DM> Depends on your OS. For example, under Linux, OS/2, and likely Win32,
 DM> fopen would work. Under DOS, however, you need to either get a com
 DM> port library, learn how to use a FOSSIL, or learn 
 GD> how to deal with the
 DM> serial port on a low level (via outportb/inportb).
 GD> Not so. The following works just fine under DOS:
 GD>   void offhook( void )
 GD>   {
 GD>     FILE *port ;
 GD>     port = fopen( "COM1", "w" );
 GD>     fprintf( port, "ATM0H1\n" );
 GD>     fclose( port );
 GD>   }
I'm not sure - I tried it with various compilers, not all of them liked it.  
It worked in some situations, not others.  Other OS's, however, force support 
of this method.
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