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Author image Paul Johnson


Devel::Cover - Code coverage metrics for Perl


version 0.93


To get coverage for an uninstalled module:

 cover -test


 cover -delete
 HARNESS_PERL_SWITCHES=-MDevel::Cover make test

To get coverage for an uninstalled module which uses Module::Build (0.26 or later):

 ./Build testcover

If the module does not use the t/*.t framework:

 PERL5OPT=-MDevel::Cover make test

If you want to get coverage for a program:

 perl -MDevel::Cover yourprog args

 perl -MDevel::Cover=-db,cover_db,-coverage,statement,time yourprog args


This module provides code coverage metrics for Perl. Code coverage metrics describe how thoroughly tests exercise code. By using Devel::Cover you can discover areas of code not exercised by your tests and determine which tests to create to increase coverage. Code coverage can be considered as an indirect measure of quality.

Although it is still being developed, Devel::Cover is now quite stable and provides many of the features to be expected in a useful coverage tool.

Statement, branch, condition, subroutine, and pod coverage information is reported. Statement coverage data should be reasonable. Branch and condition coverage data should be mostly accurate too, although not always what one might initially expect. Subroutine coverage should be as accurate as statement coverage. Pod coverage comes from Pod::Coverage. If Pod::Coverage::CountParents is available it will be used instead. Coverage data for other criteria are not yet collected.

The cover program can be used to generate coverage reports. Devel::Cover ships with a number of different reports including various types of HTML output, textual reports, a report to display missing coverage in the same format as compilation errors and a report to display coverage information within the Vim editor.

It is possible to add annotations to reports, for example you could add a column to an HTML report showing who last changed a line, as determined by git blame. Some annotation modules are shipped with Devel::Cover and you can easily create your own.

The gcov2perl program can be used to convert gcov files to Devel::Cover databases. This allows you to display your C or XS code coverage together with your Perl coverage, or to use any of the Devel::Cvoer reports to display your C coverage data.

You may find that the coverage results don't match your expectations. This could be due to a bug in Devel::Cover, but code coverage can be unintuitive to newcomers, and especially so with Perl, so you may want to reconsider your expectations as well.

Code coverage data are collected by replacing perl ops with functions which count how many times the ops are executed. These data are then mapped back to reality using the B compiler modules. There is also a statement profiling facility which should not be relied on. For proper profiling use Devel::NYTProf. Previous versions of Devel::Cover collected coverage data by replacing perl's runops function. It is still possible to switch to that mode of operation, but this now gets little testing and will probably be removed soon. You probably don't care about any of this.

The most appropriate mailing list on which to discuss this module would be perl-qa. Discussion has migrated there from perl-qa-metrics which is now defunct. See http://lists.perl.org/list/perl-qa.html.

The Devel::Cover repository can be found at http://github.com/pjcj/Devel--Cover.



  • Perl 5.6.1 or greater. Perl 5.8.8 or greater is recommended.

    Perl 5.7 is unsupported. Perl 5.8.8 or greater is recommended. Perl 5.8.7 has problems and may crash. Whilst Perl 5.6 should mostly work you will probably miss out on coverage information which would be available using a more modern version and will likely run into bugs in perl. Different versions of perl may give slightly different results due to changes in the op tree.

  • The ability to compile XS extensions.

    This means a working C compiler and make program at least. If you built perl from source you will have these already and they will be used automatically. If your perl was built in some other way, for example you may have installed it using your Operating System's packaging mechanism, you will need to ensure that the appropriate tools are installed.

  • Storable and Digest::MD5

    Both are in the core in Perl 5.8.0 and above.


Use with mod_perl

By adding use Devel::Cover; to your mod_perl startup script, you should be able to collect coverage information when running under mod_perl. You can also add any options you need at this point. I would suggest adding this as early as possible in your startup script in order to collect as much coverage information as possible.


 -blib               - "use blib" and ignore files matching \bt/ (default true
                       iff blib directory exists).
 -coverage criterion - Turn on coverage for the specified criterion.  Criteria
                       include statement, branch, condition, path, subroutine,
                       pod, time, all and none (default all available).
 -db cover_db        - Store results in coverage db (default ./cover_db).
 -dir path           - Directory in which coverage will be collected (default
 -ignore RE          - Set REs of files to ignore (default "/Devel/Cover\b").
 +ignore RE          - Append to REs of files to ignore.
 -inc path           - Set prefixes of files to ignore (default @INC).
 +inc path           - Append to prefixes of files to ignore.
 -merge val          - Merge databases, for multiple test benches (default on).
 -select RE          - Set REs of files to select (default none).
 +select RE          - Append to REs of files to select.
 -silent val         - Don't print informational messages (default off).
 -subs_only val      - Only cover code in subroutine bodies (default off).
 -replace_ops val    - Use op replacing rather than runops (default on).
 -summary val        - Print summary information iff val is true (default on).

More on Coverage Options

You can specify options to some coverage criteria. At the moment only pod coverage takes any options. These are the parameters which are passed into the Pod::Coverage constructor. The extra options are separated by dashes, and you may specify as many as you wish. For example, to specify that all subroutines containing xx are private, call Devel::Cover with the option -coverage,pod-also_private-xx.


You may select the files for which you want to collect coverage data using the select, ignore and inc options. The system uses the following procedure to decide whether a file will be included in coverage reports:

  • If the file matches a RE given as a select option, it will be included.

  • Otherwise, if it matches a RE given as an ignore option, it won't be included.

  • Otherwise, if it is in one of the inc directories, it won't be included.

  • Otherwise, it will be included.

You may add to the REs to select by using +select, or you may reset the selections using -select. The same principle applies to the REs to ignore.

The inc directories are initially populated with the contents of perl's @INC array. You may reset these directories using -inc, or add to them using +inc.

Although these options take regular expressions, you should not enclose the RE within // or any other quoting characters.

The options -coverage, [+-]select, [+-]ignore and [+-]inc can be specified multiple times, but they can also take multiple comma separated arguments. In any case you should not add a space after the comma, unless you want the argument to start with that literal space.


The -silent option is turned on when Devel::Cover is invoked via $HARNESS_PERL_SWITCHES or $PERL5OPT. Devel::Cover tries to do the right thing when $MOD_PERL is set. $DEVEL_COVER_OPTIONS is appended to any options passed into Devel::Cover.

When running Devel::Cover's own test suite, $DEVEL_COVER_DEBUG turns on debugging information, $DEVEL_COVER_GOLDEN_VERSION overrides Devel::Cover's own idea of which golden results it should test against, and $DEVEL_COVER_NO_COVERAGE runs the tests without collecting coverage. The environment variables described in this paragraph are of interest to Devel::Cover maintainers only.


Some code and ideas cribbed from:





There are things that Devel::Cover can't cover.

Absence of shared dependencies

Perl keeps track of which modules have been loaded (to avoid reloading them). Because of this, it isn't possible to get coverage for a path where a runtime import fails if the module being imported is one that Devel::Cover uses internally. For example, suppose your program has this function:

 sub foo {
     eval { require Storable };
     if ($@) {
         carp "Can't find Storable";
     # ...

You might write a test for the failure mode as

 BEGIN { @INC = () }
 # check for error message

Because Devel::Cover uses Storable internally, the import will succeed (and the test will fail) under a coverage run.

Modules used by Devel::Cover while gathering coverage:

  • B

  • B::Debug

  • B::Deparse

  • Carp

  • Cwd

  • Digest::MD5

  • File::Path

  • File::Spec

  • Storable

Redefined subroutines

If you redefine a subroutine you may find that the original subroutine is not reported on. This is because I haven't yet found a way to locate the original CV. Hints, tips or patches to resolve this will be gladly accepted.


Almost certainly.

See the BUGS file, the TODO file and the bug trackers at https://rt.cpan.org/Public/Dist/Display.html?Name=Devel-Cover and https://github.com/pjcj/Devel--Cover/issues?sort=created&direction=desc&state=open


Copyright 2001-2012, Paul Johnson (paul@pjcj.net)

This software is free. It is licensed under the same terms as Perl itself.

The latest version of this software should be available on CPAN and from my homepage: http://www.pjcj.net/.