Sembazuru:
...The Arduino has the DTE signal hardwired into the reset circuit....
so what would be usefull is that when i run my C++ code, i engage the reset on the arduino
(and then wait a while that the bootloader finishes)
so what is the chain of states do i need to put the DTE(DTR?) line ?
high, low, high/low, low/high ??
[edit : ok i got his in C++ (i use termios.h for communication)
/* sets the baud rate to 0 => (hangup) => DTR=LOW => DTE(computer) not present => arduino reset */
cfsetospeed(&oldtio, B0);
cfsetispeed(&oldtio, B0);
tcsetattr(fd,TCSANOW,&oldtio);
sleep(1);
it is actualy engaging the arduino reset 
yeay
]