problems on AMD64 requiring -fPIC

[jsamort]

Hi All,

I am having a recurring set of problems on my gentoo AMD64 box, getting errors like:

xxx can not be used when making a shared object; recompile with -fPIC

Specifically, I am developing a shared library, and am attempting to link into it several static libraries (i.e. Boost, ANTLR). On an x86 platform, this is not a problem, however, AMD64 requires the static libraries to have been compiled with the -fPIC option. Now, the general libtool default is to build shared libraries from PIC objects and static archives from non-PIC objects, so anyone attempting to do what I am trying to do runs into a non-trivial task of editing a bunch of ebuilds to get various libraries compiled with the necessary -fPIC. This can involve several time-consuming rebuilds. For example, to attempt to build Boost with the -fPIC option causes trouble with gcc (crtbeginT.o to be specific), and would also require rebuilding gcc with -fPIC.

Now, I am not an expert, and I don't know if there is a downside to compiling a static library with -fPIC versus non-PIC objects, and info on the web is fairly limited. I thought I would consult the gentoo AMD64 community and see if any guidelines exist on the subject. Currently, I am stuck with locally building some libraries, or maintaining some modified ebuilds in /usr/local/portage. I am hoping there may be a better/more accepted way to deal with this across the gentoo AMD64 distribution.

Best,
Scott

[loftwyr]

Can you show us your CFLAGS and LDFLAGS and such (emerge --info will do nicely)

You may have flags that are making things difficult for the linker.

[jsamort]

You may have flags that are making things difficult for the linker.

No, my CFLAGS are quite conservative:

Code: Select all

CFLAGS="-march=k8 -O2 -pipe -msse3"

This is a (not well) documented design decision on AMD64. You can read a bit about it here:

http://www.x86-64.org/lists/discuss/msg05760.html
http://www.physics.uq.edu.au/people/fos ... rting.html

In essence, if a user of a static library on AMD64 ever wishes to link that library to a shared library (versus an executable), it must be compiled with -fPIC. And since this is not an issue with x86, very few Gentoo static libraries are compiled with that flag. Ultimately, every ebuild for a static library should have a check for AMD64 and insert -fPIC if necessary. Here is a quote from that first link:

I.e. the rule of thumb for suppliers of libraries: If you only provide static libraries they _must_ be compiled with -fPIC, otherwise the users of them can't link them into shared libs on their own.

Best,
Scott

[jsamort]

As a follow-up, I discovered some Gentoo-specific documentation here:

http://devmanual.gentoo.org/archs/amd64/index.html

However, while it does discuss the issue with position-dependent code, it focuses on shared libraries. It is a bit redundant, as I believe all shared libraries must use -fPIC, regardless of architecture. Instead, this section should be reworked to apply to static libraries, which is where the AMD64-specific problem exists.

[friedmud]

Has there been any movement on this since?

I am trying to link a python module against mpich (pypar) and am getting these errors.... is there a Gentooish way to implement this change or do I need to go edit the mpich ebuild?

Thanks!
Friedmud