## Source control description

This next section is useful if you have not used source control systems before. I will define some of the vocabulary and explain the basic ideas of source control.

### Check out systems

When describing the older source control systems, like SCCS, I said it managed the changes for a single file and also prevented multiple people from working on the same file at the same time. This is representative of a whole class of source control systems. In these you have the idea of "checking-out" a file so you can edit it, while becoming the current "owner". At the same time, while other people using the system can see who is working on the file, they are prevented from touching it. They can get a read-only copy so they can say build software but only the "owner" can edit it. When done editing the "owner" checks it back in, after which anyone else could work on on it. At the same time the system has recorded who had it and the changes made to it.

This system works well in small groups with real time communication. A common problem is that a file is checked out by some one else and you have to make a change in it. In a small group setting, just a shout over the cube wall will solve the problem.

### Merge systems

In systems represented by CVS or Subversion the barrier is not getting a file to work on but putting it back under version control. In these systems you pull the source code files to a working directory in your area. Then you edit these files, making necessary changes. When done you commit or check them back into the repository. At this point they are back under version control and the system knows the changes from the last version to this version.