I'm not sure what you mean by "another apps tries to write to the player" . ?

The idea is to create a new device node, say.. /proc/ttyS0, which userland apps would open instead of /dev/ttyS0. Both of those map to the same serial port, but whenever /proc/ttyS0 is open, the player's /dev/ttyS0 stops sending/receiving data out the back of the empeg (still works internally for Hijack notify and Hijack serial commands though).

Cheers