I haven't tried chanclose() yet, but setting to nil the freed chan in the
alt entry is not really what I wanted since it will make alt() return -1
(better than crashing but not ideal).
In any case I found a satisfying workaround in my algorithm to solve that:
when a thread is done using a chan, I keep track of it instead of calling
chanfree(), and instead of allocating a new chan/alt when creating a new
thread, I simply reuse one of the previously "abandonned" ones (if any
is available). And I'll just free them all when the program terminates.

Thanks to all,
Mathieu