> **3.** We saw that writing `#include <file>` instead of `#include "file"` may not work if file is one that we've written. Would there be any problem with writing `$include "file"` instead of `#include <file>` if *file* is a system header?
Yes, `""` is a path relative to the current file, whereas `<>` is a path to the system's standard library headers.
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> **4.** Assume that `debug.h` is a header file with the following contents...
>(a). What is the output when the program is executed?
When `DEBUG` is defined, any instances of the `PRINT_DEBUG` token are replaced with a `printf` call during compile time, but when it's not defined, they're replaced with nothing.
> (d). Is it necessary for the `DEBUG` macro to be defined *before* `debug.h` is included in order for `PRINT_DEBUG` to have the desired effect? Justify your answer.
Macro invocations are evaluated sequentially, and so if `DEBUG` was defined after `PRINT_DEBUG`, then any usages of `PRINT_EVALUATION` would be have like `DEBUG` was not defined.
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> **5.** Suppose that a program consists of three source files - `main.c`, `f1.c`, and `f2.c`- Plus two header files, `f1.h` and `f2.h`. All three source files include `f1.h` but only `f1.c` and `f2.c` include `f2.h`. Write a makefile for this program, assuming that the compiler is `gcc` and that the executable file is to be named `demo`.
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> **6.** The following questions refer to the program described in Exercise 5.
