// ioctl #include <sys/ioctl.h> int result; unsigned int m = TIOCM_RTS; if( flag==BT_RTSOFF ){ result = ioctl(fd, TIOCMBIC, &m); }else{ result = ioctl(fd, TIOCMBIS, &m); }Man page of TTY_IOCTL</sys/ioctl.h>