MPICH's test for weak symbols is: int foo(int) __attribute__((weak,alias("__foo"))); where __foo is not defined in the same compilation unit. This fails with gcc (tested with 4.8 and 4.4): conftest.c:37:5: error: 'foo' aliased to undefined symbol '__foo' int foo(int) __attribute__((weak,alias("__foo"))); ^ This is (evidently) a change from gcc-3.x, but is intentional because knowledge of the weak alias is not desirable/meaningful in other compilation units. http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20652 My understanding of gcc's intent is that the attribute only be stated in the implementation. The caller can't do anything with the information that foo is a weak alias; it still needs to generate a PLT entry and the loader will need to perform a relocation. Now my problem is that clang accepts the test code above (i.e., it does not insist that __foo be defined in the same compilation unit) which causes #define MPICH_ATTR_WEAK_ALIAS(fname) __attribute__((weak,alias(fname))) but this breaks PMPI and when -fvisibility=hidden is used to compile user code, it generates (objdump -r) RELOCATION RECORDS FOR [.text]: OFFSET TYPE VALUE 0000000000000005 R_X86_64_PC32 __foo-0x0000000000000004 instead of the linkable (but not desired because it prevents interposing "foo") RELOCATION RECORDS FOR [.text]: OFFSET TYPE VALUE 0000000000000005 R_X86_64_PLT32 __foo-0x0000000000000004 What we actually want is an honest relocation for "foo": RELOCATION RECORDS FOR [.text]: OFFSET TYPE VALUE 0000000000000005 R_X86_64_PLT32 foo-0x0000000000000004 When you try to link the first version later, you get /usr/bin/ld.gold: error: bar.o: requires dynamic R_X86_64_PC32 reloc against '__foo' which may overflow at runtime; recompile with -fPIC which is not actually an -fPIC error, but rather a result of clang using R_X86_64_PC32 for a symbol that we really want to be R_X86_64_PLT32. I think this should probably be reported to Clang/LLVM (because it delays a confusing error condition), but I also think that MPICH should fix its usage. So in summary, we can't have __attribute__((weak,alias("PMPI_..."))) in the header because: 1. it does not compile with gcc 2. it breaks PMPI interposition with clang 3. it breaks -fvisibility=hidden with clang Instead, it should go in the implementation files.