[cfe-dev] libc++: Unexpected pointer-to-member-function behavior

Wed May 3 16:09:05 PDT 2017

This is a by-product of libc++'s use of __attribute__((always_inline,
visibility("hidden"))) on string::size (and many other functions).

std::string is externally instantiated in the dylib, this causes functions
like `string::size` to have a dylib definition.
Unfortunately this definition has hidden visibility which causes linker
failures when an out-of-line reference to `string::size` is generated.

This behavior should eventually be fixed for QoI reasons, even though the
standard forbids taking the address of a member function.

/Eric

On Fri, Apr 21, 2017 at 9:54 AM, David Blaikie via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> FWIW the C++ standard doesn't allow taking the address of standard library
> member functions (I don't know the wording - but I can go hunt for it), so
> far as I understand.
>
> That said, interesting failure - not what I'd have expected.
>
> On Thu, Apr 20, 2017 at 10:45 PM Adam C. Emerson via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> Hello. I'm not sure if this is a bug or if I'm using undefined
>> behavior or otherwise doing something screwy.
>>
>> My Minimal Working Reproducer is:
>>
>> ---------->8-----------
>>
>> #include <functional>
>> #include <iostream>
>> #include <string>
>>
>> int main(void) {
>>     auto s = ::std::string("Meow.");
>>     auto l = ::std::mem_fn(&::std::string::size);
>>
>>     ::std::cout << l(s) << ::std::endl;
>>
>>     return 0;
>> }
>>
>> ---------->8-----------
>>
>> If I compile this under g++ or with clang++ using libstdc++, this
>> compiles and works as expected. However, if I use clang++ (v4.0.0 on
>> Arch Linux x86_64) with libc++ (4.0.0), I get a link error:
>>
>> ---------->8-----------
>>
>> % clang++ -v -std=c++14 -stdlib=libc++ foo.cc
>> clang version 4.0.0 (tags/RELEASE_400/final)
>> Target: x86_64-unknown-linux-gnu
>> Thread model: posix
>> InstalledDir: /usr/bin
>> Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-pc-
>> linux-gnu/6.3.1
>> Found candidate GCC installation: /usr/bin/../lib64/gcc/x86_64-
>> pc-linux-gnu/6.3.1
>> Found candidate GCC installation: /usr/lib/gcc/x86_64-pc-linux-gnu/6.3.1
>> Found candidate GCC installation: /usr/lib64/gcc/x86_64-pc-
>> linux-gnu/6.3.1
>> Selected GCC installation: /usr/bin/../lib64/gcc/x86_64-
>> pc-linux-gnu/6.3.1
>> Candidate multilib: .;@m64
>> Candidate multilib: 32;@m32
>> Selected multilib: .;@m64
>>  "/usr/bin/clang-4.0" -cc1 -triple x86_64-unknown-linux-gnu -emit-obj
>> -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names
>> -main-file-name foo.cc -mrelocation-model static -mthread-model posix
>> -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases
>> -munwind-tables -fuse-init-array -target-cpu x86-64 -v -dwarf-column-info
>> -debugger-tuning=gdb -resource-dir /usr/bin/../lib/clang/4.0.0
>> -internal-isystem /usr/bin/../include/c++/v1 -internal-isystem
>> /usr/local/include -internal-isystem /usr/bin/../lib/clang/4.0.0/include
>> -internal-externc-isystem /include -internal-externc-isystem /usr/include
>> -std=c++14 -fdeprecated-macro -fdebug-compilation-dir /home/azure
>> -ferror-limit 19 -fmessage-length 119 -fobjc-runtime=gcc -fcxx-exceptions
>> -fexceptions -fdiagnostics-show-option -fcolor-diagnostics -o
>> /tmp/foo-1fe78b.o -x c++ foo.cc
>> clang -cc1 version 4.0.0 based upon LLVM 4.0.0 default target
>> x86_64-unknown-linux-gnu
>> ignoring nonexistent directory "/include"
>> #include "..." search starts here:
>> #include <...> search starts here:
>>  /usr/bin/../include/c++/v1
>>  /usr/local/include
>>  /usr/bin/../lib/clang/4.0.0/include
>>  /usr/include
>> End of search list.
>>  "/usr/bin/ld" --eh-frame-hdr -m elf_x86_64 -dynamic-linker
>> /lib64/ld-linux-x86-64.so.2 -o a.out /usr/bin/../lib64/gcc/x86_64-
>> pc-linux-gnu/6.3.1/../../../../lib64/crt1.o /usr/bin/../lib64/gcc/x86_64-
>> pc-linux-gnu/6.3.1/../../../../lib64/crti.o /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/crtbegin.o
>> -L/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1
>> -L/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../../../lib64
>> -L/usr/bin/../lib64 -L/lib/../lib64 -L/usr/lib/../lib64
>> -L/usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../..
>> -L/usr/bin/../lib -L/lib -L/usr/lib /tmp/foo-1fe78b.o -lc++ -lm -lgcc_s
>> -lgcc -lc -lgcc_s -lgcc /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/crtend.o
>> /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/6.3.1/../../../../lib64/crtn.o
>> /tmp/foo-1fe78b.o: In function `main':
>> foo.cc:(.text+0x44d): undefined reference to
>> `std::__1::basic_string<char, std::__1::char_traits<char>,
>> std::__1::allocator<char> >::size() const'
>> /usr/bin/ld: a.out: hidden symbol `_ZNKSt3__112basic_stringIcNS_
>> 11char_traitsIcEENS_9allocatorIcEEE4sizeEv' isn't defined
>> /usr/bin/ld: final link failed: Bad value
>> clang-4.0: error: linker command failed with exit code 1 (use -v to see
>> invocation)
>>
>> ---------->8-----------
>>
>> Thank you very much.
>> _______________________________________________
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>> cfe-dev at lists.llvm.org
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>>
>
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