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

How to contribute to this document

You may mail corrections, additions, and suggestions by following the instructions set forth in this document and submitting a patch :).


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

What to patch

Generally speaking you should patch the latest development release of perl. The maintainers of the individual branches will see to it that patches are picked up and applied as appropriate.

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 ensures 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.

When GNU diff is available, the pumpkins would prefer you use -u -p (--unified --show-c-function) as arguments for optimal control. The examples below will only use -u.

The preferred method for creating a unified diff suitable for feeding to the patch program is:

        diff -u old-file new-file > patch-file

Note the order of files. See below for how to create a patch from two directory trees.

If your patch is for wider consumption, it may be better to create it as a context diff as some machines have broken patch utilities that choke on unified diffs. A context diff is made using diff -c rather than diff -u.

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 -p old/file new/file
    % diff -u -F '^[_a-zA-Z0-9]+ *(' old/file new/file

    # show sub name in perl files and modules
    % diff -u -F '^sub' old/ new/

    # show header in doc patches
    % diff -u -F '^=head' old/file.pod new/file.pod
Derived Files

Many files in the distribution are derivative--avoid patching them. Patch the originals instead. Most utilities (like perldoc) are in this category, i.e. patch utils/perldoc.PL rather than utils/perldoc. Similarly, don't create patches for files under $src_root/ext from their copies found in $install_root/lib. If you are unsure about the proper location of a file that may have gotten copied while building the source distribution, consult the MANIFEST.


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.dist 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 -ruN old-dir new-dir to create all the patches at once.


IMPORTANT: Patches should be generated from the source root directory, not from the directory that the patched file resides in. This ensures that the maintainer patches the proper file.

For larger patches that are dealing with multiple files or directories, Johan Vromans has written a powerful utility: makepatch. See the JV directory on CPAN for the current version. If you have this program available, it is recommended to create a duplicate of the perl directory tree against which you are intending to provide a patch and let makepatch figure out all the changes you made to your copy of the sources. As perl comes with a MANIFEST file, you need not delete object files and other derivative files from the two directory trees, makepatch is smart about them.

Say, you have created a directory perl-5.7.1@8685/ for the perl you are taking as the base and a directory perl-5.7.1@8685-withfoo/ where you have your changes, you would run makepatch as follows:

    makepatch -oldman perl-5.7.1@8685/MANIFEST \
              -newman perl-5.7.1@8685-withfoo/MANIFEST \
              -diff "diff -u" \
              perl-5.7.1@8685 perl-5.7.1@8685-withfoo
Binary Files

Since the patch(1) utility cannot deal with binary files, it's important that you either avoid the use of binary files in your patch, generate the files dynamically, or that you encode any binary files using the utility.

Assuming you needed to include a gzip-encoded file for a module's test suite, you might do this as follows using the utility:

    $ perl -v -p -D lib/Some/Module/t/src/t.gz
    Writing lib/Some/Module/t/src/t.gz into lib/Some/Module/t/src/t.gz.packed

This will replace the t.gz file with an encoded counterpart. During make test, before any tests are run, perl's Makefile will restore all the .packed files mentioned in the MANIFEST to their original name. This means that the test suite does not need to be aware of this packing scheme and will not need to be altered.

Try it yourself

Just to make sure your patch "works", be sure to apply it to the Perl distribution, rebuild everything, and make sure the testsuite runs without incident.

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.

Directions 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.


In general, please follow the particular style of the code you are patching.

In particular, follow these general guidelines for patching Perl sources:

    8-wide tabs (no exceptions!)
    4-wide indents for code, 2-wide indents for nested CPP #defines
    try hard not to exceed 79-columns
    ANSI C prototypes
    uncuddled elses and "K&R" style for indenting control constructs
    no C++ style (//) comments, most C compilers will choke on them
    mark places that need to be revisited with XXX (and revisit often!)
    opening brace lines up with "if" when conditional spans multiple
        lines; should be at end-of-line otherwise
    in function definitions, name starts in column 0 (return value is on
        previous line)
    single space after keywords that are followed by parens, no space
        between function name and following paren
    avoid assignments in conditionals, but if they're unavoidable, use
        extra paren, e.g. "if (a && (b = c)) ..."
    "return foo;" rather than "return(foo);"
    "if (!foo) ..." rather than "if (foo == FALSE) ..." etc.

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 numbers whenever possible (the 
          EXPECTED/GOT found in t/op/tie.t is much more maintainable, 
          and gives better failure reports).
        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 the 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 perl5.7.42/MANIFEST.old perl5.7.42/MANIFEST > mypatch
      (testing the patch:)
      mv perl5.7.42/MANIFEST perl5.7.42/
      cp perl5.7.42/MANIFEST.old perl5.7.42/MANIFEST
      patch -p < mypatch
      (should succeed)
      diff perl5.7.42/MANIFEST perl5.7.42/
      (should produce no output)

Submitting your patch


Please, please, please (get the point? 8-) don't use a mailer that word wraps your patch. This leaves the patch essentially worthless to the maintainers.

Unfortunately many mailers word wrap the main text of messages, but luckily you can usually send your patches as email attachments without them getting "helpfully" word wrapped.

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.x.x AREA] Description

where the x's are replaced by the appropriate version number. The description should be a very brief but accurate summary of the problem (don't forget this is an email header).


    [PATCH 5.6.4 DOC] fix minor typos

    [PATCH 5.7.9 CORE] New warning for foo() when frobbing

    [PATCH 5.7.16 CONFIG] Added support for fribnatz 1.5

The name of the file being patched makes for a poor subject line if no other descriptive text accompanies it.

Where to send your patch

If your patch is for a specific bug in the Perl core, it should be sent using the perlbug utility. Don't forget to describe the problem and the fix adequately.

If it is a patch to a module that you downloaded from CPAN you should submit your patch to that module's author.

If your patch addresses one of the items described in perltodo.pod, please discuss your approach before you make the patch at <>. Be sure to browse the archives of past discussions (see perltodo.pod for archive locations).

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 N argument to patch (where N is the number of path components to skip in the files found in the headers). This helps reconcile differing paths between the machine the patch was created on and the machine on which it is being applied.

Be sure to use the Larry Wall version of patch. Some Operating Systems (HP-UX amongst those) have a patch command that does something completely different. The correct version of patch will show Larry's name several times when invoked as patch --version.

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 almost always screws up the line numbers and offsets in the patch, making it useless.

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 no one can use because no one 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-)

Author and Copyright Information

Copyright (c) 1998-2002 Daniel Grisinger

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

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