CODEWITHSUNDEEP
gcccmakeportability
spraff
spraff

Reputation: 33445

Why is the CMake solution for "R_X86_64_32 can not be used when making a shared object" specific to x86_64?

I just got this error when compiling some plugins for my new toy (on gcc/g++ on Linux):

relocation R_X86_64_32 can not be used when making a shared object; recompile with -fPIC

I basically understand why PIC is needed but, within the CMake system, the solution seems to be this:

IF (CMAKE_SYSTEM_PROCESSOR STREQUAL "x86_64")
    SET_TARGET_PROPERTIES (${PLUGIN_BASE_LIB} PROPERTIES COMPILE_FLAGS "-fPIC")
ENDIF (CMAKE_SYSTEM_PROCESSOR STREQUAL "x86_64")

I don't understand why this solution is conditional.

The follow-up seems to suggest that -fPIC should be used basically everywhere except 32-bit Linux, which suggests that the above is not portable.

Should I always use -fPIC? Will there be any adverse effects?

${PLUGIN_BASE_LIB} needs to be statically linked to both the main executable, and statically the various shared libraries which are the plugins.

Upvotes: 4

Views: 840

Answers (1)

yugr
yugr

Reputation: 21954

Ideally you want to build two versions of code: one for the main executable and one for the library. The first will need to be compiled with -fPIE (which is default in modern distros) and the second with -fPIC. As you point out this does not depend on target architecture.

You can compile only one version with -fPIC but then main executable will be suboptimal because -fPIC forces compiler to obey symbol interposition rules which significantly limits it's ability to optimize code e.g. inline and clone functions.

Upvotes: 0

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