foo.c: In function 'main':
foo.c:4:5: warning: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
     scanf("%d%d", &a, &b);
     ^~~~~
foo.c:4:5: warning: incompatible implicit declaration of built-in function 'scanf'
foo.c:4:5: note: include '<stdio.h>' or provide a declaration of 'scanf'
foo.c:5:5: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
     printf("%d\n", a + b);
     ^~~~~~
foo.c:5:5: warning: incompatible implicit declaration of built-in function 'printf'
foo.c:5:5: note: include '<stdio.h>' or provide a declaration of 'printf'