If your change includes a new file that doesn't exist in the code, then of course you have to send the whole file. But if you're modifying an already-existing file, don't send the whole file. Send a diff instead; specifically, send the output of the diff(1) command run to compare the baseline distributed version against your modified version.

The diff(1) command and its dual, patch(1), are the most basic tools of open-source development. Diffs are better than whole files because the developer you're sending a patch to may have changed the baseline version since you got your copy. By sending him a diff you save him the effort of separating your changes from his; you show respect for his time.

It is both counterproductive and rude to send a maintainer patches against the code as it existed several releases ago, and expect him to do all the work of determining which changes duplicate things he has since done, versus which things are actually novel in your patch.

As a patch submitter, it is your responsibility to track the state of the source and send the maintainer a minimal patch that expresses what you want done to the main-line codebase. That means sending a patch against the current version.

Some people put special tokens in their source files that are expanded by the version-control system when the file is checked in: the $Id$ construct used by RCS and CVS, for example.

If you're using a local version-control system yourself, your changes may alter these tokens. This isn't really harmful, because when your recipient checks his code back in after applying your patch the tokens will be re-expanded in accordance with the maintainer's version-control status. But those extra patch bands are noise. They're distracting. It's more considerate not to send them.

This is another minor error. You'll be forgiven for it if you get the big things right. But you want to avoid it anyway.

The default (-e) format of diff(1) is very brittle. It doesn't include any context, so the patch tool can't cope if any lines have been inserted or deleted in the baseline code since you took the copy you modified.

Getting an -e diff is annoying, and suggests that the sender is either an extreme newbie, careless, or clueless. Most such patches get tossed out without a second thought.

This is very important. If your patch makes a user-visible addition or change to the software's features, include changes to the appropriate man pages and other documentation files in your patch . Do not assume that the recipient will be happy to document your code for you, or to have undocumented features lurking in the code.

Documenting your changes well demonstrates some good things. First, it's considerate to the person you are trying to persuade. Second, it shows that you understand the ramifications of your change well enough to explain it to somebody who can't see the code. Third, it demonstrates that you care about the people who will ultimately use the software.

Good documentation is usually the most visible sign of what separates a solid contribution from a quick and dirty hack. If you take the time and care necessary to produce it, you'll find you're already 85% of the way to having your patch accepted by most developers.

Your patch should include cover notes explaining why you think the patch is necessary or useful. This is explanation directed not to the users of the software but to the maintainer to whom you are sending the patch.

The note can be short — in fact, some of the most effective cover notes I've ever seen just said “See the documentation updates in this patch”. But it should show the right attitude.

The right attitude is helpful, respectful of the maintainer's time, quietly confident but unassuming. It's good to display understanding of the code you're patching. It's good to show that you can identify with the maintainer's problems. It's also good to be up front about any risks you perceive in applying the patch. Here are some examples of the sorts of explanatory comments that experienced developers send:

“I've seen two problems with this code, X and Y. I fixed problem X, but I didn't try addressing problem Y because I don't think I understand the part of the code that I believe is involved”.

“Fixed a core dump that can happen when one of the foo inputs is too long. While I was at it, I went looking for similar overflows elsewhere. I found a possible one in blarg.c, near line 666. Are you sure the sender can't generate more than 80 characters per transmission?”

“Have you considered using the Foonly algorithm for this problem? There is a good implementation at <https://www.example.com/~jsmith/foonly.html>”.

“This patch solves the immediate problem, but I realize it complicates the memory allocation in an unpleasant way. Works for me, but you should probably test it under heavy load before shipping”.

“This may be featuritis, but I'm sending it anyway. Maybe you'll know a cleaner way to implement the feature”.

A maintainer will want to have strong confidence that he understands your changes before merging them in. This isn't an invariable rule; if you have a track record of good work with the maintainer, he may just run a casual eye over the changes before checking them in semiautomatically. But everything you can do to help him understand your code and decrease his uncertainty increases your chances that your patch will be accepted.

Good comments in your code help the maintainer understand it. Bad comments don't.

Here's an example of a bad comment:

/* norman newbie fixed this 13 Aug 2001 */

This conveys no information. It's nothing but a muddy territorial bootprint you're planting in the middle of the maintainer's code. If he takes your patch (which you've made less likely) he will almost certainly strip out this comment. If you want a credit, include a patch band for the project NEWS or HISTORY file. He's more likely to take that.

Here's an example of a good comment:

/*
 * This conditional needs to be guarded so that crunch_data() never
 * gets passed a NULL pointer.  <[email protected]>
 */

This comment shows that you understand not only the maintainer's code but the kind of information that he needs to have confidence in your changes. This kind of comment gives him confidence in your changes.

There are lots of reasons a patch can be rejected that don't reflect on you. Remember that most maintainers are under heavy time pressure, and have to be conservative in what they accept lest the project code get broken. Sometime resubmitting with improvements will help. Sometimes it won't. Life is hard.