'fopen': This function or variable may be unsafe

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_terr，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：warningC4996: 'fopen': This function or variable may be unsafe. Consider using fopen_sinstead. To disable deprecation, use_CRT_SECURE_NO_WARNINGS. See online helpfor 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,strcpystrcpy_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 <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' wasnot opened\n" );
   else
      printf( "The file 'data2' wasopened\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 closedby _fcloseall: %u\n", numclosed );
}