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3.1 Configuring the Source

Before building and installing mod_perl you will have to configure it, as you would configure any other Perl module:

panic% perl Makefile.PL [parameters].

Perl Installation Requirements

Make sure you have Perl installed! Use the latest stable version, if possible. To determine your version of Perl, run the following command on the command line:

panic% perl -v

You will need at least Perl Version 5.004. If you don't have it, install it. Follow the instructions in the distribution's INSTALL file. The only thing to watch for is that during the configuration stage (while running ./Configure) you make sure you can dynamically load Perl module extensions. That is, answer YES to the following question:

Do you wish to use dynamic loading? [y]

In this section, we will explain each of the parameters accepted by the Makefile.PL file for mod_perl First, however, lets talk about how the mod_perl configuration dovetails with Apache's configuration. The source configuration mechanism in Apache 1.3 provides four major features (which of course are available to mod_perl):

  • Apache modules can use per-module configuration scripts to link themselves into the Apache configuration process. This feature lets you automatically adjust the configuration and build parameters from the Apache module sources. It is triggered by ConfigStart/ConfigEnd sections inside modulename.module files (e.g., see the file libperl.module in the mod_perl distribution).

  • The APache AutoConf-style Interface (APACI) is the top-level configure script from Apache 1.3; it provides a GNU Autoconf-style interface to the Apache configuration process. APACI is useful for configuring the source tree without manually editing any src/Configuration files. Any parameterization can be done via command-line options to the configure script. Internally, this is just a nifty wrapper over the old src/Configure script.

    Since Apache 1.3, APACI is the best way to install mod_perl as cleanly as possible. However, the complete Apache 1.3 source configuration mechanism is available only under Unix at this writing—it doesn't work on Win32.

  • Dynamic shared object (DSO) support is one of the most interesting features in Apache 1.3. It allows Apache modules to be built as so-called DSOs (usually named, which can be loaded via the LoadModule directive in Apache's httpd.conf file. The benefit is that the modules become part of the httpd executable only on demand; they aren't loaded into the address space of the httpd executable until the user asks for them to be. The benefits of DSO support are most evident in relation to memory consumption and added flexibility (in that you won't have to recompile your httpd each time you want to add, remove, or upgrade a module).

    The DSO mechanism is provided by Apache's mod_so module, which needs to be compiled into the httpd binary with:

    panic% ./configure --enable-module=so

    The usage of any —enable-shared option automatically implies an —enable-module=so option, because the bootstrapping module mod_so is always needed for DSO support. So if, for example, you want the module mod_dir to be built as a DSO, you can write:

    panic% ./configure --enable-shared=dir

    and the DSO support will be added automatically.

  • The APache eXtension Support tool (APXS) is a tool from Apache 1.3 that can be used to build an Apache module as a DSO even outside the Apache source tree. APXS is to Apache what MakeMaker and XS are to Perl.[1] It knows the platform-dependent build parameters for making DSO files and provides an easy way to run the build commands with them.

    [1] MakeMaker allows easy, automatic configuration, building, testing, and installation of Perl modules, while XS allows you to call functions implemented in C/C++ from Perl code.

Pros and Cons of Building mod_perl as a DSO

As of Apache 1.3, the configuration system supports two optional features for taking advantage of the modular DSO approach: compilation of the Apache core program into a DSO library for shared usage, and compilation of the Apache modules into DSO files for explicit loading at runtime.

Should you build mod_perl as a DSO? Let's study the pros and cons of this installation method, so you can decide for yourself.


  • The server package is more flexible because the actual server executable can be assembled at runtime via LoadModule configuration commands in httpd.conf instead of via AddModule commands in the Configuration file at build time. This allows you to run different server instances (e.g., standard and SSL servers, or servers with and without mod_perl) with only one Apache installation; the only thing you need is different configuration files (or, by judicious use of IfDefine, different startup scripts).

  • The server package can easily be extended with third-party modules even after installation. This is especially helpful for vendor package maintainers who can create an Apache core package and additional packages containing extensions such as PHP, mod_perl, mod_fastcgi, etc.

  • DSO support allows easier Apache module prototyping, because with the DSO/APXS pair you can work outside the Apache source tree and need only an apxs -i command followed by an apachectl restart to bring a new version of your currently developed module into the running Apache server.


  • The DSO mechanism cannot be used on every platform, because not all operating systems support shared libraries.

  • The server starts up approximately 20% slower because of the overhead of the symbol-resolving the Unix loader now has to do.

  • The server runs approximately 5% slower on some platforms, because position-independent code (PIC) sometimes needs complicated assembler tricks for relative addressing, which are not necessarily as fast as those for absolute addressing.

  • Because DSO modules cannot be linked against other DSO-based libraries (ld -lfoo) on all platforms (for instance, a.out-based platforms usually don't provide this functionality, while ELF-based platforms do), you cannot use the DSO mechanism for all types of modules. In other words, modules compiled as DSO files are restricted to use symbols only from the Apache core, from the C library (libc) and from any other dynamic or static libraries used by the Apache core, or from static library archives (libfoo.a) containing position-independent code. The only way you can use other code is to either make sure the Apache core itself already contains a reference to it, load the code yourself via dlopen( ), or enable the SHARED_CHAIN rule while building Apache (if your platform supports linking DSO files against DSO libraries). This, however, won't be of much significance to you if you're writing modules only in Perl.

  • Under some platforms (e.g., many SVR4 systems), there is no way to force the linker to export all global symbols for use in DSOs when linking the Apache httpd executable program. But without the visibility of the Apache core symbols, no standard Apache module could be used as a DSO. The only workaround here is to use the SHARED_CORE feature, because in this way the global symbols are forced to be exported. As a consequence, the Apache src/Configure script automatically enforces SHARED_CORE on these platforms when DSO features are used in the Configuration file or on the configure command line.

Together, these four features provide a way to integrate mod_perl into Apache in a very clean and smooth way. No patching of the Apache source tree is usually required, and for APXS support, not even the Apache source tree is needed.

To benefit from the above features, a hybrid build environment was created for the Apache side of mod_perl. See Section 3.5, later in this chapter, for details.

Once the overview of the four building steps is complete, we will return to each of the above configuration mechanisms when describing different installation passes.

3.1.1 Controlling the Build Process

The configuration stage of the build is performed by the command perl Makefile.PL, which accepts various parameters. This section covers all of the configuration parameters, grouped by their functionality.

Of course, you should keep in mind that these options are cumulative. We display only one or two options being used at once, but you should use the ones you want to enable all at once, in one call to perl Makefile.PL.


These four parameters are tightly interconnected, as they control the way in which the Apache source is handled.

Typically, when you want mod_perl to be compiled statically with Apache without adding any extra components, you specify the location of the Apache source tree using the APACHE_SRC parameter and use the DO_HTTPD=1 parameter to tell the installation script to build the httpd executable:

panic% perl Makefile.PL APACHE_SRC=../apache_1.3.xx/src DO_HTTPD=1

If no APACHE_SRC is specified, Makefile.PL makes an intelligent guess by looking at the directories at the same level as the mod_perl sources and suggesting a directory with the highest version of Apache found there.

By default, the configuration process will ask you to confirm whether the location of the source tree is correct before continuing. If you use DO_HTTPD=1 or NO_HTTPD=1, the first Apache source tree found or the one you specified will be used for the rest of the build process.

If you don't use DO_HTTPD=1, you will be prompted by the following question:

Shall I build httpd in ../apache_1.3.xx/src for you?

Note that if you set DO_HTTPD=1 but do not use APACHE_SRC=../apache_1.3.xx/src, the first Apache source tree found will be used to configure and build against. Therefore, you should always use an explicit APACHE_SRC parameter, to avoid confusion.

If you don't want to build the httpd in the Apache source tree because you might need to add extra third-party modules, you should use NO_HTTPD=1 instead of DO_HTTPD=1. This option will install all the files that are needed to build mod_perl in the Apache source tree, but it will not build httpd itself.

PREP_HTTPD=1 is similar to NO_HTTPD=1, but if you set this parameter you will be asked to confirm the location of the Apache source directory even if you have specified the APACHE_SRC parameter.

If you choose not to build the binary, you will have to do that manually. Building an httpd binary is covered in an upcoming section. In any case, you will need to run make install in the mod_perl source tree so the Perl side of mod_perl will be installed. Note that mod_perl's make test won't work until you have built the server.


When Apache and mod_perl are installed, you may need to build other Perl modules that use Apache C functions, such as HTML::Embperl or Apache::Peek. These modules usually will fail to build if Apache header files aren't installed in the Perl tree. By default, the Apache source header files are installed into the $Config{sitearchexp}/auto/Apache/include directory.[2] If you don't want or need these headers to be installed, you can change this behavior by using the APACHE_HEADER_INSTALL=0 parameter.

[2] %Config is defined in the file in your Perl installation.


The USE_APACI parameter tells mod_perl to configure Apache using the flexible APACI. The alternative is the older system, which required a file named src/Configuration to be edited manually. To enable APACI, use:

panic% perl Makefile.PL USE_APACI=1

When you use the USE_APACI=1 parameter, you can tell Makefile.PL to pass any arguments you want to the Apache ./configure utility. For example:

panic% perl Makefile.PL USE_APACI=1 \
    APACI_ARGS='--sbindir=/home/httpd/httpd_perl/sbin, \

Note that the APACI_ARGS argument must be passed as a single long line if you work with a C-style shell (such as csh or tcsh), as those shells seem to corrupt multi-lined values enclosed inside single quotes.

Of course, if you want the default Apache directory layout but a different root directory (/home/httpd/httpd_perl/, in our case), the following is the simplest way to do so:

panic% perl Makefile.PL USE_APACI=1 \

This parameter enables building of built-in Apache modules. For example, to enable the mod_rewrite and mod_proxy modules, you can do the following:

panic% perl Makefile.PL ADD_MODULE=proxy,rewrite

If you are already using APACI_ARGS, you can add the usual Apache ./configure directives as follows:

panic% perl Makefile.PL USE_APACI=1 \
    APACI_ARGS='--enable-module=proxy --enable-module=rewrite'

As an alternative to:


you can use the APACHE_PREFIX parameter. When USE_APACI is enabled, this attribute specifies the same —prefix option.

Additionally, the APACHE_PREFIX option automatically executes make install in the Apache source directory, which makes the following commands:

panic% perl Makefile.PL APACHE_SRC=../apache_1.3.xx/src \
panic% make && make test
panic# make install
panic# cd ../apache_1.3.xx
panic# make install

equivalent to these commands:

panic% perl Makefile.PL APACHE_SRC=../apache_1.3.xx/src \
panic% make && make test
panic# make install

Normally, if a C code extension is statically linked with Perl, it is listed in's $Config{static_exts}, in which case mod_perl will also statically link this extension with httpd. However, if an extension is statically linked with Perl after it is installed, it will not be listed in You can either edit and add these extensions, or configure mod_perl like this:

panic% perl Makefile.PL "PERL_STATIC_EXTS=DBI DBD::Oracle"

This option tells mod_perl to build the Apache::* API extensions as shared libraries. The default is to link these modules statically with the httpd executable. This can save some memory if you use these API features only occasionally. To enable this option, use:

panic% perl Makefile.PL DYNAMIC=1

If this option is enabled, mod_perl will be built using the APXS tool. This tool is used to build C API modules in a way that is independent of the Apache source tree. mod_perl will look for the apxs executable in the location specified by WITH_APXS; otherwise, it will check the bin and sbin directories relative to APACHE_PREFIX. To enable this option, use:

panic% perl Makefile.PL USE_APXS=1

This attribute tells mod_perl the location of the apxs executable. This is necessary if the binary cannot be found in the command path or in the location specified by APACHE_PREFIX. For example:

panic% perl Makefile.PL USE_APXS=1 WITH_APXS=/home/httpd/bin/apxs

This option tells mod_perl to build itself as a DSO. Although this reduces the apparent size of the httpd executable on disk, it doesn't actually reduce the memory consumed by each httpd process. This is recommended only if you are going to be using the mod_perl API only occasionally, or if you wish to experiment with its features before you start using it in a production environment. To enable this option, use:

panic% perl Makefile.PL USE_DSO=1

When building against a mod_ssl-enabled server, this option will tell Apache where to look for the SSL include and lib subdirectories. For example:

panic% perl Makefile.PL SSL_BASE=/usr/share/ssl

When the Perl interpreter shuts down, this level enables additional checks during server shutdown to make sure the interpreter has done proper bookkeeping. The default is 0. A value of 1 enables full destruction, and 2 enables full destruction with checks. This value can also be changed at runtime by setting the environment variable PERL_DESTRUCT_LEVEL. We will revisit this parameter in Chapter 5.


To enable mod_perl debug tracing, configure mod_perl with the PERL_TRACE option:

panic% perl Makefile.PL PERL_TRACE=1

To see the diagnostics, you will also need to set the MOD_PERL_TRACE environment variable at runtime.

We will use mod_perl configured with this parameter enabled to show a few debugging techniques in Chapter 21.


This option builds mod_perl and the Apache server with C source code debugging enabled (the -g switch). It also enables PERL_TRACE, sets PERL_DESTRUCT_LEVEL to 2, and links against the debuggable libperld Perl interpreter if one has been installed. You will be able to debug the Apache executable and each of its modules with a source-level debugger, such as the GNU debugger gdb. To enable this option, use:

panic% perl Makefile.PL PERL_DEBUG=1

We will discuss this option in Chapter 21, as it is extremely useful to track down bugs or report problems.

3.1.2 Activating Callback Hooks

A callback hook (also known simply as a callback) is a reference to a subroutine. In Perl, we create subroutine references with the following syntax:

$callback = \&subroutine;

In this example, $callback contains a reference to the subroutine called subroutine. Another way to create a callback is to use an anonymous subroutine:

$callback = sub { 'some code' };

Here, $callback contains a reference to the anonymous subroutine. Callbacks are used when we want some action (subroutine call) to occur when some event takes place. Since we don't know exactly when the event will take place, we give the event handler a reference to the subroutine we want to be executed. The handler will call our subroutine at the right time, effectively calling back that subroutine.

By default, most of the callback hooks except for PerlHandler, PerlChildInitHandler, PerlChildExitHandler, PerlConnectionApi, and PerlServerApi are turned off. You may enable them via options to Makefile.PL.

Here is the list of available hooks and the parameters that enable them. The Apache request prcessing phases were explained in Chapter 1.

Directive/Hook              Configuration Option
PerlPostReadRequestHandler  PERL_POST_READ_REQUEST
PerlTransHandler            PERL_TRANS
PerlInitHandler             PERL_INIT
PerlHeaderParserHandler     PERL_HEADER_PARSER
PerlAuthenHandler           PERL_AUTHEN
PerlAuthzHandler            PERL_AUTHZ
PerlAccessHandler           PERL_ACCESS
PerlTypeHandler             PERL_TYPE
PerlFixupHandler            PERL_FIXUP
PerlHandler                 PERL_HANDLER
PerlLogHandler              PERL_LOG
PerlCleanupHandler          PERL_CLEANUP
PerlChildInitHandler        PERL_CHILD_INIT
PerlChildExitHandler        PERL_CHILD_EXIT
PerlDispatchHandler         PERL_DISPATCH

As with any parameters that are either defined or not, use OPTION_FOO=1 to enable them (e.g., PERL_AUTHEN=1).

To enable all callback hooks, use:


There are a few more hooks that won't be enabled by default, because they are experimental.

If you are using:

panic% perl Makefile.PL EVERYTHING=1 ...

it already includes the ALL_HOOKS=1 option.

3.1.3 Activating Standard API Features

The following options enable various standard features of the mod_perl API. While not absolutely needed, they're very handy and there's little penalty in including them. Unless specified otherwise, these options are all disabled by default. The EVERYTHING=1 or DYNAMIC=1 options will enable them en masse. If in doubt, include these.


Enables the Apache::File class, which helps with the handling of files under mod_perl.


Enables the Apache::Table class, which provides tied access to the Apache Table structure (used for HTTP headers, among others).


Enables the Apache::Log class. This class allows you to access Apache's more advanced logging features.


Enables the Apache::URI class, which deals with the parsing of URIs in a similar way to the Perl URI::URL module, but much faster.


Enables the Apache::Util class, allowing you to use various functions such as HTML escaping or date parsing, but implemented in C.


Enables the Apache::Connection class. This class is enabled by default. Set the option to 0 to disable it.


Enables the Apache::Server class. This class is enabled by default. Set the option to 0 to disable it.

Please refer to Lincoln Stein and Doug MacEachern's Writing Apache Modules with Perl and C (O'Reilly) for more information about the Apache API.

3.1.4 Enabling Extra Features

mod_perl comes with a number of other features. Most of them are disabled by default. This is the list of features and options to enable them:

  • <Perl> sections give you a way to configure Apache using Perl code in the httpd.conf file itself. See Chapter 4 for more information.

    panic% perl Makefile.PL PERL_SECTIONS=1 ...
  • With the PERL_SSI option, the mod_include module can be extended to include a #perl directive.

    panic% perl Makefile.PL PERL_SSI=1

    By enabling PERL_SSI, a new #perl element is added to the standard mod_include functionality. This element allows server-side includes to call Perl subroutines directly. This feature works only when mod_perl is not built as a DSO (i.e., when it's built statically).

  • If you develop an Apache module in Perl and you want to create custom configuration directives[3] to be recognized in httpd.conf, you need to use Apache::ModuleConfig and Apache::CmdParms. For these modules to work, you will need to enable this option:

    [3] See Chapters 8 and 9 of Writing Apache Modules with Perl and C (O'Reilly).

    panic% perl Makefile.PL PERL_DIRECTIVE_HANDLERS=1
  • The stacked handlers feature explained in Chapter 4 requires this parameter to be enabled:

    panic% perl Makefile.PL PERL_STACKED_HANDLERS=1
  • The method handlers feature discussed in Chapter 4 requires this parameter to be enabled:

    panic% perl Makefile.PL PERL_METHOD_HANDLERS=1
  • To enable all phase callback handlers, all API modules, and all miscellaneous features, use the "catch-all" option we used when we first compiled mod_perl:

    panic% perl Makefile.PL EVERYTHING=1

3.1.5 Reusing Configuration Parameters

When you have to upgrade the server, it's sometimes hard to remember what parameters you used in the previous mod_perl build. So it's a good idea to save them in a file.

One way to save parameters is to create a file (e.g., ~/.mod_perl_build_options) with the following contents:

APACHE_SRC=../apache_1.3.xx/src DO_HTTPD=1 USE_APACI=1 \

Then build the server with the following command:

panic% perl Makefile.PL `cat ~/.mod_perl_build_options`
panic% make && make test
panic# make install

But mod_perl has a standard method to perform this trick. If a file named makepl_args.mod_perl is found in the same directory as the mod_perl build location, it will be read in by Makefile.PL. Parameters supplied at the command line will override the parameters given in this file.

The makepl_args.mod_perl file can also be located in your home directory or in the ../ directory relative to the mod_perl distribution directory. The filename can also start with a dot (.makepl_args.mod_perl), so you can keep it nicely hidden along with the rest of the dot files in your home directory. So, Makefile.PL will look for the following files (in this order), using the first one it comes across:


For example:

panic% ls -1 /home/stas/src

panic% cat makepl_args.mod_perl

panic% cd mod_perl-1.xx
panic% perl Makefile.PL
panic% make && make test
panic# make install

Now the parameters from the makepl_args.mod_perl file will be used automatically, as if they were entered directly.

In the sample makepl_args.mod_perl file in the eg/ directory of the mod_perl distribution package, you might find a few options enabling some experimental features for you to play with, too!

If you are faced with a compiled Apache and no trace of the parameters used to build it, you can usually still find them if make clean was not run on the sources. You will find the Apache-specific parameters in apache_1.3.xx/config.status and the mod_perl parameters in mod_perl-1.xx/apaci/mod_perl.config.

3.1.6 Discovering Whether a Feature Was Enabled

mod_perl Version 1.25 introduced Apache::MyConfig, which provides access to the various hooks and features set when mod_perl was built. This circumvents the need to set up a live server just to find out if a certain callback hook is available.

To see whether some feature was built in or not, check the %Apache::MyConfig::Setup hash. For example, suppose we install mod_perl with the following options:

panic% perl Makefile.PL EVERYTHING=1

but the next day we can't remember which callback hooks were enabled. We want to know whether the PERL_LOG callback hook is available. One of the ways to find an answer is to run the following code:

panic% perl -MApache::MyConfig -e 'print $Apache::MyConfig::Setup{PERL_LOG}'

If it prints 1, that means the PERL_LOG callback hook is enabled (which it should be, as EVERYTHING=1 enables them all).

Another approach is to configure Apache::Status (see Chapter 9) and run http://localhost/perl-status?hooks to check for enabled hooks.

If you want to check for the existence of various hooks within your handlers, you can use the script shown in Example 3-1.

Example 3-1.
use mod_perl_hooks;

for my $hook (mod_perl::hooks( )) {
    if (mod_perl::hook($hook)) {
        print "$hook is enabled\n";
    else {
        print "$hook is not enabled\n";

You can also try to look at the symbols inside the httpd executable with the help of nm(1) or a similar utility. For example, if you want to see whether you enabled PERL_LOG=1 while building mod_perl, you can search for a symbol with the same name but in lowercase:

panic% nm httpd | grep perl_log
08071724 T perl_logger

This shows that PERL_LOG=1 was enabled. But this approach will work only if you have an unstripped httpd binary. By default, make install strips the binary before installing it, thus removing the symbol names to save space. Use the —without-execstrip ./configure option to prevent stripping during the make install phase. [4]

[4] You might need the unstripped version for debugging reasons too.

Yet another approach that will work in most cases is to try to use the feature in question. If it wasn't configured, Apache will give an error message.

3.1.7 Using an Alternative Configuration File

By default, mod_perl provides its own copy of the Configuration file to Apache's configure utility. If you want to pass it your own version, do this:

panic% perl Makefile.PL CONFIG=Configuration.custom

where Configuration.custom is the pathname of the file relative to the Apache source tree you build against.

3.1.8 perl Makefile.PL Troubleshooting

During the configuration (perl Makefile.PL) stage, you may encounter some of these problems. To help you avoid them, let's study them, find out why they happened, and discuss how to fix them. A test compilation with your Makefile configuration failed...

When you see the following error during the perl Makefile.PL stage:

** A test compilation with your Makefile configuration
** failed. This is most likely because your C compiler
** is not ANSI. Apache requires an ANSI C Compiler, such
** as gcc. The above error message from your compiler
** will also provide a clue.

it's possible that you have a problem with a compiler. It may be improperly installed or not installed at all. Sometimes the reason is that your Perl executable was built on a different machine, and the software installed on your machine is not the same. Generally this happens when you install prebuilt packages, such as rpm or deb. You may find that the dependencies weren't properly defined in the Perl binary package and you were allowed to install it even though some essential packages were not installed.

The most frequent pitfall is a missing gdbm library (see the next section).

But why guess, when we can actually see the real error message and understand what the real problem is? To get a real error message, edit the Apache src/Configure script. Around line 2140, you should see a line like this:

if ./helpers/TestCompile sanity; then

Add the -v option, as follows:

if ./helpers/TestCompile -v sanity; then

and try again. Now you should get a useful error message. Missing or misconfigured

On some Red Hat Linux systems, you might encounter a problem during the perl Makefile.PL stage, when Perl was installed from an rpm package built with the gdbm library, but libgdbm isn't actually installed. If this happens to you, make sure you install it before proceeding with the build process.

You can check how Perl was built by running the perl -V command:

panic% perl -V | grep libs

You should see output similar to this:

libs=-lnsl -lndbm -lgdbm -ldb -ldl -lm -lc -lposix -lcrypt

Sometimes the problem is even more obscure: you do have libgdbm installed, but it's not installed properly. Do this:

panic% ls /usr/lib/*

If you get at least three lines, like we do:

lrwxrwxrwx   /usr/lib/ ->
lrwxrwxrwx   /usr/lib/ ->
-rw-r--r--   /usr/lib/

you are all set. On some installations, the symbolic link is missing, so you get only:

lrwxrwxrwx   /usr/lib/ ->
-rw-r--r--   /usr/lib/

To fix this problem, add the missing symbolic link:

panic% cd /usr/lib
panic% ln -s

Now you should be able to build mod_perl without any problems.

Note that you might need to prepare this symbolic link as well:

lrwxrwxrwx   /usr/lib/ ->

with the command:

panic% ln -s

Of course, if a new version of the libgdbm library was released between the moment we wrote this sentence and the moment you're reading it, you will have to adjust the version numbers. We didn't use the usual xx.xx version replacement here, to make it easier to understand how the symbolic links should be set.

About the gdbm, db, and ndbm Libraries

If you need to have the dbm library linked in, you should know that both the gdbm and db libraries offer ndbm emulation, which is the interface that Apache actually uses. So when you build mod_perl, you end up using whichever library was linked first by the Perl compilation. If you build Apache without mod_perl, you end up with whatever appears to be be your ndbm library, which will vary between systems, and especially Linux distributions. So you may have to work a bit to get both Apache and Perl to use the same library, and you are likely to have trouble copying the dbm file from one system to another or even using it after an upgrade. Undefined reference to `PL_perl_destruct_level'

When manually building mod_perl using the shared library:

panic% cd mod_perl-1.xx
panic% perl Makefile.PL PREP_HTTPD=1
panic% make && make test
panic# make install

panic% cd ../apache_1.3.xx
panic% ./configure --with-layout=RedHat --target=perlhttpd 

you might see the following output:

gcc -c  -I./os/unix -I./include   -DLINUX=2 -DTARGET=\"perlhttpd\"
-DUSE_HSREGEX -DUSE_EXPAT -I./lib/expat-lite `./apaci` buildmark.c
-I./lib/expat-lite `./apaci`    \
      -o perlhttpd buildmark.o modules.o modules/perl/libperl.a 
modules/standard/libstandard.a main/libmain.a ./os/unix/libos.a ap/libap.a 
regex/libregex.a lib/expat-lite/libexpat.a  -lm -lcrypt
modules/perl/libperl.a(mod_perl.o): In function `perl_shutdown':
mod_perl.o(.text+0xf8): undefined reference to `PL_perl_destruct_level'
mod_perl.o(.text+0x102): undefined reference to `PL_perl_destruct_level'
mod_perl.o(.text+0x10c): undefined reference to `PL_perl_destruct_level'
mod_perl.o(.text+0x13b): undefined reference to `Perl_av_undef'
[more errors snipped]

This happens when Perl was built statically linked, with no shared libperl.a. Build a dynamically linked Perl (with libperl.a) and the problem will disappear.

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