foo.c: In function ‘input’:
foo.c:27:1: warning: control reaches end of non-void function [-Wreturn-type]
   27 | }
      | ^
foo.c: In function ‘output’:
foo.c:35:1: warning: control reaches end of non-void function [-Wreturn-type]
   35 | }
      | ^
foo.c: In function ‘func’:
foo.c:44:1: warning: control reaches end of non-void function [-Wreturn-type]
   44 | }
      | ^