[Pharo-users] Understanding the role of the sources file

Dimitris Chloupis kilon.alios at gmail.com
Wed Jan 13 06:58:17 EST 2016

So I am correct that the image does not store the source code, and that the
source code is stored in sources and changes. The only diffirence is that
the objects have a source variable that points to the right place for
finding the source code.

This is the final text if you find anything incorrect please correct me


1. The virtual machine (VM) is the only component that is different for
each operating system. The purpose of the VM is to take Pharo bytcode that
is generated each time user accepts a piece of code and convert it to
machine code in order to be executed. Pharo 4 comes with the Cog VM a very
fast JIT VM. The VM executable is named:

• Pharo.exe for Windows; • pharo for Linux ; and

• Pharo for OSX (inside a package also named Pharo.app).
The other components below are portable across operating systems, and

can be copied and run on any appropriate virtual machine.

2. The sources file contains source code for parts of Pharo that don’t
change frequently. Sources file is important because the image file format
stores only the bytecode of live objects and not their source code.
Typically a new sources file is generated once per major release of Pharo.
For Pharo 4.0, this file is named PharoV40.sources.

3. The changes file logs of all source code modifications since the .sources
file was generated. This facilitates a per method history for diffs or re-
verting.That means that even if you dont manage to save the image file on a
crash or you just forgot you can recover your changes from this file. Each
release provides a near empty file named for the release, for example

4. The image file provides a frozen in time snapshot of a running Pharo
system. This is the file where the Pharo bytecode is stored and as such its
a cross platform format. This is the heart of Pharo, containing the live
state of all objects in the system (including classes and methods, since
they are objects too). The file is named for the release (like

The .image and .changes files provided by a Pharo release are the starting
point for a live environment that you adapt to your needs. Essentially the
image file containes the compiler of the language (not the VM) , the
language parser, the IDE tools, many libraries and acts a bit like a
virtual Operation System that runs on top of a Virtual Machine (VM),
similarly to ISO files.

As you work in Pharo, these files are modified, so you need to make sure
that they are writable. The .image and .changes files are intimately linked
and should always be kept together, with matching base filenames. Never
edit them directly with a text editor, as .images holds your live object
runtime memory, which indexes into the .changes files for the source. It is
a good idea to keep a backup copy of the downloaded .image and .changes files
so you can always start from a fresh image and reload your code. However
the most efficient way for backing up code is to use a version control
system that will provide an easier and powerful way to back up and track
your changes.

The four main component files above can be placed in the same directory,
although it’s also possible to put the Virtual Machine and sources file in
a separate directory where everyone has read-only access to them.

If more than one image file is present in the same directory pharo will
prompt you to choose an image file you want to load.

Do whatever works best for your style of working and your operating system.

On Wed, Jan 13, 2016 at 12:13 PM Sven Van Caekenberghe <sven at stfx.eu> wrote:

> > On 13 Jan 2016, at 10:57, Dimitris Chloupis <kilon.alios at gmail.com>
> wrote:
> >
> > I was adding a short description to the UPBE about sources file , I
> always thought that the sources file is the file that contains the source
> code of the image because the image file itself stores only the bytecode.
> >
> > However its just came to my attention that the sources file does not
> contain code that is recently installed in the image.
> >
> > So how exactly the sources file works and what it is ?
> The main perspective is from the object point of view: methods are just
> objects like everything else. In order to be executable they know their
> byte codes (which might be JIT compiled on execution, but that is an
> implementation detail) and they know their source code.
> Today we would probably just store the source code strings in the image
> (maybe compressed) as memory is pretty cheap. But way back when Smalltalk
> started, that was not the case. So they decided to map the source code out
> to files.
> So method source code is a magic string (RemoteString) that points to some
> position in a file. There are 2 files in use: the sources file and the
> changes file.
> The sources file is a kind of snapshot of the source code of all methods
> at the point of release of a major new version. That is why there is a Vxy
> in their name. The source file never changes once created or renewed (a
> process called generating the sources, see PharoSourcesCondenser).
> While developing and creating new versions of methods, the new source code
> is appended to another file called the changes file, much like a
> transaction log. This is also a safety mechanism to recover 'lost' changes.
> The changes file can contain multiple versions of a method. This can be
> reduced in size using a process called condensing the changes, see
> PharoChangesCondenser.
> On a new release, the changes file will be (almost) empty.
> HTH,
> Sven
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