foo.c: In function 'main':
foo.c:7:2: warning: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
  scanf("%d", &a);
  ^~~~~
foo.c:7:2: warning: incompatible implicit declaration of built-in function 'scanf'
foo.c:7:2: note: include '<stdio.h>' or provide a declaration of 'scanf'
foo.c:15:7: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
       printf("%d+%d=%d\n", b, c, a);
       ^~~~~~
foo.c:15:7: warning: incompatible implicit declaration of built-in function 'printf'
foo.c:15:7: note: include '<stdio.h>' or provide a declaration of 'printf'

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