foo.c: In function 'main':
foo.c:15:9: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   15 |         printf("%d\n", ret);
      |         ^~~~~~
foo.c:1:1: note: include '<stdio.h>' or provide a declaration of 'printf'
  +++ |+#include <stdio.h>
    1 | int DigitSum(int x)
foo.c:15:9: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
   15 |         printf("%d\n", ret);
      |         ^~~~~~
foo.c:15:9: note: include '<stdio.h>' or provide a declaration of 'printf'

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