added an example

docs/using_eessi/building_on_eessi.md

@@ -143,13 +143,98 @@ shell before building. To do this:
 * Set manually `LD_RUN_PATH` to resolve libraries at runtime. `LIBRARY_PATH` should contain all the paths we need, and we also need to include the path to
   `libstdc++` from our GCC installation to avoid picking up the one from the host:
   ```sh
-      export LD_RUN_PATH=$LIBRARY_PATH:$EBROOTGCCcore/lib64
+      export LD_RUN_PATH=$LIBRARY_PATH:$EBROOTGCCCORE/lib64
   ```
 * Compile and make sure the library resolution points to the EESSI stack. For this, `ldd` from compatibility layer and **not** `/usr/bin/ldd` should be used
   when checking the binary.
 
 * Run! 
 
+To exemplify this, take the classic MPI Hello World example code:
+
+```
+/*The Parallel Hello World Program*/
+#include <stdio.h>
+#include <mpi.h>
+
+int main(int argc, char **argv)
+{
+   int node;
+
+   MPI_Init(&argc,&argv);
+   MPI_Comm_rank(MPI_COMM_WORLD, &node);
+
+   printf("Hello World from Node %d\n",node);
+
+   MPI_Finalize();
+}
+
+```
+
+As described in the steps above, prepare the environment and load the required dependencies. For this case, we will use `gompi/2023b as the toolchain to compile it.
+
+```
+# Starting the environment
+$ source /cvmfs/software.eessi.io/versions/2023.06/init/bash
+
+# Loading the toolchain
+{EESSI 2023.06} $ m̀l gompi/2023b
+```
+
+Now, set the `LD_RUN_PATH` environment variable for all the libraries to point to the runtime libraries, then compile the code. From the previous text, we are assuming that you are using GCC as the compiler, but please change the `$EBROOTGCCCORE variable accordingly in case you are using any other kind of compiler that you want to prevent picking from the host.
+
+```
+# Setting LD_RUN_PATH
+{EESSI 2023.06}$ export LD_RUN_PATH=$LIBRARY_PATH:$EBROOTGCCCORE/lib64
+
+# Compile the code manually
+{EESSI 2023.06} $ mpicc -o HelloWorld mpi.c
+```
+
+This is the moment to check if the compiler picked all the libraries from the compatibility layer.
+
+Look at the difference on the library solving when using the compatibility layer ldd from the host one:
+
+```sh
+
+# ldd from the compatibility layer, notice how all libraries are resolved from the software layer -> "/cvmfs/software.eessi.io/versions/2023.06/software", the libc and the interpreter point to the compatibility layer, so all good to go!
+
+{EESSI 2023.06} $ ldd HelloWorld
+	linux-vdso.so.1 (0x00007ffce03af000)
+	libmpi.so.40 => /cvmfs/software.eessi.io/versions/2023.06/software/linux/x86_64/intel/skylake_avx512/software/OpenMPI/4.1.6-GCC-13.2.0/lib/libmpi.so.40 (0x00007fadd9e84000)
+	libc.so.6 => /cvmfs/software.eessi.io/versions/2023.06/compat/linux/x86_64/lib64/libc.so.6 (0x00007fadd9ca8000)
+[...]
+	libevent_pthreads-2.1.so.7 => /cvmfs/software.eessi.io/versions/2023.06/software/linux/x86_64/intel/skylake_avx512/software/libevent/2.1.12-GCCcore-13.2.0/lib64/libevent_pthreads-2.1.so.7 (0x00007fadd98f0000)
+	libm.so.6 => /cvmfs/software.eessi.io/versions/2023.06/compat/linux/x86_64/lib/../lib64/libm.so.6 (0x00007fadd9810000)
+	/cvmfs/software.eessi.io/versions/2023.06/compat/linux/x86_64/lib64/ld-linux-x86-64.so.2 (0x00007fadd9fab000)
+
+```
+
+```sh
+
+# ldd from the host, even though the libraries point to the software layer, now the linker ld-linux-x86-64.so.2 from the compat layer directly points to "/lib64/ld-linux-x86-64.so.2" from the host to do the resolving, resulting in the GLIBC mismatch as libc is also resolved in the host and not the compat layer
+
+{EESSI 2023.06} $ /usr/bin/ldd HelloWorld
+./HelloWorld: /lib64/libc.so.6: version `GLIBC_2.36' not found (required by /cvmfs/software.eessi.io/versions/2023.06/software/linux/x86_64/intel/skylake_avx512/software/libevent/2.1.12-GCCcore-13.2.0/lib64/libevent_core-2.1.so.7)
+./HelloWorld: /lib64/libc.so.6: version `GLIBC_ABI_DT_RELR' not found (required by /cvmfs/software.eessi.io/versions/2023.06/compat/linux/x86_64/lib/../lib64/libm.so.6)
+	linux-vdso.so.1 (0x00007fffe4fd3000)
+	libmpi.so.40 => /cvmfs/software.eessi.io/versions/2023.06/software/linux/x86_64/intel/skylake_avx512/software/OpenMPI/4.1.6-GCC-13.2.0/lib/libmpi.so.40 (0x00007f1fdf571000)
+	libc.so.6 => /lib64/libc.so.6 (0x00007f1fdf200000)
+	[...]
+	libevent_pthreads-2.1.so.7 => /cvmfs/software.eessi.io/versions/2023.06/software/linux/x86_64/intel/skylake_avx512/software/libevent/2.1.12-GCCcore-13.2.0/lib64/libevent_pthreads-2.1.so.7 (0x00007f1fdf420000)
+	libm.so.6 => /cvmfs/software.eessi.io/versions/2023.06/compat/linux/x86_64/lib/../lib64/libm.so.6 (0x00007f1fdeeb1000)
+	/cvmfs/software.eessi.io/versions/2023.06/compat/linux/x86_64/lib64/ld-linux-x86-64.so.2 => /lib64/ld-linux-x86-64.so.2 (0x00007f1fdf698000)
+
+```
+Now is the moment of truth, if everything looks right when checking with ldd, you should be fine to run the program:
+
+```
+{EESSI 2023.06} $ mpirun -n 2 HelloWorld
+Hello World from Node 0
+Hello World from Node 1
+
+```
+Even when closing the shell and restarting the environment, the libraries should point to the directories we set in `LD_RUN_PATH`, still, remember to load the required dependencies before running the binary.
 
 !!! warning