Tuesday, March 22, 2011

How to fix -fPIC errors

A very good article on fPIC error. See 3. HOWTO fix -fPIC errors by Gentoo Linux

If you have problem like " relocation R_X86_64_32 against `a local symbol' can not be used when making a shared object; recompile with -fPIC .libs/assert.o: could not read symbols: Bad value ".

The article lists 4 cases of fPIC

Case 1: Broken Compiler
At least GCC 3.4 is known to have a broken implementation of the -fvisibility-inlines-hidden flag. The use of this flag is therefore highly discouraged, reported bugs are usually marked as RESOLVED INVALID. See bug 108872 for an example of a typical error message caused by this flag."
Case 2: Broken `-fPIC' support checks in configure
Many configure tools check whether the compiler supports the -fPIC flag or not. They do so by compiling a minimalistic program with the -fPIC flag and checking stderr. If the compiler prints *any* warnings, it is assumed that the -fPIC flag is not supported by the compiler and is therefore abandoned. Unfortunately, if the user specifies a non-existing flag (i.e. C++-only flags in CFLAGS or flags introduced by newer versions of GCC but unknown to older ones), GCC prints a warning too, resulting in borkage.

To prevent this kind of breakage, the AMD64 profiles use a bashrc that filters out invalid flags in C[XX]FLAGS

Case 3: Lack of `-fPIC' flag in the software to be built
This is the most common case. It is a real bug in the build system and should be fixed in the ebuild, preferably with a patch that is sent upstream. Assuming the error message looks like this:

Code Listing 6.1: A sample error message
.libs/assert.o: relocation R_X86_64_32 against `a local symbol' can not be used
when making a shared object; recompile with -fPIC .libs/assert.o: could not
read symbols: Bad value

This means that the file assert.o was not compiled with the -fPIC flag, which it should. When you fix this kind of error, make sure only objects that are used in shared libraries are compiled with -fPIC.
In this case, globally adding -fPIC to C[XX]FLAGS resolves the issue, although this practice is discouraged because the executables end up being PIC-enabled, too.

 Case 4: Linking dynamically against static archives
Sometimes a package tries to build shared libraries using statically built archives which are not PIC-enabled. There are two main reasons why this happens:
Often it is the result of mixing USE=static and USE=-static. If a library package can be built statically by setting USE=static, it usually doesn't create a .so file but only a .a archive. However, when GCC is given the -l flag to link to said (dynamic or static) library, it falls back to the static archive when it can't find a shared lib. In this case, the preferred solution is to build the static library using the -fPIC flag too.

Sometimes it is also the case that a library isn't intended to be a shared library at all, e.g. because it makes heavy usage of global variables. In this case the solution is to turn the to-be-built shared library into a static one.

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