Symbolic link to compile the code using 32-bit libraries in 64-bit Ubuntu 12.10 - matlab

Question

I am trying to mex C code using 32-bit 2012a Matlab on 64-bit Ubuntu 12.10
I've downloaded all possible libraries (gcc 4.7, build-essential,libs-32 etc.) however I am getting the following error
/usr/bin/ld: skipping incompatible /usr/lib/gcc/x86_64-linux-gnu/4.7/libstdc++.so when searching for -lstdc++
/usr/bin/ld: skipping incompatible /usr/lib/gcc/x86_64-linux-gnu/4.7/libstdc++.a when searching for -lstdc++
/usr/bin/ld: cannot find -lstdc++
/usr/bin/ld: skipping incompatible /usr/lib/gcc/x86_64-linux-gnu/4.7/libgomp.so when searching for -lgomp
/usr/bin/ld: skipping incompatible /usr/lib/gcc/x86_64-linux-gnu/4.7/libgomp.a when searching for -lgomp
/usr/bin/ld: cannot find -lgomp
/usr/bin/ld: skipping incompatible /usr/lib/gcc/x86_64-linux-gnu/4.7/libgcc_s.so when searching for -lgcc_s
/usr/bin/ld: cannot find -lgcc_s
collect2: error: ld returned 1 exit status
I've found that this problem can be solved by setting symbolic link from 64-bit libraries to the 32-bit ones. I tried to create different links but couldn't finish the compilation.
Thank you in advance.
UPDATE 1
gcc-multilib was missing, so after
sudo apt-get install gcc-multilib
I get the following errors
Warning: You are using gcc version "4.7.2-2ubuntu1)". The version
currently supported with MEX is "4.4.6".
For a list of currently supported compilers see:
http://www.mathworks.com/support/compilers/current_release/
/usr/bin/ld: skipping incompatible /usr/lib/gcc/x86_64-linux-gnu/4.7/libstdc++.so when searching for -lstdc++
/usr/bin/ld: skipping incompatible /usr/lib/gcc/x86_64-linux-gnu/4.7/libstdc++.a when searching for -lstdc++
/usr/bin/ld: cannot find -lstdc++
collect2: error: ld returned 1 exit status
My mexopts.sh looks like
CC='gcc'
CFLAGS='-ansi -D_GNU_SOURCE'
CFLAGS="$CFLAGS -fPIC -pthread -m32"
CFLAGS="$CFLAGS -fexceptions"
CFLAGS="$CFLAGS -D_FILE_OFFSET_BITS=64"
CLIBS="$RPATH $MLIBS -lm"
COPTIMFLAGS='-O -DNDEBUG'
CDEBUGFLAGS='-g'
CLIBS="$CLIBS -lstdc++"
the -m32 is there, however I am not sure if it should be written in that way.
Can someone elaborate on how to edit mexopts.sh to make matlab look at 32-bit libraries?
UPDATE 2
after looking at Linking using g++ fails searching for -lstdc++
I tried to install g++-multilib
sudo apt-get install g++-multilib
Now, errors have the form:
/usr/bin/ld: i386:x86-64 architecture of input file `bin/fv_cache.o' is incompatible with i386 output
/usr/bin/ld: i386:x86-64 architecture of input file `bin/obj_func.o' is incompatible with i386 output
...

Answer 1

First make sure you can build 32-bit executable from command prompt by executing gcc (more info here: 32bit application on 64 bit Linux)
In matlab command prompt execute mex -setup and choose gcc as a compiler. At the end you will get a message about location of 'mexopts.sh' (usually ~/.matlab//mexopts.sh)
Try to "mex".
If not successful open mexopts.sh and check if option -m32 is in CFLAGS. If not, add it.