fopen和fopen_s用法的比较  

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）

#include

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 );
}

fscanf和fscanf_s用法的比较 

fscanf 和 fscanf_s

      fscanf用法：fscanf(fp,"%d",&var)

      fscanf_s用法：fscanf(fp,"%d",&var,sizeof(int))

      区别：fscanf_s需要指定长度

 fscanf函数：

fscanf(fp,"%s",temp_str);和fscanf(fp,"%lf",&min_snr);

fscanf就是从文件中读取数据，保存到第三个参数开始的变量里，fp是一个FILE类型的指针。fscanf(fp,"%s",temp_str);  // 就是从文件指针fp里面读取一个字符串，保存到temp_str里面，跟scanf差不多，只是scanf是从键盘输入，fscanf是从文件里读取fscanf(fp,"%lf",&min_snr);