patching.pod - Appropriate format for patches to the perl source tree

Where to get this document

The latest version of this document is available from

How to contribute to this document

You may mail corrections, additions, and suggestions to me at but the preferred method would be to follow the instructions set forth in this document and submit a patch 8-).


Why this document exists

As an open source project Perl relies on patches and contributions from its users to continue functioning properly and to root out the inevitable bugs. But, some users are unsure as to the right way to prepare a patch and end up submitting seriously malformed patches. This makes it very difficult for the current maintainer to integrate said patches into their distribution. This document sets out usage guidelines for patches in an attempt to make everybody's life easier.

Common problems

The most common problems appear to be patches being mangled by certain mailers (I won't name names, but most of these seem to be originating on boxes running a certain popular commercial operating system). Other problems include patches not rooted in the appropriate place in the directory structure, and patches not produced using standard utilities (such as diff).

Proper Patch Guidelines

How to prepare your patch

Creating your patch

First, back up the original files. This can't be stressed enough, back everything up _first_.

Also, please create patches against a clean distribution of the perl source. This insures that everyone else can apply your patch without clobbering their source tree.


While individual tastes vary (and are not the point here) patches should be created using either -u or -c arguments to diff. These produce, respectively, unified diffs (where the changed line appears immediately next to the original) and context diffs (where several lines surrounding the changes are included). See the manpage for diff for more details.

Also, the preferred method for patching is -

diff [<CODE>-c</CODE> | <CODE>-u</CODE>] <old-file> <new-file>

Note the order of files.

Also, if your patch is to the core (rather than to a module) it is better to create it as a context diff as some machines have broken patch utilities that choke on unified diffs.

GNU diff has many desirable features not provided by most vendor-supplied diffs. Some examples using GNU diff:

    # generate a patch for a newly added file
    % diff -u /dev/null new/file
    # generate a patch to remove a file (patch > v2.4 will remove it cleanly)
    % diff -u old/goner /dev/null
    # get additions, deletions along with everything else, recursively
    % diff -ruN olddir newdir
    # ignore whitespace
    % diff -bu a/file b/file
    # show function name in every hunk (safer, more informative)
    % diff -u -F '^[_a-zA-Z0-9]+ *(' old/file new/file

Patches should be generated from the source root directory, not from the directory that the patched file resides in. This insures that the maintainer patches the proper file and avoids name collisions (especially common when trying to apply patches to files that appear in both $src_root/ext/* and $src_root/lib/*). It is better to diff the file in $src_root/ext than the file in $src_root/lib.


The most usual convention when submitting patches for a single file is to make your changes to a copy of the file with the same name as the original. Rename the original file in such a way that it is obvious what is being patched ($file~ or $file.old seem to be popular).

If you are submitting patches that affect multiple files then you should backup the entire directory tree (to $source_root.old/ for example). This will allow diff <CODE>-c</CODE> <old-dir> <new-dir> to create all the patches at once.

What to include in your patch

Description of problem

The first thing you should include is a description of the problem that the patch corrects. If it is a code patch (rather than a documentation patch) you should also include a small test case that illustrates the bug.

Direction for application

You should include instructions on how to properly apply your patch. These should include the files affected, any shell scripts or commands that need to be run before or after application of the patch, and the command line necessary for application.

If you have a code patch

If you are submitting a code patch there are several other things that you need to do.
Comments, Comments, Comments

Be sure to adequately comment your code. While commenting every line is unnecessary, anything that takes advantage of side effects of operators, that creates changes that will be felt outside of the function being patched, or that others may find confusing should be documented. If you are going to err, it is better to err on the side of adding too many comments than too few.


Please follow the indentation style and nesting style in use in the block of code that you are patching.


When submitting a patch you should make every effort to also include an addition to perl's regression tests to properly exercise your patch. Your testsuite additions should generally follow these guidelines (courtesy of Gurusamy Sarathy (

        Know what you're testing.  Read the docs, and the source.
        Tend to fail, not succeed.
        Interpret results strictly.
        Use unrelated features (this will flush out bizarre interactions).
        Use non-standard idioms (otherwise you are not testing TIMTOWTDI).
        Avoid using hardcoded test umbers whenever possible (the EXPECTED/GOT style
          found in t/op/tie.t is much more maintainable, and gives better failure
        Give meaningful error messages when a test fails.
        Avoid using qx// and system() unless you are testing for them.  If you
          do use them, make sure that you cover _all_ perl platforms.
        Unlink any temporary files you create.
        Promote unforeseen warnings to errors with $SIG{__WARN__}.
        Be sure to use the libraries and modules shipped with version being tested,
          not those that were already installed.
        Add comments to the code explaining what you are testing for.
        Make updating the '1..42' string unnecessary.  Or make sure that you update it.
        Test _all_ behaviors of a given operator, library, or function-
          All optional arguments
          Return values in various contexts (boolean, scalar, list, lvalue)
          Use both global and lexical variables
          Don't forget the exceptional, pathological cases.
Test your patch

Apply your patch to a clean distribution, compile, and run the regression test suite (you did remember to add one for your patch, didn't you).

An example patch creation

This should work for most patches-

      emacs MANIFEST
      (make changes)
      cd ..
      diff -c perl5.008_42/MANIFEST.old perl5.008_42/MANIFEST > mypatch
      (testing the patch:)
      mv perl5.008_42/MANIFEST perl5.008_42/
      cp perl5.008_42/MANIFEST.old perl5.008_42/MANIFEST
      patch -p < mypatch
      (should succeed)
      diff perl5.008_42/MANIFEST perl5.008_42/
      (should produce no output)

Submitting your patch


Please, please, please (get the point? 8-) don't use a mailer that word wraps your patch or that MIME encodes it. Both of these leave the patch essentially worthless to the maintainer.

If you have no choice in mailers and no way to get your hands on a better one there is, of course, a perl solution. Just do this-

      perl -ne 'print pack("u*",$_)' patch > patch.uue

and post patch.uue with a note saying to unpack it using

      perl -ne 'print unpack("u*",$_)' patch.uue > patch
Subject lines for patches

The subject line on your patch should read

[PATCH]5.xxx_xx (Area) Description

where the x's are replaced by the appropriate version number, area is a short keyword identifying what area of perl you are patching, and description is a very brief summary of the problem (don't forget this is an email header).


[PATCH]5.004_04 (DOC) fix minor typos

[PATCH]5.004_99 (CORE) New warning for foo() when frobbing

[PATCH]5.005_42 (CONFIG) Added support for fribnatz 1.5

Where to send your patch

If your patch is for the perl core it should be sent If it is a patch to a module that you downloaded from CPAN you should submit your patch to that module's author.

Applying a patch

General notes on applying patches

The following are some general notes on applying a patch to your perl distribution.
patch -p

It is generally easier to apply patches with the -p argument to patch. This helps reconcile differing paths between the machine the patch was created on and the machine on which it is being applied.

Cut and paste

_Never_ cut and paste a patch into your editor. This usually clobbers the tabs and confuses patch.

Hand editing patches

Avoid hand editing patches as this frequently screws up the whitespace in the patch and confuses the patch program.

Final notes

If you follow these guidelines it will make everybody's life a little easier. You'll have the satisfaction of having contributed to perl, others will have an easy time using your work, and it should be easier for the maintainers to coordinate the occasionally large numbers of patches received.

Also, just because you're not a brilliant coder doesn't mean that you can't contribute. As valuable as code patches are there is always a need for better documentation (especially considering the general level of joy that most programmers feel when forced to sit down and write docs). If all you do is patch the documentation you have still contributed more than the person who sent in an amazing new feature that noone can use because noone understands the code (what I'm getting at is that documentation is both the hardest part to do (because everyone hates doing it) and the most valuable).

Mostly, when contributing patches, imagine that it is you receiving hundreds of patches and that it is your responsibility to integrate them into the source. Obviously you'd want the patches to be as easy to apply as possible. Keep that in mind. 8-)

Last Modified

Last modified 21 May 1998 by Daniel Grisinger <>

Author and Copyright Information

Copyright (c) 1998 Daniel Grisinger

Adapted from a posting to perl5-porters by Tim Bunce (

I'd like to thank the perl5-porters for their suggestions.


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