Problems with MySQL libraries while upgrading rails

I’ve just upgraded to Rails 2.3.2 on a RedHat Linux machine, but I had a
lot of problems. Eventually I got there. I’ve seen other people
posting questions about similar problems, but no answers so far, so here

As specified in the instructions I did this:

gem install rails

No errors, but when I started my rails app, it fell over with error
messages advising me to look at the mkmf.log for errors during the
installation of the mysql connection package.

The next hurdle is to find the appropriate mkmf.log file. I had to use
the UNIX find command to search the whole filestore:

$ find / -name mkmf.log 2>/dev/null

This produced a long list of log files. Looking at the directory names
I found one for mysql. Mine was called


It complained that mysql-devel package was missing from my MySQL
installation. I think this is an extension to MySQL containing stuff
for developers. Rails didn’t use to use it, now it does. I fixed that
problem by finding the appropriate RPM for my version of Linux and
installing it.

Now the gem install threw another error, advising me to install the
Rails MySQL connection software, but that led to further errors:

# gem install mysql
Building native extensions.  This could take a while...
ERROR:  Error installing mysql:
        ERROR: Failed to build gem native extension.

/usr/local/bin/ruby extconf.rb
checking for mysql_query() in -lmysqlclient... no

Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers.  Check the mkmf.log file for

details. You may need configuration options.

Provided configuration options:

I’ve removed lots of detail from the above, in particular, it gives a
long list of configuration options. As you will see, these are the key
to the solution. You just need to know which option to supply and how
to set it …

Looking again at the mkmf.log, I could see that the errors were caused
by failures when gem tried to run a makefile:

more /usr/local/lib/ruby/gems/1.8/gems/mysql-2.7/mkmf.log

find_library: checking for mysql_query() in -lmysqlclient… -------- no

“gcc -o conftest -I. -I/usr/local/lib/ruby/1.8/i686-linux -I.
de -g -O2 conftest.c -L’/usr/local/lib’ -Wl,-R’/usr/local/lib’
ib’ -Wl,-R’/usr/local/lib’ -lruby-static -lmysqlclient -ldl
-lcrypt -lm
/usr/bin/ld: cannot find -lmysqlclient

What’s happening here is that gem is calling the standard UNIX make
command. This is trying to find a library of C functions, and failing,
because it’s looking in the wrong place. This is basic C programming
under UNIX stuff, nothing to do with Ruby. The solution is to get gems
to call make and hand it the correct information. This is where the
config options come in.

With a bit more searching on the web, I found this solution:

gem install mysql – --with-mysql-config=/usr/bin/mysql_config

(Note the extra “–”. You need this to use the config options.)

What you are doing here is handing the name of a MySQL configuration
file to gem. On your system, there should be a file called
mysql-config, possibly in /usr/bin, possibly somewhere else. You need
to find it and supply the right pathname. Basically, this file tells
tools like gem where to find all the bits and pieces that make up your
MySQL system.

This installed the rails MySQL component.

Finally, complete the rails upgrade:

gem install rails

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