+#include <stdio.h>

RelayCapeOne.c:9:5:
     fprintf( stderr, "open: %m\n" );
     ^~~~~~~
RelayCapeOne.c:9:14: error: ‘stderr’ undeclared (first use in this function)
     fprintf( stderr, "open: %m\n" );
              ^~~~~~
RelayCapeOne.c:9:14: note: ‘stderr’ is defined in header ‘<stdio.h>’; did you forget to ‘#include <stdio.h>’?
RelayCapeOne.c:9:14: note: each undeclared identifier is reported only once for each function it appears in