fPIC and its effect on BASIN dependencies

Hi Basin Group,

I've looked into the fPIC issue and how it relates to BASIN. I've listed
my findings below. I plan to add this information to the wiki once we
have the troubleshooting section created.

A -fpic or -fPIC flag should be used when compiling source files that
will eventually be included in a shared library.

On normal x86 systems, -fpic should suffice for code compiled by gcc.
The issue is that on amd64 systems, -fPIC (yes, the capital letters are
the only difference) should be used to support that system's architecture.

This information is important to Basin when it comes time to build our
code and put it into shared libraries (which is the default mode for
building basin).

Thankfully, we are using libtool which takes care of the decision of
what fpic flags to use. So far, it seems that the correct choice is made
on the systems we build basin for.

However, some dependencies of basin that are *static* libraries need to
be taken into consideration when doing this.

Namely, MPICH2 builds its C++ library as a static library. This means
that on amd64 systems MPICH2 should compile its source using the -fPIC
flag. It appears that the configure file for MPICH2 does not handle this
automatically. Therefore, it is necessary to use something like the
following to properly configure MPICH2 for shared libraries on amd64
systems:
        CFLAGS="-fPIC" CXXFLAGS="-fPIC" ./configure <options>

If MPICH2 is *not* built using -fPIC on amd64, then it will only be
possible to build *static* libraries for any source that relies on
MPICH2, including basin.

The situation becomes very complicated when you consider that packages
such as HDF5 and FFTW rely on MPICH2 when they build. These packages
will most likely default to building shared libraries, causing errors on
amd64 systems using MPICH2 without fPIC.

To make matters worse, an error will occur when the code that is
building against MPICH2 tries to create a shared library. That means
that you may see build errors when compiling HDF5, FFTW, or basin that
are the result of an incorrectly built MPICH2 installation.

So, on amd64 systems there are two general paths to follow.

1) Force MPICH2 to build with -fPIC flags as listed above. All packages
can then build using their default options, which is likely a shared
library.

2) Build only *static* libraries for anything that relies on MPICH2.
This includes basin. To build the basin_kernel using static libraries
one should do the following
        ./configure <options> --enable-shared=no
Other packages that depend on MPICH2 will likely also need similar
changes to their configure flags. However, the flag to use is not
universal so the documentation for each package will need to be
consulted to find the appropriate information.
Received on Thu Nov 29 13:51:13 2007