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/libgdbm.so*
If you get at least three lines, like we do:
lrwxrwxrwx /usr/lib/libgdbm.so -> libgdbm.so.2.0.0 lrwxrwxrwx /usr/lib/libgdbm.so.2 -> libgdbm.so.2.0.0 -rw-r--r-- /usr/lib/libgdbm.so.2.0.0
lrwxrwxrwx /usr/lib/libgdbm.so.2 -> libgdbm.so.2.0.0 -rw-r--r-- /usr/lib/libgdbm.so.2.0.0
To fix this problem, add the missing symbolic link:
panic% cd /usr/lib panic% ln -s libgdbm.so.2.0.0 libgdbm.so
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/libgdbm.so.2 -> libgdbm.so.2.0.0
with the command:
panic% ln -s libgdbm.so.2.0.0 libgdbm.so.2
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.