Challenge 2: Omitting math_utils.o From the Link Command — Possible Solution
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$ gcc -c main.c -o main.o
$ gcc main.o -o program

Error:

    /usr/bin/ld: main.o: in function `main':
    main.c:(.text+0x15): undefined reference to `add'
    collect2: error: ld returned 1 exit status

Explanation: main.c compiled successfully into main.o without any
error at all -- the compiler only needed math_utils.h's DECLARATION of
add to accept the call add(2, 3) as valid syntax with the correct
signature. The problem only surfaces at the LINKING step: main.o
contains a call to a function named add with no actual body attached
to it (an unresolved external symbol), and since math_utils.o -- the
only object file that contains add's real DEFINITION -- was never
included in the final gcc command, the linker has nothing to match
that call against. It reports "undefined reference to `add'" because
it can find a reference (the call) but no corresponding definition
anywhere in the files it was given.

WHY THIS WORKS AS AN ANSWER
------------------------------
This shows the exact error message, correctly identifies it as a
LINKER error rather than a compile error (compilation of main.c
succeeded on its own), and explains precisely why the header alone was
enough for compilation to succeed while the missing object file caused
linking specifically to fail -- exactly the declaration-vs-definition
distinction the chapter's tip-box describes.