I'm preparing a talk on Open Source for a group of project managers, and my contact mentioned that being clear about what source code actually is would help.

So, here we go. I'm sharing this below for the benefit of mankind my two readers.

Apart from that, I'm going to tell them How the ASF works, point them to a paper by the European Working Group on Free Software (mainly about licences but there are lots of good things in there), and talk about the lightweight tools that we use to reach our goals.

Here's the blurb about source code - WDYT?

What is "source code"?

Wikipedia says:

Source code (commonly just source or code) is any series of statements written in some human-readable computer programming language.

and

A computer program's source code is the collection of files that can be converted from human-readable form to an equivalent computer-executable form. The source code is either converted into executable by a compiler for a particular computer architecture, or executed from the human readable form with the aid of an interpreter.

An example in the C language

Here's the source code of a very simple program, written in a programming language called "C":

main() { printf("Hello, World!\n"); }

When compiled and executed, this program displays Hello, World! on screen.

Real programs are much more complex, of course, often consisting of thousands of such source code files, totalizing tens of thousands or even millions of lines of source code.

The compilation process creates a machine-readable binary version of the program, optimized for quick execution on a given computer system.

Before Open Source, this binary version was the only one people would get when they bought software: a static, frozen version of the software.

Here's a binary listing of the compiled version of the above program (on mac OSX, using the od -h command):

0000000 feed face 0000 0012 0000 0000 0000 0002 0000020 0000 000a 0000 0500 0000 0085 0000 0001 0000040 0000 0038 5f5f 5041 4745 5a45 524f 0000 0000060 0000 0000 0000 0000 0000 1000 0000 0000 ... about 400 more lines like this ... 0026360 6e64 5f64 796c 645f 7265 6d6f 7665 5f69 0026400 6d61 6765 5f68 6f6f 6b00 0000 0026414

(Note that "feed face" don't mean "give food to your head", they're hexadecimal values ;-)

As you can imagine, modifying the program from the source code (for example to change the message) is relatively easy, but doing the same change on the binary, compiled version of the program, although theoretically possible, is much harder, and in most practical cases impossible.

Update: thanks to Ezra for pointing out my mistake, the binary was a dump of the source code instead of the compiled version, it is corrected now. Also, the three comments seem to indicate that you readers are indeed more than two. I stand corrected on both points.