dup与dup2函数

彻底理解dup(...)和dup2(...)这两个函数的作用

dup函数的作用：复制一个现有的句柄，产生一个与“源句柄特性”完全一样的新句柄（也即生成一个新的句柄号，并关联到同一个设备）

dup2函数的作用：复制一个现有的句柄到另一个句柄上，目标句柄的特性与“源句柄特性”完全一样（也即首先关闭目标句柄，与设备断连，接着从源句柄完全拷贝复制到目标句柄）
dup和dup2都是系统服务，window平台对应DuplicateHandle函数
/* DUP.C: This program uses the variable old to save
 * the original stdout. It then opens a new file named
 * new and forces stdout to refer to it. Finally, it
 * restores stdout to its original state.
 */

＃i nclude <io.h>
＃i nclude <stdlib.h>
＃i nclude <stdio.h>

void main( void )
{
   int old;
   FILE *new;

   old = _dup( 1 );   /* "old" now refers to "stdout" */
                      /* Note:  file handle 1 == "stdout" */
   if( old == -1 )
   {
      perror( "_dup( 1 ) failure" );
      exit( 1 );
   }
   write( old, "This goes to stdout first\r\n", 27 );
   if( ( new = fopen( "data", "w" ) ) == NULL )
   {
      puts( "Can't open file 'data'\n" );
      exit( 1 );
   }

   /* stdout now refers to file "data" */
   if( -1 == _dup2( _fileno( new ), 1 ) )
   {
      perror( "Can't _dup2 stdout" );
      exit( 1 );
   }
   puts( "This goes to file 'data'\r\n" );

   /* Flush stdout stream buffer so it goes to correct file */
   fflush( stdout );
   fclose( new );

   /* Restore original stdout */
   _dup2( old, 1 );
   puts( "This goes to stdout\n" );
   puts( "The file 'data' contains:" );
   system( "type data" );
}

Output

This goes to stdout first
This goes to file 'data'

This goes to stdout

The file 'data' contains:

This goes to file 'data'