#include <sys/time.h>  #include <sys/types.h> 
#include <unistd.h>  
#define STDIN 0 /*±ê×?ê?è????t?èê?·?*/  
  main()  
   ????{  
   ???? struct timeval tv;  
   ???? fd_set readfds;  
   ???? tv.tv_sec = 2;  
   ???? tv.tv_usec = 500000;  
   ???? FD_ZERO(&readfds);  
   ???? FD_SET(STDIN,&readfds);  
   ???? /* ?aà?2?1?D?D????toíòì3￡??àí???t?èê?·??ˉo? */  
   ???? select(STDIN+1￡? &readfds￡? NULL￡? NULL￡? &tv);  
   ???? if (FD_ISSET(STDIN￡? &readfds)) printf("A key was pressed!");  
 else printf("Timed out.");  
  }