Ruby Gems: Missing libraries or headers - ruby

Question

I need to install a Ruby gem on my Debian server. This fails with an error message which is not very helpful to me as a non-ruby programmer. Can you help me diagnose the problem? What are the "necessary libraries and/or headers" that are missing?
root ~ # gem install schleuder
Building native extensions. This could take a while...
ERROR: Error installing schleuder:
ERROR: Failed to build gem native extension.
/usr/bin/ruby1.8 extconf.rb
checking for magic_open() in -lmagic... no
*** ERROR: missing required library to compile this module
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers. Check the mkmf.log file for more
details. You may need configuration options.
Provided configuration options:
--with-opt-dir
--without-opt-dir
--with-opt-include
--without-opt-include=${opt-dir}/include
--with-opt-lib
--without-opt-lib=${opt-dir}/lib
--with-make-prog
--without-make-prog
--srcdir=.
--curdir
--ruby=/usr/bin/ruby1.8
--with-magic-dir
--without-magic-dir
--with-magic-include
--without-magic-include=${magic-dir}/include
--with-magic-lib
--without-magic-lib=${magic-dir}/lib
--with-magiclib
--without-magiclib
Gem files will remain installed in /var/lib/gems/1.8/gems/ruby-filemagic-0.4.2 for inspection.
Results logged to /var/lib/gems/1.8/gems/ruby-filemagic-0.4.2/ext/gem_make.out
The logfile (assuming this is the right one) says:
root ~ # cat /var/lib/gems/1.8/gems/ruby-filemagic-0.4.2/ext/mkmf.log
have_library: checking for magic_open() in -lmagic... -------------------- no
"gcc -o conftest -I. -I/usr/lib/ruby/1.8/x86_64-linux -I. -fno-strict-aliasing -g -g -O2 -fPIC conftest.c -L. -L/usr/lib -L. -rdynamic -Wl,-export-dynamic -lruby1.8-static -lmagic -lpthread -lrt -ldl -lcrypt -lm -lc"
conftest.c: In function ‘t’:
conftest.c:3: error: ‘magic_open’ undeclared (first use in this function)
conftest.c:3: error: (Each undeclared identifier is reported only once
conftest.c:3: error: for each function it appears in.)
checked program was:
/* begin */
1: /*top*/
2: int main() { return 0; }
3: int t() { void ((*volatile p)()); p = (void ((*)()))magic_open; return 0; }
/* end */
"gcc -o conftest -I. -I/usr/lib/ruby/1.8/x86_64-linux -I. -fno-strict-aliasing -g -g -O2 -fPIC conftest.c -L. -L/usr/lib -L. -rdynamic -Wl,-export-dynamic -lruby1.8-static -lmagic -lpthread -lrt -ldl -lcrypt -lm -lc"
/usr/bin/ld: cannot find -lmagic
collect2: ld returned 1 exit status
checked program was:
/* begin */
1: /*top*/
2: int main() { return 0; }
3: int t() { magic_open(); return 0; }
/* end */
--------------------

Answer 1

Some Ruby gems come with "native extensions", meaning that they aren't entirely written in Ruby, but contain bindings to some external libraries written in C. Often this is for performance reasons, or so as to use well-established libraries rather than reinventing the wheel.
In your case it can't compile the native extension because the header files are missing. You probably need to install the libmagic-dev Debian package. Something like
sudo apt-get install libmagic-dev
would do the trick, depending on your package manager!