Describe examples 2 - 4

Add links to GNU Make online resources.

doc/exercise.md

@@ -7,24 +7,97 @@
 In this exercise we will execute `make` on different makefiles.
 You will also edit an Makefile and try to create your own Makefile.
 
-**Step 1**: Logon on to a Linux system like Dardel
-**Step 2**: Go to [makefile-examples](https://github.com/coderefinery/makefile-examples) repository
+### Example 1
+- **Step 1**: Logon on to a Linux system like Dardel
+- **Step 2**: Go to [makefile-examples](https://github.com/coderefinery/makefile-examples) repository
 and clone the repository to your user space on the Linux system.
-**Step 3**: Change to the subdirectory `make-examples`, list the files. You will see that there is
+- **Step 3**: Change to the subdirectory `make-examples`, list the files. You will see that there is
 4 makefile examples in the repository:
 ```sh
 git clone https://github.com/coderefinery/makefile-examples.git
 cd makefile-examples
 ls
 ```
-**Step 4**: Change to subdirectory `example_1` and execute make. You will see that `make` executes
+- **Step 4**: Change to subdirectory `example_1`. Take a look at the `makefile` and execute make. You will see that `make` executes
 the command to build the executable. Test the executable and then remove it.
 ```sh
 cd example_1
+cat Makefile
 make
 ./hello.exe
 rm hello.exe
 ```
-**Step 5**: Introduce an error in the `example_1` makefile. Open the `Makefile` in an editor and
+- **Step 5**: Introduce an error in the `example_1` makefile. Open the `Makefile` in an editor and
 replace the tab in front of the command for the target with spaces. Save the file and execute
-`make` again. This time you get an error:Makefile:2: *** missing separator.  Stop.
+`make` again. This time you get an error:`Makefile:2: *** missing separator.  Stop.`
+
+### Example 2
+- **Step 1**: Change directory to `example_2`. Take a look at the `Makefile` and execute `make`.
+See how different object files are created in the subdirectory.
+```sh
+# from subdirectory example_1
+cd ../example_2
+ls
+cat Makefile
+make
+ls
+```
+
+- **Step 2**: Remove a object file and rerun `make`. Observe how `make` only build the missing
+object file and the rebuilds the executable since it is dependent on the newly built object file.
+```sh
+rm module.o
+make
+```
+
+- **Step 3**: Remove the executable and rerun `make`. Observe that only the step to build the
+executable is taken. The object files that the executable is dependent on is untouched.
+```sh
+rm hello.exe
+make
+```
+
+### Example 3
+Here you will try to create a makefile. Change into the subdirectory and observe that you have
+source files in the subdirectory `src`. How will you start out? The `Makefile` in example 2
+build an executable from source files in a `src` subdirectory. Let us see if we can use it as a
+starting point.
+
+- **Step 1**: Copy the `Makefile` from example 2 and execute make. You see that you get an error message
+from make: `make: *** No rule to make target `hello.o', needed by `hello.exe'.  Stop.`
+- **Step 2**: Open an editor (nano, vim) and replace references to `hello.exe` in the `Makefile`
+with `calculation.exe`, both as a target as a dependency. Execute `make` again and observe the
+erro message: `src/calculation.c:4:10: fatal error: 'example_math.h' file not found`
+This is an error message from the compilation of `calculation.c`. The compiler cannot find
+the include file `example_math.h` which resides in the subdirectory `include`.
+- **Step 3**: To find the include file, the compile needs to be told to look in the `include` subdirectory.
+We do this by adding the CFLAGS=-I include to the `Makefile`. Add it at the top of the `Makefile`, after
+the .PHONY statement, like this:
+```makefile
+.PHONY: clean all install
+CFLAGS=-I include
+```
+- **Step 4**: Rerun `make` and observe how the build of the executable completes:
+```sh
+make
+ls
+./calculation.exe
+make clean
+make
+```
+
+### Example 4
+In this example the builds, the object files and the executable, ends up in its own subdirectory `bin`.
+This separates the source files and the top `Makefile` from the binaries. This is very tidy and useful.
+Take a look at the make file, see how a function creates the necessary subdirectory, and how binaries
+are placed in an own subdirectory:
+```sh
+cd ../example_4
+ls
+cat Makefile
+make
+touch src/cos.c  # simulate a update of src/cos.c
+make
+make clean
+ls
+```

doc/index.rst

@@ -14,3 +14,4 @@ Welcome to Makefile's documentation!
    rules.md
    other_makefiles.md
    exercise.md
+   resources.md

doc/resources.md

@@ -0,0 +1,4 @@
+# Resources
+
+- The GNU Make documentation page, https://www.gnu.org/software/make/manual/
+- Managing Projects with GNU Make, Third Edition, https://www.oreilly.com/openbook/make3/book/index.csp