About Makefiles – Part I

Hello,

Today I will briefly dissert about Makefiles and make, an efficient automation tool to compiles many programs at once.

I am writing this because when doing my SPO600 Project, I encountered many Makefiles with different functions and I was curious on how they work.

What I noticed first is that not all the Makefiles compiles things, most of them just have flags and configurations like this:
A common use of Makefiles is to set variables to be included in other Makefiles:

But first, lets understand how hey work from the beginning:

Whats the purpose of `Makefile`?

A Makefile is a set of instructions that will be read when the make command is called.

Make, then will read the Makefile follow its instructions and compile and link the program it wants to do.

A function of the Makefile is to create several make scripts like:

make: will run the Makefile
make clean: will erase the binaries compiled by the Makefile and others
make install will create and install tar packages and others
make distclean and make realclean will remove same files as make clean but TAGS, makefiles and config.status files, make realclean also removes info files generated by .texinfo

The makefile can also tell make how to run miscellaneous commands when explicitly asked (for example, to remove certain files as a clean-up operation).

Source: GNU

How `Make` works?

The command make will read the directory and search for a Makefile. Then it will start processing the first rule. A rule is a instruction with a target, a prerequisite and a recipe.

A target is usually the name of a file that is generated by a program; examples of targets are executable or object files. A target can also be the name of an action to carry out, such as ‘clean’ (see Phony Targets).
A prerequisite is a file that is used as input to create the target. A target often depends on several files.
A recipe is an action that make carries out. A recipe may have more than one command, either on the same line or each on its own line. Please note: you need to put a tab character at the beginning of every recipe line! This is an obscurity that catches the unwary. If you prefer to prefix your recipes with a character other than tab, you can set the .RECIPEPREFIX variable to an alternate character (see Special Variables).

Source: GNU

Looking easy right?
Lets take a look at this next screenshot and change our minds:

As we can see, it is really hard to understand what’s going on because there is a lot of variables being used.

Can we use Variables to make our life easier?

Yes we can, to avoid duplication and reduce redundancy we can set a variable.
If we take a look at the screenshot above, we can see that the second line is declaring a variable and assigning a value to it.