/in/foo.c: In function 'main':
/in/foo.c:16:2: warning: implicit declaration of function 'mul1' [-Wimplicit-function-declaration]
  mul1(x, m);
  ^~~~
/in/foo.c:17:2: warning: implicit declaration of function 'mul2' [-Wimplicit-function-declaration]
  mul2(y, n);
  ^~~~
/in/foo.c: In function 'mul1':
/in/foo.c:56:1: warning: control reaches end of non-void function [-Wreturn-type]
 }int mul2(int x, int n)
 ^
/in/foo.c: In function 'mul2':
/in/foo.c:78:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^