/in/foo.c: In function 'main':
/in/foo.c:13:2: warning: implicit declaration of function 'sort' [-Wimplicit-function-declaration]
  sort(a,length);
  ^~~~
/in/foo.c:17:9: warning: implicit declaration of function 'pop' [-Wimplicit-function-declaration]
    tmp+=pop(a,length--);
         ^~~
/in/foo.c:20:3: warning: implicit declaration of function 'push' [-Wimplicit-function-declaration]
   push(a,length++,tmp);
   ^~~~
/in/foo.c: In function 'sort':
/in/foo.c:29:3: warning: implicit declaration of function 'sink' [-Wimplicit-function-declaration]
   sink(arr,i,length-1);
   ^~~~
/in/foo.c: In function 'sink':
/in/foo.c:53:3: warning: implicit declaration of function 'swap' [-Wimplicit-function-declaration]
   swap(arr,k,j);
   ^~~~
/in/foo.c: In function 'push':
/in/foo.c:44:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^