$ cc -O -Wall -o span_panic span_panic.c
span_panic.c: In function 'die':
span_panic.c:34:2: warning: implicit declaration of function 'exit' [-Wimplicit-function-declaration]
span_panic.c:34:2: warning: incompatible implicit declaration of built-in function 'exit'
span_panic.c:34:2: note: include '<stdlib.h>' or provide a declaration for 'exit'
span_panic.c:30:1:
+#include <stdlib.h>
span_panic.c:34:2: warning: incompatible implicit declaration of built-in function 'exit'
BUILD_RC=0 (binary produced)