Incorrect $SystemID in Mathematica - c++

Question

I am trying to link Mathlink libraries in mathematica. I am using a x86_64-suse-linux OS, but when I check $SystemID in Mathematica, it tells me I am using 'Linux' instead of 'Linux-x86-64'.
This is a problem because if I try to link the library as
g++ cpp2mma.cpp -L/usr/local/Wolfram/Mathematica/8.0/SystemFiles/Links/MathLink/DeveloperKit/Linux/CompilerAdditions/ -lML32i3 -lpthread -lstdc++ -lm -lrt
(the 32 bit version - and the only library in Linux/)
I get the following error:
/usr/lib64/gcc/x86_64-suse-linux/4.5/../../../../x86_64-suse-linux/bin/ld: skipping incompatible
/usr/local/Wolfram/Mathematica/8.0/SystemFiles/Links/MathLink/DeveloperKit/Linux/CompilerAdditions//libML32i3.so when searching for -lML32i3
/usr/lib64/gcc/x86_64-suse-linux/4.5/../../../../x86_64-suse-linux/bin/ld: skipping incompatible
/usr/local/Wolfram/Mathematica/8.0/SystemFiles/Links/MathLink/DeveloperKit/Linux/CompilerAdditions//libML32i3.a when searching for -lML32i3
/usr/lib64/gcc/x86_64-suse-linux/4.5/../../../../x86_64-suse-linux/bin/ld: cannot find -lML32i3
collect2: ld returned 1 exit status
however, if I try to link it as
g++ cpp2mma.cpp -L/usr/local/Wolfram/Mathematica/8.0/SystemFiles/Links/MathLink/DeveloperKit/Linux-x86-64/CompilerAdditions/ -lML64i3 -lpthread -lstdc++ -lm -lrt
it compiles just fine, but if I try to run it I get the following error:
./a.out: error while loading shared libraries: libML64i3.so: cannot open shared object file: No such file or directory
This file obviously exists as running 'locate libML64i3.so' yields:
/usr/local/Wolfram/Mathematica/8.0/SystemFiles/Libraries/Linux-x86-64/libML64i3.so
/usr/local/Wolfram/Mathematica/8.0/SystemFiles/Links/MathLink/DeveloperKit/Linux-x86-64/CompilerAdditions/libML64i3.so
Am I correct in assuming that the problem lies in the fact that Mathematica thinks that I am not running a 32 bit system? If so, what do I do?

Answer 1

The problem is not in Mathematica. The kernel is never running -- from the perspective of e system you are compiling and launching an ELF executable. libML64i3 will start a kernel for you, but its not getting loaded here.
There are several ways to get the linker to find libML64i3.so at runtime
Export the directory containing libML64i3.so through LD_LIBRARY_PATH (generally not a good idea)
use an entry in /etc/ld.so.conf.d or /etc/ld.so.conf to indicate that /usr/local/Wolfram/Mathematica/8.0/SystemFiles/Links/MathLink/DeveloperKit/Linux-x86-64/CompilerAdditions should be part of the library search path -- look at ldconfig(8).
Symlink libML64i3.so somewhere already in the library search path
Link to a static version of libML, as described in this StackOverflow post.
To deal with the compile-time linker errors, you could compile your application 32-bit (by passing -m32 to g++). If you did this, you would get a runtime message griping about not being able to find the 32-bit version of libML, which would require the same solution as above.
I'm not sure why $SystemID is returning a value indicating you are running the 32-bit kernel. That probably has something to do with how you are starting Mathematica. On CentOS 5-7-x86_64, I have
host 11% ls -l $(which math)
lrwxrwxrwx 1 root root 51 Nov 8 16:27 /usr/local/bin/math -> \
/usr/local/Wolfram/Mathematica/8.0/Executables/math
host 12% math
Mathematica 8.0 for Linux x86 (64-bit)
Copyright 1988-2011 Wolfram Research, Inc.
In[1]:= $SystemID
Out[1]= Linux-x86-64