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Compilation course

Lab 0 - Git crash course

Setup

For all the compilation labs we will use git. A git repository tracks a project. Each repository may be cloned on different local or remote computers.

This document is under CC by-nc-sa licence, inspired by the lecture Gerez vos codes souces avec git from M. Nebra.

Introduction to Git

Git is able to follow the changes inside a text file for each line of code. It can record all the intermediate versions and changes in your project. But Git is not only a backup tool, it offers many other features such as:

  • Git records who made each change to each file and why.

  • If two people work on the same file simultaneously, Git is capable of merging their modifications and avoids loosing any contributions.

Git is a “distributed” version management software. Each of the developers participating in the same project has its own git repository. Developers can exchange versions and commits (changes) in a peer-to-peer fashion. Often a central repository is created on a server. The central repository serves as a meeting point and facilitates exchanges between developers.

It is in this last mode that we will work in this course.

Creating an account on gitlab-chps.ens.uvsq.fr

To save your work between lab sessions we will use the server at gitlab-chps.ens.uvsq.fr.

▶ An account with your university email first.last@ens.uvsq.fr has been created. The initial password is everything before the @ in your email. For example, if your email is first.last@ens.uvsq.fr, you will use the password first.last.

▶ Immediatly after logging change your password to secure your account. Click on the drop-down menu on the top-right, then go to Preferences > Password.

▶ When you log on the server, you will see that a project (coa/firstname-lastname) has been created for you.

▶ (optional) To clone and push on your repository without typing your password each time, create and add an ssh key to your account. First look how to create an ssh-key on your system. Then click on the drop-down menu, then choose Settings > SSH Keys to upload your public key pair.

Cloning the repository

▶ To start working on your git repository, you’ll have to clone it. Log into gitlab-chps.ens.uvsq.fr and copy the address of the repository. Select the protocol (HTTP or SSH) on the drop-down menu at the center of the interface (do not choose SSH unless you have added an SSH key on your profile). Then copy the address that is located nearby.

▶ Now go to the terminal and type the following commands:

    # WARNING: please replace pablo-oliveira by your own name

    # clone the repository 
    $ git clone http://gitlab-chps.ens.uvsq.fr/coa/pablo-oliveira.git 
    warning: You appear to have cloned an empty repository.
    Checking connectivity... done.

    # enter the repository 
    $ cd pablo-oliveira/

    # Now create a lab0 folder and enter it 
    $ mkdir lab0
    $ cd lab0

If you want to clone this repository on other machines; the procedure is the same.

In the rest of the lab, you will work inside the lab0 directory that you just created.

Adding and committing files

First configure git with your personal information,

$ git config --global user.name "Pablo Oliveira"
$ git config --global user.email pablo.oliveira@uvsq.fr

Now write a test.cc program that displays “Hello World!” on the standard output and a Makefile file to compile it with clang++-3.9. Write your program in C++.

To commit the files first add them in git with

$ git add Makefile test.cc
$ git status
On branch master

Initial commit

Changes to be committed:
  (use "git rm --cached <file>..." to unstage)

	new file:   Makefile
	new file:   test.cc

The git status command allows you to see what’s new from the last save (or commit) in the repository. Here git status shows that both files are ready to be committed.

The git commit command opens an editor that allows you to type a a message explaining the changes.

For commit messages use the following convention,

  • the first line must contain a short and informative message that summarize your changes

  • the second line must be empty

  • next you can describe in detail your changes

For example:

    lab0 (exo1): write a hello world program

    test.cc prints "Hello World!" to the standard
    output. Also added a makefile.

Using the log

Change your program again. This time change the program so that it segfaults. You can be creative ☺

Then type git diff to see your changes. Then commit your changes with git add test.cc followed by git commit.

You can view the journal of changes with git log

$ git log
commit 66ea53ab301045fc5a990fac3672f842dc75b2e8
Author: Pablo Oliveira <pablo.oliveira@uvsq.fr>
Date:   Thu Sep 14 22:59:15 2023 +0200

    Harmless modification that makes the program segfault

commit b35c2b6551d4b3738170637634702ed4d82aa501
Author: Pablo Oliveira <pablo.oliveira@uvsq.fr>
Date:   Thu Sep 14 22:55:05 2023 +0200

    lab0 (exo1): write a hello world program

    test.cc prints "Hello World!" to the standard
    output. Also added a makefile.

Each commit is followed by a unique hexadecimal code that identifies it. You can view changes between two versions with git diff b35c2b6 66ea53a, there is no need to type all the numbers, the first are sufficient (except in case of conflict). Try !

The different versions form a branch of versions. In git the main branch is usually called master.

Git allows you to work on several branches at the same time, go back to previous versions of code, and many more things but that goes beyond the scope of this introduction.

Sharing code with others

To send your changes to another repository, you will need to use two commands:

  • git pull retrieves changes from another repository

  • git push sends your changes to another repository

Since you already clone your repository from gitlab-chps.ens.uvsq.fr; this server is already configure as the default push server.

To send your changes do,

$ git push origin master

Counting objects: 3, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (2/2), done.
Writing objects: 100% (3/3), 271 bytes | 0 bytes/s, done.
Total 3 (delta 0), reused 0 (delta 0)
To http://pablo-oliveira@gitlab-chps.ens.uvsq.fr/coa/pablo-oliveira.git
* [new branch]      master -> master