Signed and unsigned , g++5 compoiler warning

I got the problem in a particular function.

I know there are new and come directly to ask for help, but to help and I in my turn.

So get this warning from the compiler (g ++ 5 under FreeBSD):

fdwatch.c: In function 'void fdwatch_clear_event(LPFDWATCH, socket_t, unsigned int)':
fdwatch.c:163:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     assert(event_idx < fdw->nfiles * 2);

Here is the fdwatch.c file : http://ift.tt/1a06h8g

THX