 }

 fclose(fp);
 putchar(10);
 
 while(1);

 return 0;
}

实例2：

#include <stdio.h>

FILE *stream, *stream2;

int main( void )
{
   int numclosed;
   errno_t err;

   // Open for read (will fail if file "crt_fopen_s.c" does not exist)
   if( (err  = fopen_s( &stream, "crt_fopen_s.c", "r" )) !=0 )
      printf( "The file 'crt_fopen_s.c' was not opened\n" );
   else
      printf( "The file 'crt_fopen_s.c' was opened\n" );

   // Open for write 
   if( (err = fopen_s( &stream2, "data2", "w+" )) != 0 )
      printf( "The file 'data2' was not opened\n" );
   else
      printf( "The file 'data2' was opened\n" );

   // Close stream if it is not NULL 
   if( stream)
   {
      if ( fclose( stream ) )
      {
         printf( "The file 'crt_fopen_s.c' was not closed\n" );
      }
   }

   // All other files are closed:
   numclosed = _fcloseall( );
   printf( "Number of files closed by _fcloseall: %u\n", numclosed );
}