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Author image Todd Rinaldo
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Pumpkin - Notes on handling the Perl Patch Pumpkin And Porting Perl


There is no simple synopsis, yet.


This document attempts to begin to describe some of the considerations involved in patching, porting, and maintaining perl.

This document is still under construction, and still subject to significant changes. Still, I hope parts of it will be useful, so I'm releasing it even though it's not done.

For the most part, it's a collection of anecdotal information that already assumes some familiarity with the Perl sources. I really need an introductory section that describes the organization of the sources and all the various auxiliary files that are part of the distribution.

Where Do I Get Perl Sources and Related Material?

The Comprehensive Perl Archive Network (or CPAN) is the place to go. There are many mirrors, but the easiest thing to use is probably http://www.cpan.org/README.html , which automatically points you to a mirror site "close" to you.

Perl5-porters mailing list

The mailing list perl5-porters@perl.org is the main group working with the development of perl. If you're interested in all the latest developments, you should definitely subscribe. The list is high volume, but generally has a fairly low noise level.

To subscribe to perl5-porters, send an email to


Archives of the list are held at:


How are Perl Releases Numbered?

Beginning with v5.6.0, even versions will stand for maintenance releases and odd versions for development releases, i.e., v5.6.x for maintenance releases, and v5.7.x for development releases. Before v5.6.0, subversions _01 through _49 were reserved for bug-fix maintenance releases, and subversions _50 through _99 for unstable development versions.

For example, in v5.6.1, the revision number is 5, the version is 6, and 1 is the subversion.

For compatibility with the older numbering scheme the composite floating point version number continues to be available as the magic variable $], and amounts to $revision + $version/1000 + $subversion/100000. This can still be used in comparisons.

        print "You've got an old perl\n" if $] < 5.005_03;

In addition, the version is also available as a string in $^V.

        print "You've got a new perl\n" if $^V and $^V ge v5.6.0;

You can also require particular version (or later) with:

        use 5.006;

or using the new syntax available only from v5.6 onward:

        use v5.6.0;

At some point in the future, we may need to decide what to call the next big revision. In the .package file used by metaconfig to generate Configure, there are two variables that might be relevant: $baserev=5 and $package=perl5.

Perl releases produced by the members of perl5-porters are usually available on CPAN in the src/5.0/maint and src/5.0/devel directories.

Maintenance and Development Subversions

The first rule of maintenance work is "First, do no harm."

Trial releases of bug-fix maintenance releases are announced on perl5-porters. Trial releases use the new subversion number (to avoid testers installing it over the previous release) and include a 'local patch' entry in patchlevel.h. The distribution file contains the string MAINT_TRIAL to make clear that the file is not meant for public consumption.

In general, the names of official distribution files for the public always match the regular expression:


$1 in the pattern is always an even number for maintenance versions, and odd for developer releases.

In the past, release managers sometimes invented naming conventions on the fly. If you are releasing perl, before you invent a new name for any of the three types of perl distributions, please inform the people from the CPAN who are doing indexing and provide the trees of symlinks and the like. They will have to know in advance what you decide.

Why is it called the patch pumpkin?

Chip Salzenberg gets credit for that, with a nod to his cow orker, David Croy. We had passed around various names (baton, token, hot potato) but none caught on. Then, Chip asked:

[begin quote]

   Who has the patch pumpkin?

To explain: David Croy once told me once that at a previous job, there was one tape drive and multiple systems that used it for backups. But instead of some high-tech exclusion software, they used a low-tech method to prevent multiple simultaneous backups: a stuffed pumpkin. No one was allowed to make backups unless they had the "backup pumpkin".

[end quote]

The name has stuck.

Philosophical Issues in Patching and Porting Perl

There are no absolute rules, but there are some general guidelines I have tried to follow as I apply patches to the perl sources. (This section is still under construction.)

Solve problems as generally as possible

Never implement a specific restricted solution to a problem when you can solve the same problem in a more general, flexible way.

For example, for dynamic loading to work on some SVR4 systems, we had to build a shared libperl.so library. In order to build "FAT" binaries on NeXT 4.0 systems, we had to build a special libperl library. Rather than continuing to build a contorted nest of special cases, I generalized the process of building libperl so that NeXT and SVR4 users could still get their work done, but others could build a shared libperl if they wanted to as well.

Contain your changes carefully. Assume nothing about other operating systems, not even closely related ones. Your changes must not affect other platforms.

Spy shamelessly on how similar patching or porting issues have been settled elsewhere.

If feasible, try to keep filenames 8.3-compliant to humor those poor souls that get joy from running Perl under such dire limitations. There's a script, check83.pl, for keeping your nose 8.3-clean. In a similar vein, do not create files or directories which differ only in case (upper versus lower).

Seek consensus on major changes

If you are making big changes, don't do it in secret. Discuss the ideas in advance on perl5-porters.

Keep the documentation up-to-date

If your changes may affect how users use perl, then check to be sure that the documentation is in sync with your changes. Be sure to check all the files pod/*.pod and also the INSTALL document.

Consider writing the appropriate documentation first and then implementing your change to correspond to the documentation.

Avoid machine-specific #ifdef's

To the extent reasonable, try to avoid machine-specific #ifdef's in the sources. Instead, use feature-specific #ifdef's. The reason is that the machine-specific #ifdef's may not be valid across major releases of the operating system. Further, the feature-specific tests may help out folks on another platform who have the same problem.

Machine-specific files

source code

If you have many machine-specific #defines or #includes, consider creating an "osish.h" (os2ish.h, vmsish.h, and so on) and including that in perl.h. If you have several machine-specific files (function emulations, function stubs, build utility wrappers) you may create a separate subdirectory (djgpp, win32) and put the files in there. Remember to update MANIFEST when you add files.

If your system supports dynamic loading but none of the existing methods at ext/DynaLoader/dl_*.xs work for you, you must write a new one. Study the existing ones to see what kind of interface you must supply.

build hints

There are two kinds of hints: hints for building Perl and hints for extensions. The former live in the hints subdirectory, the latter in ext/*/hints subdirectories.

The top level hints are Bourne-shell scripts that set, modify and unset appropriate Configure variables, based on the Configure command line options and possibly existing config.sh and Policy.sh files from previous Configure runs.

The extension hints are written in Perl (by the time they are used miniperl has been built) and control the building of their respective extensions. They can be used to for example manipulate compilation and linking flags.

build and installation Makefiles, scripts, and so forth

Sometimes you will also need to tweak the Perl build and installation procedure itself, like for example Makefile.SH and installperl. Tread very carefully, even more than usual. Contain your changes with utmost care.

test suite

Many of the tests in t subdirectory assume machine-specific things like existence of certain functions, something about filesystem semantics, certain external utilities and their error messages. Use the $^O and the Config module (which contains the results of the Configure run, in effect the config.sh converted to Perl) to either skip (preferably not) or customize (preferable) the tests for your platform.


Certain standard modules may need updating if your operating system sports for example a native filesystem naming. You may want to update some or all of the modules File::Basename, File::Spec, File::Path, and File::Copy to become aware of your native filesystem syntax and peculiarities.

Remember to have a $VERSION in the modules. You can use the Porting/checkVERSION.pl script for checking this.


If your operating system comes from outside UNIX you almost certainly will have differences in the available operating system functionality (missing system calls, different semantics, whatever). Please document these at pod/perlport.pod. If your operating system is the first not to have a system call also update the list of "portability-bewares" at the beginning of pod/perlfunc.pod.

A file called README.youros at the top level that explains things like how to install perl at this platform, where to get any possibly required additional software, and for example what test suite errors to expect, is nice too. Such files are in the process of being written in pod format and will eventually be renamed INSTALL.youros.

You may also want to write a separate .pod file for your operating system to tell about existing mailing lists, os-specific modules, documentation, whatever. Please name these along the lines of perlyouros.pod. [unfinished: where to put this file (the pod/ subdirectory, of course: but more importantly, which/what index files should be updated?)]

Allow for lots of testing

We should never release a main version without testing it as a subversion first.

We should never release a main version without testing whether or not it breaks various popular modules and applications. A partial list of such things would include majordomo, metaconfig, apache, Tk, CGI, libnet, and libwww, to name just a few. Of course it's quite possible that some of those things will be just plain broken and need to be fixed, but, in general, we ought to try to avoid breaking widely-installed things.

Automated generation of derivative files

The embed.h, keywords.h, opcode.h, regcharclass.h, l1_char_class_tab.h, and perltoc.pod files are all automatically generated by perl scripts. In general, don't patch these directly; patch the data files instead.

Also Makefile is automatically produced from Makefile.SH. In general, look out for all *.SH files.

Finally, the sample files config.sh and config_H in the Porting/ subdirectory are generated by the script Porting/mksample.

Files generated by metaconfig

Configure, config_h.SH and Porting/Glossary are generated by metaconfig (see below for more information on how to use this system) and direct changes to these files should in general not be pushed to blead.

The exceptions are:

  • security fixes

  • changes pre-approved by the metaconfig maintainers

Such changes should also be notified to the metaconfig maintainers by creating an issue at <https://github.com/Perl/metaconfig/issues>.

Alternatively, do consider if the *ish.h files or the hint files might be a better place for your changes.

Working with metaconfig

Information about how to use metaconfig can be found in the README and README_U files in the metaconfig repository containing Perl's metaconfig units:

        # anonymous clone
        git clone https://github.com/Perl/metaconfig.git
        # or using a registered github.com identity with ssh
        git clone github.com:Perl/metaconfig.git

Since metaconfig is hard to change, running correction scripts after this generation is sometimes needed. Configure gained complexity over time, and the order in which config_h.SH is generated can cause havoc when compiling perl. Therefor, you need to run Porting/config_h.pl after that generation. All that and more is described in the README files that come with the metaunits.

How to Make a Distribution

This section has now been expanded and moved into its own file, Porting/release_managers_guide.pod.

I've kept some of the subsections here for now, as they don't directly relate to building a release any more, but still contain what might be useful information - DAPM 7/2009.


If you are using metaconfig to regenerate Configure, then you should note that metaconfig actually uses MANIFEST.new, so you want to be sure MANIFEST.new is up-to-date too. I haven't found the MANIFEST/MANIFEST.new distinction particularly useful, but that's probably because I still haven't learned how to use the full suite of tools in the dist distribution.

Run Configure

This will build a config.sh and config.h. You can skip this if you haven't changed Configure or config_h.SH at all. I use the following command

    sh Configure -Dprefix=/opt/perl -Doptimize=-O -Dusethreads \
        -Dcf_by='yourname' \
        -Dcf_email='yourname@yourhost.yourplace.com' \
        -Dperladmin='yourname@yourhost.yourplace.com' \
        -Dmydomain='.yourplace.com' \
        -Dmyhostname='yourhost' \

Update Porting/config.sh and Porting/config_H

[XXX This section needs revision. We're currently working on easing the task of keeping the vms, win32, and plan9 config.sh info up-to-date. The plan is to use keep up-to-date 'canned' config.sh files in the appropriate subdirectories and then generate 'canned' config.h files for vms, win32, etc. from the generic config.sh file. This is to ease maintenance. When Configure gets updated, the parts sometimes get scrambled around, and the changes in config_H can sometimes be very hard to follow. config.sh, on the other hand, can safely be sorted, so it's easy to track (typically very small) changes to config.sh and then propagate them to a canned 'config.h' by any number of means, including a perl script in win32/ or carrying config.sh and config_h.SH to a Unix system and running sh config_h.SH.) Vms uses configure.com to generate its own config.sh and config.h. If you want to add a new variable to config.sh check with vms folk how to add it to configure.com too. XXX]

The Porting/config.sh and Porting/config_H files are provided to help those folks who can't run Configure. It is important to keep them up-to-date. If you have changed config_h.SH, those changes must be reflected in config_H as well. (The name config_H was chosen to distinguish the file from config.h even on case-insensitive file systems.) Simply edit the existing config_H file; keep the first few explanatory lines and then copy your new config.h below.

It may also be necessary to update win32/config.?c, and plan9/config.plan9, though you should be quite careful in doing so if you are not familiar with those systems. You might want to issue your patch with a promise to quickly issue a follow-up that handles those directories.

make regen_perly

If perly.y has been edited, it is necessary to run this target to rebuild perly.h, perly.act and perly.tab. In fact this target just runs the Perl script regen_perly.pl. Note that perly.c is not rebuilt; this is just a plain static file now.

This target relies on you having Bison installed on your system. Running the target will tell you if you haven't got the right version, and if so, where to get the right one. Or if you prefer, you could hack regen_perly.pl to work with your version of Bison. The important things are that the regexes can still extract out the right chunks of the Bison output into perly.act and perly.tab, and that the contents of those two files, plus perly.h, are functionally equivalent to those produced by the supported version of Bison.

Note that in the old days, you had to do make run_byacc instead.

make regen_all

This target takes care of the regen_headers target. (It used to also call the regen_pods target, but that has been eliminated.)

make regen_headers

The embed.h, keywords.h, and opcode.h files are all automatically generated by perl scripts. Since the user isn't guaranteed to have a working perl, we can't require the user to generate them. Hence you have to, if you're making a distribution.

I used to include rules like the following in the makefile:

    # The following three header files are generated automatically
    # The correct versions should be already supplied with the perl kit,
    # in case you don't have perl or 'sh' available.
    # The - is to ignore error return codes in case you have the source
    # installed read-only or you don't have perl yet.
    keywords.h: keywords.pl
            @echo "Don't worry if this fails."
            - perl keywords.pl

However, I got lots of mail consisting of people worrying because the command failed. I eventually decided that I would save myself time and effort by manually running make regen_headers myself rather than answering all the questions and complaints about the failing command.

globvar.sym, and perlio.sym

Make sure these files are up-to-date. Read the comments in these files and in perl_exp.SH to see what to do.

Binary compatibility

If you do change embed.fnc think carefully about what you are doing. To the extent reasonable, we'd like to maintain source and binary compatibility with older releases of perl. That way, extensions built under one version of perl will continue to work with new versions of perl.

Of course, some incompatible changes may well be necessary. I'm just suggesting that we not make any such changes without thinking carefully about them first. If possible, we should provide backwards-compatibility stubs. There's a lot of XS code out there. Let's not force people to keep changing it.


dist/Devel-PPPort/PPPort.pm needs to be synchronized to include all new macros added to .h files (normally perl.h and XSUB.h, but others as well). Since chances are that when a new macro is added the committer will forget to update PPPort.pm, it's the best to diff for changes in .h files when making a new release and making sure that PPPort.pm contains them all.

The Steering Council can delegate the synchronization responsibility to anybody else, but the release process is the only place where we can make sure that no new macros fell through the cracks.


The Porting/todo.pod file contains a roughly-categorized unordered list of aspects of Perl that could use enhancement, features that could be added, areas that could be cleaned up, and so on. During your term as pumpkin-holder, you will probably address some of these issues, and perhaps identify others which, while you decide not to address them this time around, may be tackled in the future. Update the file to reflect the situation as it stands when you hand over the pumpkin.

You might like, early in your pumpkin-holding career, to see if you can find champions for particular issues on the to-do list: an issue owned is an issue more likely to be resolved.

There are also some more porting-specific "Todo" items later in this file.

OS/2-specific updates

In the os2 directory is diff.configure, a set of OS/2-specific diffs against Configure. If you make changes to Configure, you may want to consider regenerating this diff file to save trouble for the OS/2 maintainer.

You can also consider the OS/2 diffs as reminders of portability things that need to be fixed in Configure.

VMS-specific updates

The Perl revision number appears as "perl5" in configure.com. It is courteous to update that if necessary.

Making a new patch

I find the makepatch utility quite handy for making patches. You can obtain it from any CPAN archive under https://www.cpan.org/authors/id/J/JV/JV/. There are a couple of differences between my version and the standard one. I have mine do a

        # Print a reassuring "End of Patch" note so people won't
        # wonder if their mailer truncated patches.
        print "\n\nEnd of Patch.\n";

at the end. That's because I used to get questions from people asking if their mail was truncated.

It also writes Index: lines which include the new directory prefix (change Index: print, approx line 294 or 310 depending on the version, to read: print PATCH ("Index: $newdir$new\n");). That helps patches work with more POSIX conformant patch programs.

Here's how I generate a new patch. I'll use the hypothetical 5.004_07 to 5.004_08 patch as an example.

        # unpack perl5.004_07/
        gzip -d -c perl5.004_07.tar.gz | tar -xof -
        # unpack perl5.004_08/
        gzip -d -c perl5.004_08.tar.gz | tar -xof -
        makepatch perl5.004_07 perl5.004_08 > perl5.004_08.pat

Makepatch will automatically generate appropriate rm commands to remove deleted files. Unfortunately, it will not correctly set permissions for newly created files, so you may have to do so manually. For example, patch 5.003_04 created a new test t/op/gv.t which needs to be executable, so at the top of the patch, I inserted the following lines:

        # Make a new test
        touch t/op/gv.t
        chmod +x t/opt/gv.t

Now, of course, my patch is now wrong because makepatch didn't know I was going to do that command, and it patched against /dev/null.

So, what I do is sort out all such shell commands that need to be in the patch (including possible mv-ing of files, if needed) and put that in the shell commands at the top of the patch. Next, I delete all the patch parts of perl5.004_08.pat, leaving just the shell commands. Then, I do the following:

        cd perl5.004_07
        sh ../perl5.004_08.pat
        cd ..
        makepatch perl5.004_07 perl5.004_08 >> perl5.004_08.pat

(Note the append to preserve my shell commands.) Now, my patch will line up with what the end users are going to do.

Testing your patch

It seems obvious, but be sure to test your patch. That is, verify that it produces exactly the same thing as your full distribution.

        rm -rf perl5.004_07
        gzip -d -c perl5.004_07.tar.gz | tar -xf -
        cd perl5.004_07
        sh ../perl5.004_08.pat
        patch -p1 -N < ../perl5.004_08.pat
        cd ..
        gdiff -r perl5.004_07 perl5.004_08

where gdiff is GNU diff. Other diff's may also do recursive checking.

More testing

Again, it's obvious, but you should test your new version as widely as you can. You can be sure you'll hear about it quickly if your version doesn't work on both ANSI and pre-ANSI compilers, and on common systems such as SunOS 4.1.[34], Solaris, and Linux.

If your changes include conditional code, try to test the different branches as thoroughly as you can. For example, if your system supports dynamic loading, you can also test static loading with

        sh Configure -Uusedl

You can also hand-tweak your config.h to try out different #ifdef branches.

Other tests

gcc -ansi -pedantic

Configure -Dgccansipedantic [ -Dcc=gcc ] will enable (via the cflags script, not $Config{ccflags}) the gcc strict ANSI C flags -ansi and -pedantic for the compilation of the core files on platforms where it knows it can do so (like Linux, see cflags.SH for the full list), and on some platforms only one (Solaris can do only -pedantic, not -ansi). The flag -DPERL_GCC_PEDANTIC also gets added, since gcc does not add any internal cpp flag to signify that -pedantic is being used, as it does for -ansi (__STRICT_ANSI__).

Note that the -ansi and -pedantic are enabled only for version 3 (and later) of gcc, since even gcc version 2.95.4 finds lots of seemingly false "value computed not used" errors from Perl.

The -ansi and -pedantic are useful in catching at least the following nonportable practices:

  • gcc-specific extensions

  • lvalue casts

  • // C++ comments

  • enum trailing commas

The -Dgccansipedantic should be used only when cleaning up the code, not for production builds, since otherwise gcc cannot inline certain things.

Common Gotchas

Probably Prefer POSIX

It's often the case that you'll need to choose whether to do something the BSD-ish way or the POSIX-ish way. It's usually not a big problem when the two systems use different names for similar functions, such as memcmp() and bcmp(). The perl.h header file handles these by appropriate #defines, selecting the POSIX mem*() functions if available, but falling back on the b*() functions, if need be.

More serious is the case where some brilliant person decided to use the same function name but give it a different meaning or calling sequence :-). getpgrp() and setpgrp() come to mind. These are a real problem on systems that aim for conformance to one standard (e.g. POSIX), but still try to support the other way of doing things (e.g. BSD). My general advice (still not really implemented in the source) is to do something like the following. Suppose there are two alternative versions, fooPOSIX() and fooBSD().

    #ifdef HAS_FOOPOSIX
        /* use fooPOSIX(); */
    #  ifdef HAS_FOOBSD
        /* try to emulate fooPOSIX() with fooBSD();
           perhaps with the following:  */
    #    define fooPOSIX fooBSD
    #  else
    #  /* Uh, oh.  We have to supply our own. */
    #    define fooPOSIX Perl_fooPOSIX
    #  endif
Think positively

If you need to add an #ifdef test, it is usually easier to follow if you think positively, e.g.

        #ifdef HAS_NEATO_FEATURE
            /* use neato feature */
            /* use some fallback mechanism */

rather than the more impenetrable

            /* Not missing it, so we must have it, so use it */
            /* Are missing it, so fall back on something else. */

Of course for this toy example, there's not much difference. But when the #ifdef's start spanning a couple of screen fulls, and the #else's are marked something like

        #else /* !MISSING_NEATO_FEATURE */

I find it easy to get lost.

Providing Missing Functions -- Problem

Not all systems have all the neat functions you might want or need, so you might decide to be helpful and provide an emulation. This is sound in theory and very kind of you, but please be careful about what you name the function. Let me use the pause() function as an illustration.

Perl5.003 has the following in perl.h

    #ifndef HAS_PAUSE
    #define pause() sleep((32767<<16)+32767)

Configure sets HAS_PAUSE if the system has the pause() function, so this #define only kicks in if the pause() function is missing. Nice idea, right?

Unfortunately, some systems apparently have a prototype for pause() in unistd.h, but don't actually have the function in the library. (Or maybe they do have it in a library we're not using.)

Thus, the compiler sees something like

    extern int pause(void);
    /* . . . */
    #define pause() sleep((32767<<16)+32767)

and dies with an error message. (Some compilers don't mind this; others apparently do.)

To work around this, 5.003_03 and later have the following in perl.h:

    /* Some unistd.h's give a prototype for pause() even though
       HAS_PAUSE ends up undefined.  This causes the #define
       below to be rejected by the compiler.  Sigh.
    #ifdef HAS_PAUSE
    #  define Pause     pause
    #  define Pause() sleep((32767<<16)+32767)

This works.

The curious reader may wonder why I didn't do the following in util.c instead:

    #ifndef HAS_PAUSE
    void pause()

That is, since the function is missing, just provide it. Then things would probably be been alright, it would seem.

Well, almost. It could be made to work. The problem arises from the conflicting needs of dynamic loading and namespace protection.

For dynamic loading to work on AIX (and VMS) we need to provide a list of symbols to be exported. This is done by the script perl_exp.SH, which reads embed.fnc. Thus, the pause symbol would have to be added to embed.fnc So far, so good.

On the other hand, one of the goals of Perl5 is to make it easy to either extend or embed perl and link it with other libraries. This means we have to be careful to keep the visible namespace "clean". That is, we don't want perl's global variables to conflict with those in the other application library. Although this work is still in progress, the way it is currently done is via the embed.h file. This file is built from the embed.fnc file, since those files already list the globally visible symbols. If we had added pause to embed.fnc, then embed.h would contain the line

    #define pause       Perl_pause

and calls to pause in the perl sources would now point to Perl_pause. Now, when ld is run to build the perl executable, it will go looking for perl_pause, which probably won't exist in any of the standard libraries. Thus the build of perl will fail.

Those systems where HAS_PAUSE is not defined would be ok, however, since they would get a Perl_pause function in util.c. The rest of the world would be in trouble.

And yes, this scenario has happened. On SCO, the function chsize is available. (I think it's in -lx, the Xenix compatibility library.) Since the perl4 days (and possibly before), Perl has included a chsize function that gets called something akin to

    #ifndef HAS_CHSIZE
    I32 chsize(fd, length)
    /*  . . . */

When 5.003 added

    #define chsize      Perl_chsize

to embed.h, the compile started failing on SCO systems.

The "fix" is to give the function a different name. The one implemented in 5.003_05 isn't optimal, but here's what was done:

    #ifdef HAS_CHSIZE
    # ifdef my_chsize      /* Probably #defined to Perl_my_chsize */
    #   undef my_chsize    /* in embed.h */
    # endif
    # define my_chsize chsize

My explanatory comment in patch 5.003_05 said:

    Undef and then re-define my_chsize from Perl_my_chsize to
    just plain chsize if this system HAS_CHSIZE.  This probably only
    applies to SCO.  This shows the perils of having internal
    functions with the same name as external library functions :-).

Now, we can safely put my_chsize in embed.fnc, export it, and hide it with embed.h.

To be consistent with what I did for pause, I probably should have called the new function Chsize, rather than my_chsize. However, the perl sources are quite inconsistent on this (Consider New, Mymalloc, and Myremalloc, to name just a few.)

There is a problem with this fix, however, in that Perl_chsize was available as a libperl.a library function in 5.003, but it isn't available any more (as of 5.003_07). This means that we've broken binary compatibility. This is not good.

Providing missing functions -- some ideas

We currently don't have a standard way of handling such missing function names. Right now, I'm effectively thinking aloud about a solution. Some day, I'll try to formally propose a solution.

Part of the problem is that we want to have some functions listed as exported but not have their names mangled by embed.h or possibly conflict with names in standard system headers. We actually already have such a list at the end of perl_exp.SH (though that list is out-of-date):

    # extra globals not included above.
    cat <<END >> perl.exp

This still needs much thought, but I'm inclined to think that one possible solution is to prefix all such functions with perl_ in the source and list them along with the other perl_* functions in perl_exp.SH.

Thus, for chsize, we'd do something like the following:

    /* in perl.h */
    #ifdef HAS_CHSIZE
    #  define perl_chsize chsize

then in some file (e.g. util.c or doio.c) do

    #ifndef HAS_CHSIZE
    I32 perl_chsize(fd, length)
    /* implement the function here . . . */

Alternatively, we could just always use chsize everywhere and move chsize from embed.fnc to the end of perl_exp.SH. That would probably be fine as long as our chsize function agreed with all the chsize function prototypes in the various systems we'll be using. As long as the prototypes in actual use don't vary that much, this is probably a good alternative. (As a counter-example, note how Configure and perl have to go through hoops to find and use get Malloc_t and Free_t for malloc and free.)

At the moment, this latter option is what I tend to prefer.

All the world's a VAX

Sorry, showing my age:-). Still, all the world is not BSD 4.[34], SVR4, or POSIX. Be aware that SVR3-derived systems are still quite common (do you have any idea how many systems run SCO?) If you don't have a bunch of v7 manuals handy, the metaconfig units (by default installed in /usr/local/lib/dist/U) are a good resource to look at for portability.

Miscellaneous Topics


Why does perl use a metaconfig-generated Configure script instead of an autoconf-generated configure script?

Metaconfig and autoconf are two tools with very similar purposes. Metaconfig is actually the older of the two, and was originally written by Larry Wall, while autoconf is probably now used in a wider variety of packages. The autoconf info file discusses the history of autoconf and how it came to be. The curious reader is referred there for further information.

Overall, both tools are quite good, I think, and the choice of which one to use could be argued either way. In March, 1994, when I was just starting to work on Configure support for Perl5, I considered both autoconf and metaconfig, and eventually decided to use metaconfig for the following reasons:

Compatibility with Perl4

Perl4 used metaconfig, so many of the #ifdef's were already set up for metaconfig. Of course metaconfig had evolved some since Perl4's days, but not so much that it posed any serious problems.

Metaconfig worked for me

My system at the time was Interactive 2.2, an SVR3.2/386 derivative that also had some POSIX support. Metaconfig-generated Configure scripts worked fine for me on that system. On the other hand, autoconf-generated scripts usually didn't. (They did come quite close, though, in some cases.) At the time, I actually fetched a large number of GNU packages and checked. Not a single one configured and compiled correctly out-of-the-box with the system's cc compiler.

Configure can be interactive

With both autoconf and metaconfig, if the script works, everything is fine. However, one of my main problems with autoconf-generated scripts was that if it guessed wrong about something, it could be very hard to go back and fix it. For example, autoconf always insisted on passing the -Xp flag to cc (to turn on POSIX behavior), even when that wasn't what I wanted or needed for that package. There was no way short of editing the configure script to turn this off. You couldn't just edit the resulting Makefile at the end because the -Xp flag influenced a number of other configure tests.

Metaconfig's Configure scripts, on the other hand, can be interactive. Thus if Configure is guessing things incorrectly, you can go back and fix them. This isn't as important now as it was when we were actively developing Configure support for new features such as dynamic loading, but it's still useful occasionally.


At the time, autoconf-generated scripts were covered under the GNU Public License, and hence weren't suitable for inclusion with Perl, which has a different licensing policy. (Autoconf's licensing has since changed.)


Metaconfig builds up Configure from a collection of discrete pieces called "units". You can override the standard behavior by supplying your own unit. With autoconf, you have to patch the standard files instead. I find the metaconfig "unit" method easier to work with. Others may find metaconfig's units clumsy to work with.

Why isn't there a directory to override Perl's library?

Mainly because no one's gotten around to making one. Note that "making one" involves changing perl.c, Configure, config_h.SH (and associated files, see above), and documenting it all in the INSTALL file.

Apparently, most folks who want to override one of the standard library files simply do it by overwriting the standard library files.


In the perl.c sources, you'll find an undocumented APPLLIB_EXP variable, sort of like PRIVLIB_EXP and ARCHLIB_EXP (which are documented in config_h.SH). Here's what APPLLIB_EXP is for, from a mail message from Larry:

    The main intent of APPLLIB_EXP is for folks who want to send out a
    version of Perl embedded in their product.  They would set the
    symbol to be the name of the library containing the files needed
    to run or to support their particular application.  This works at
    the "override" level to make sure they get their own versions of
    any library code that they absolutely must have configuration
    control over.

    As such, I don't see any conflict with a sysadmin using it for a
    override-ish sort of thing, when installing a generic Perl.  It
    should probably have been named something to do with overriding
    though.  Since it's undocumented we could still change it...  :-)

Given that it's already there, you can use it to override distribution modules. One way to do that is to add

        ccflags="$ccflags -DAPPLLIB_EXP=\"/my/override\""

to your config.over file. (You have to be particularly careful to get the double quotes in. APPLLIB_EXP must be a valid C string. It might actually be easier to just #define it yourself in perl.c.)

Then perl.c will put /my/override ahead of ARCHLIB and PRIVLIB. Perl will also search architecture-specific and version-specific subdirectories of APPLLIB_EXP.

Shared libperl.so location

Why isn't the shared libperl.so installed in /usr/lib/ along with "all the other" shared libraries? Instead, it is installed in $archlib, which is typically something like


and is architecture- and version-specific.

The basic reason why a shared libperl.so gets put in $archlib is so that you can have more than one version of perl on the system at the same time, and have each refer to its own libperl.so.

Three examples might help. All of these work now; none would work if you put libperl.so in /usr/lib.

  1. Suppose you want to have both threaded and non-threaded perl versions around. Configure will name both perl libraries "libperl.so" (so that you can link to them with -lperl). The perl binaries tell them apart by having looking in the appropriate $archlib directories.

  2. Suppose you have perl5.004_04 installed and you want to try to compile it again, perhaps with different options or after applying a patch. If you already have libperl.so installed in /usr/lib/, then it may be either difficult or impossible to get ld.so to find the new libperl.so that you're trying to build. If, instead, libperl.so is tucked away in $archlib, then you can always just change $archlib in the current perl you're trying to build so that ld.so won't find your old libperl.so. (The INSTALL file suggests you do this when building a debugging perl.)

  3. The shared perl library is not a "well-behaved" shared library with proper major and minor version numbers, so you can't necessarily have perl5.004_04 and perl5.004_05 installed simultaneously. Suppose perl5.004_04 were to install /usr/lib/libperl.so.4.4, and perl5.004_05 were to install /usr/lib/libperl.so.4.5. Now, when you try to run perl5.004_04, ld.so might try to load libperl.so.4.5, since it has the right "major version" number. If this works at all, it almost certainly defeats the reason for keeping perl5.004_04 around. Worse, with development subversions, you certainly can't guarantee that libperl.so.4.4 and libperl.so.4.55 will be compatible.

    Anyway, all this leads to quite obscure failures that are sure to drive casual users crazy. Even experienced users will get confused :-). Upon reflection, I'd say leave libperl.so in $archlib.

Indentation style

Over the years Perl has become a mishmash of various indentation styles, but the original "Larry style" can probably be restored with (GNU) indent somewhat like this:

    indent -kr -nce -psl -sc

A more ambitious solution would also specify a list of Perl specific types with -TSV -TAV -THV .. -TMAGIC -TPerlIO ... but that list would be quite ungainly. Also note that GNU indent also doesn't do aligning of consecutive assignments, which would truly wreck the layout in places like sv.c:Perl_sv_upgrade() or sv.c:Perl_clone_using(). Similarly nicely aligned &&s, ||s and ==s would not be respected.

Upload Your Work to CPAN

You can upload your work to CPAN if you have a CPAN id. Check out http://www.cpan.org/modules/04pause.html for information on _PAUSE_, the Perl Author's Upload Server.

I typically upload both the patch file, e.g. perl5.004_08.pat.gz and the full tar file, e.g. perl5.004_08.tar.gz.

If you want your patch to appear in the src/5.0/unsupported directory on CPAN, send e-mail to the CPAN master librarian. (Check out http://www.cpan.org/CPAN.html ).

Help Save the World

You should definitely announce your patch on the perl5-porters list.


Here, in no particular order, are some Configure and build-related items that merit consideration. This list isn't exhaustive, it's just what I came up with off the top of my head.

Adding missing library functions to Perl

The perl Configure script automatically determines which headers and functions you have available on your system and arranges for them to be included in the compilation and linking process. Occasionally, when porting perl to an operating system for the first time, you may find that the operating system is missing a key function. While perl may still build without this function, no perl program will be able to reference the missing function. You may be able to write the missing function yourself, or you may be able to find the missing function in the distribution files for another software package. In this case, you need to instruct the perl configure-and-build process to use your function. Perform these steps.

  • Code and test the function you wish to add. Test it carefully; you will have a much easier time debugging your code independently than when it is a part of perl.

  • Here is an implementation of the POSIX truncate function for an operating system (VOS) that does not supply one, but which does supply the ftruncate() function.

      /* Beginning of modification history */
      /* Written 02-01-02 by Nick Ing-Simmons (nick@ing-simmons.net) */
      /* End of modification history */
      /* VOS doesn't supply a truncate function, so we build one up
         from the available POSIX functions.  */
      #include <fcntl.h>
      #include <sys/types.h>
      #include <unistd.h>
      truncate(const char *path, off_t len)
       int fd = open(path,O_WRONLY);
       int code = -1;
       if (fd >= 0) {
         code = ftruncate(fd,len);
       return code;

    Place this file into a subdirectory that has the same name as the operating system. This file is named perl/vos/vos.c

  • If your operating system has a hints file (in perl/hints/XXX.sh for an operating system named XXX), then start with it. If your operating system has no hints file, then create one. You can use a hints file for a similar operating system, if one exists, as a template.

  • Add lines like the following to your hints file. The first line (d_truncate="define") instructs Configure that the truncate() function exists. The second line (archobjs="vos.o") instructs the makefiles that the perl executable depends on the existence of a file named "vos.o". (Make will automatically look for "vos.c" and compile it with the same options as the perl source code). The final line ("test -h...") adds a symbolic link to the top-level directory so that make can find vos.c. Of course, you should use your own operating system name for the source file of extensions, not "vos.c".

      # VOS does not have truncate() but we supply one in vos.c
      # Help gmake find vos.c
      test -h vos.c || ln -s vos/vos.c vos.c

    The hints file is a series of shell commands that are run in the top-level directory (the "perl" directory). Thus, these commands are simply executed by Configure at an appropriate place during its execution.

  • At this point, you can run the Configure script and rebuild perl. Carefully test the newly-built perl to ensure that normal paths, and error paths, behave as you expect.

Good ideas waiting for round tuits

Configure -Dsrc=/blah/blah

We should be able to emulate configure --srcdir. Tom Tromey tromey@creche.cygnus.com has submitted some patches to the dist-users mailing list along these lines. They have been folded back into the main distribution, but various parts of the perl Configure/build/install process still assume src='.'.

Hint file fixes

Various hint files work around Configure problems. We ought to fix Configure so that most of them aren't needed.

Hint file information

Some of the hint file information (particularly dynamic loading stuff) ought to be fed back into the main metaconfig distribution.

Probably good ideas waiting for round tuits

GNU configure --options

I've received sensible suggestions for --exec_prefix and other GNU configure --options. It's not always obvious exactly what is intended, but this merits investigation.

Try gcc if cc fails

Currently, we just give up.

bypassing safe*alloc wrappers

On some systems, it may be safe to call the system malloc directly without going through the util.c safe* layers. (Such systems would accept free(0), for example.) This might be a time-saver for systems that already have a good malloc. (Recent Linux libc's apparently have a nice malloc that is well-tuned for the system.)

Vague possibilities

gconvert replacement

Maybe include a replacement function that doesn't lose data in rare cases of coercion between string and numerical values.

Improve makedepend

The current makedepend process is clunky and annoyingly slow, but it works for most folks. Alas, it assumes that there is a filename $firstmakefile that the make command will try to use before it uses Makefile. Such may not be the case for all make commands, particularly those on non-Unix systems.

Probably some variant of the BSD .depend file will be useful. We ought to check how other packages do this, if they do it at all. We could probably pre-generate the dependencies (with the exception of malloc.o, which could probably be determined at Makefile.SH extraction time.

GNU Makefile standard targets

GNU software generally has standardized Makefile targets. Unless we have good reason to do otherwise, I see no reason not to support them.

File locking

Somehow, straighten out, document, and implement lockf(), flock(), and/or fcntl() file locking. It's a mess. See $d_fcntl_can_lock in recent config.sh files though.

The following is based on the consensus of a couple of IPR lawyers, but it is of course not a legally binding statement, just a common sense summary.

  • Tacking on copyright statements is unnecessary to begin with because of the Berne convention. But assuming you want to go ahead...

  • The right form of a copyright statement is

            Copyright (C) Year, Year, ... by Someone

    The (C) is not required everywhere but it doesn't hurt and in certain jurisdictions it is required, so let's leave it in. (Yes, it's true that in some jurisdictions the "(C)" is not legally binding, one should use the true ringed-C. But we don't have that character available for Perl's source code.)

    The years must be listed out separately. Year-Year is not correct. Only the years when the piece has changed 'significantly' may be added.

  • One cannot give away one's copyright trivially. One can give one's copyright away by using public domain, but even that requires a little bit more than just saying 'this is in public domain'. (What it exactly requires depends on your jurisdiction.) But barring public domain, one cannot "transfer" one's copyright to another person or entity. In the context of software, it means that contributors cannot give away their copyright or "transfer" it to the "owner" of the software.

    Also remember that in many cases if you are employed by someone, your work may be copyrighted to your employer, even when you are contributing on your own time (this all depends on too many things to list here). But the bottom line is that you definitely can't give away a copyright you may not even have.

    What is possible, however, is that the software can simply state

            Copyright (C) Year, Year, ... by Someone and others

    and then list the "others" somewhere in the distribution. And this is exactly what Perl does. (The "somewhere" is AUTHORS and the Changes* files.)

  • Split files, merged files, and generated files are problematic. The rule of thumb: in split files, copy the copyright years of the original file to all the new files; in merged files make an union of the copyright years of all the old files; in generated files propagate the copyright years of the generating file(s).

  • The files of Perl source code distribution do carry a lot of copyrights, by various people. (There are many copyrights embedded in perl.c, for example.) The most straightforward thing for perl releasers to do is to simply update Larry's copyrights at the beginning of the *.[hcy], *.pl, and README files, and leave all other copyrights alone. Doing more than that requires quite a bit of tracking.


Original author: Andy Dougherty doughera@lafayette.edu . Additions by Chip Salzenberg chip@perl.com, Tim Bunce and the perl5 development team.

All opinions expressed herein are those of the author(s).


2017-10-13 Dominic Hargreaves