Question [Q] Where is iSerial on S5839i ?

I am working on a utility app which relies on device serial number (and if possible a unique one...). However, I have just bought a Galaxy Ace (S5839i) which has a default / uninitialized serial number:
$adb devices
List of devices attached 
0123456789ABCDEF	device
I am overwriting the iSerial file as described below to overcome this on my other devices:
echo 'XXX' > /sys/class/android_usb/android0/iSerial
However, on the Galaxy Ace, this file is not present. Do you have already encountered the issue ?
Do you know if there is a solution to read / modify the serial on this particular device ?

Thanks for your help,