$ cc -O2 -o harness harness.c
harness.c: In function 'sig_segv':
harness.c:112:2: warning: implicit declaration of function 'write'; did you mean 'fwrite'?
  write(2, buf, n);
  ^~~~~
harness.c:113:2: warning: implicit declaration of function '_exit'; did you mean '_exit'?
  _exit(133);
  ^~~~~
$ echo BUILD_OK
BUILD_OK