/in/foo.c:2:6: warning: return type of 'main' is not 'int' [-Wmain]
 void main()
      ^~~~
/in/foo.c: In function 'main':
/in/foo.c:17:3: warning: 'd' may be used uninitialized in this function [-Wmaybe-uninitialized]
   printf("%d%c%d=%d\n",a,c,b,d);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~