Makefile

Syntax

A Rule in makefile looks like this:

targets: prerequisites
 recipe

Typically, the recipe operates of pre-requisites (other targets/files) and produces a new file with same name as current target

MUST USE TABS, not spaces

Recipe

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

Ref: https://makefiletutorial.com/

Each command is run in a new shell (or at least the effect is as such)

all: 
    cd ..
    # The cd above does not affect this line, because each command is effectively run in a new shell
    echo `pwd`
 
    # This cd command affects the next because they are on the same line
    cd ..;echo `pwd`
 
    # Same as above
    cd ..; \
    echo `pwd`

Multiple Targets and Default Target

There can be multiple targets in the same Rule. $@ automatic variable autoamtically picks the target name which is being called

The very first target in the file is the default target. It runs when you only call make with no arguments

What does running a target mean?

Assume X is a target in the Makefile

It will be run when:

1. It is called diretly using make X
2. X is a pre-requisite of another target say Y

Once target is run:

1. If the commands in the target X do not create a file X, then make will run the recipe in the target X everytime. This behaviour is similar to .PHONY
2. If X is an existing file in the repo, then make will first check if the existing file X is outdated or not. If its outdated, make will run the recipe in the target X

How does make verify if files are outdated?

Ref: https://makefiletutorial.com/

To make this happen, it uses the filesystem timestamps as a proxy to determine if something has changed. This is a reasonable heuristic, because file timestamps typically will only change if the files are modified. But it’s important to realize that this isn’t always the case. You could, for example, modify a file, and then change the modified timestamp of that file to something old. If you did, Make would incorrectly guess that the file hadn’t changed and thus could be ignored.

Variables

Variables can be initialised with either := or =
files := file1 file2
Single or double quotes have no meaning to Make, they are simply characters that are assigned to the variable
but those might be meaningful to bash

Automatic Variables

https://www.gnu.org/software/make/manual/html_node/Automatic-Variables.html

NOTE: they only have values within the recipe. In targets/prerequisites they will resolve to empty strings.

# outputs the active target name
$@
 
# all prerequisites newer than the target
$?
 
# Outputs all prerequisites with spaces
$^
 
# The name of the first prerequisite.
$<
 

Wildcards

* and % are some of the wildcard symbos, used for autocompletion.

* searches your filesystem for matching filenames.
* may be used in the target, prerequisites, variable assignment or in the wildcard function.
For safety, always use it with wildcard function, e.g. $(wildcard *.o)

% wildcard is used for matching and replacing
% is most often used in rule definitions and in some specific functions.

Dependency files

Note: todo not sure why we do it clear purpose

https://stackoverflow.com/questions/19114410/what-is-d-file-after-building-with-make

More details can be found gcc documentation for preprocessor args: link

Usually, -MD or -MMD is used to generate a dependency output file as a side effect of the compilation process

References

1. https://makefiletutorial.com/