Linker error using gcc and clang together on macos sierra - macos

Question

I have to compile c++ code with g++ 6.4.0 (Homebrew g++-6) to a static lib, which is then wrapped into a C static lib (Homebrew gcc-6) and linked to a clang++ (clang 8.1.0) app on macos sierra. So the picture is:
c++ (gcc) wrapped in c (gcc) linked to clang app.
As a testcase I use shared-lib.cpp:
#include <iostream>
using namespace std;
void foo()
{
cerr << "Hi from the shared lib" << endl;
}
together with shared-lib.h
extern void foo();
and wrapper-lib.c
#include "shared-lib.h"
int wrapper()
{
foo();
return 123;
}
along with wrapper-lib.h
#ifdef __cplusplus
extern "C"
{
#endif
extern int wrapper();
#ifdef __cplusplus
}
#endif
The main.cpp that uses all the libs looks like
#include <iostream>
#include <string>
#include "shared-lib.h"
#include "wrapper-lib.h"
using namespace std;
int main()
{
auto s = "Hello world from main";
cout << s << endl;
foo(); // from c++ lib
int result = wrapper(); // from c wrapper lib
cout << "wrapper returned " << result << endl;
return 0;
}
My test built script is
g++-6 --version
echo -----------------------
echo build shared-lib .o with g++
g++-6 -c -Wall -fpic -std=c++11 shared-lib.cpp
echo build a wrapper library in C with gcc
gcc-6 -c -Wall -fpic wrapper-lib.c
echo build static libshared-lib.a
ar rcs libshared-lib.a shared-lib.o
echo build static libwrapper-lib.a
ar rcs libwrapper-lib.a wrapper-lib.o
echo build main with clang
clang++ --version
echo ----------------------
clang++ -v -L/Users/worker -Wall -std=c++11 -stdlib=libstdc++ -lwrapper-lib -lshared-lib main.cpp -o main
echo start the app
./main
If I only call the gcc c++ function foo() then everything works fine.
If I call the C wrapper function wrapper(), then clang comes up with:
Undefined symbols for architecture x86_64:
"_foo", referenced from:
_wrapper in libwrapper-lib.a(wrapper-lib.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Maybe someone can simply spot, what's wrong with my workflow?
Note, for completeness the whole build script output
Note2, since ar in the gcc#6 toolchain does not work (liblto_plugin.so missing) I use clang's ar tool...
mac-mini:~ worker$ ./build-test.sh
g++-6 (Homebrew GCC 6.4.0) 6.4.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
-----------------------
build shared-lib .o with g++
build a wrapper library in C with gcc
build static libshared-lib.a
build static libwrapper-lib.a
build main with clang
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 8.1.0 (clang-802.0.41)
Target: x86_64-apple-darwin16.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
----------------------
Apple LLVM version 8.1.0 (clang-802.0.41)
Target: x86_64-apple-darwin16.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
clang: warning: libstdc++ is deprecated; move to libc++ [-Wdeprecated]
"/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang" -cc1 -triple x86_64-apple-macosx10.12.0 -Wdeprecated-objc-isa-usage -Werror=deprecated-objc-isa-usage -emit-obj -mrelax-all -disable-free -disable-llvm-verifier -discard-value-names -main-file-name main.cpp -mrelocation-model pic -pic-level 2 -mthread-model posix -mdisable-fp-elim -masm-verbose -munwind-tables -target-cpu penryn -target-linker-version 278.4 -v -dwarf-column-info -debugger-tuning=lldb -resource-dir /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/8.1.0 -stdlib=libstdc++ -Wall -std=c++11 -fdeprecated-macro -fdebug-compilation-dir /Users/worker -ferror-limit 19 -fmessage-length 166 -stack-protector 1 -fblocks -fobjc-runtime=macosx-10.12.0 -fencode-extended-block-signature -fcxx-exceptions -fexceptions -fmax-type-align=16 -fdiagnostics-show-option -fcolor-diagnostics -o /var/folders/18/m18t0kxx03d7__31kg3wrsr40000gq/T/main-337db7.o -x c++ main.cpp
clang -cc1 version 8.1.0 (clang-802.0.41) default target x86_64-apple-darwin16.7.0
ignoring nonexistent directory "/usr/include/c++/4.2.1/i686-apple-darwin10/x86_64"
ignoring nonexistent directory "/usr/include/c++/4.0.0"
ignoring nonexistent directory "/usr/include/c++/4.0.0/i686-apple-darwin8/"
ignoring nonexistent directory "/usr/include/c++/4.0.0/backward"
#include "..." search starts here:
#include <...> search starts here:
/usr/include/c++/4.2.1
/usr/include/c++/4.2.1/backward
/usr/local/include
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/8.1.0/include
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include
/usr/include
/System/Library/Frameworks (framework directory)
/Library/Frameworks (framework directory)
End of search list.
"/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld" -demangle -lto_library /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/libLTO.dylib -no_deduplicate -dynamic -arch x86_64 -macosx_version_min 10.12.0 -o main -L/Users/worker -lwrapper-lib -lshared-lib /var/folders/18/m18t0kxx03d7__31kg3wrsr40000gq/T/main-337db7.o -lstdc++ -lSystem /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../lib/clang/8.1.0/lib/darwin/libclang_rt.osx.a
Undefined symbols for architecture x86_64:
"_foo", referenced from:
_wrapper in libwrapper-lib.a(wrapper-lib.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Answer 1

You compile shared-lib.cpp with:
g++-6 -c -Wall -fpic -std=c++11 shared-lib.cpp
And you compile wrapper-lib.c with:
gcc-6 -c -Wall -fpic wrapper-lib.c
Have a look at the symbol table of shared-lib.o. It's something like:
$ readelf -s shared-lib.o
Symbol table '.symtab' contains 24 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FILE LOCAL DEFAULT ABS shared-lib.cpp
2: 0000000000000000 0 SECTION LOCAL DEFAULT 1
3: 0000000000000000 0 SECTION LOCAL DEFAULT 3
4: 0000000000000000 0 SECTION LOCAL DEFAULT 4
5: 0000000000000000 0 SECTION LOCAL DEFAULT 5
6: 0000000000000000 1 OBJECT LOCAL DEFAULT 5 _ZStL19piecewise_construc
7: 0000000000000000 1 OBJECT LOCAL DEFAULT 4 _ZStL8__ioinit
8: 0000000000000032 73 FUNC LOCAL DEFAULT 1 _Z41__static_initializati
9: 000000000000007b 21 FUNC LOCAL DEFAULT 1 _GLOBAL__sub_I_shared_lib
10: 0000000000000000 0 SECTION LOCAL DEFAULT 6
11: 0000000000000000 0 SECTION LOCAL DEFAULT 9
12: 0000000000000000 0 SECTION LOCAL DEFAULT 10
13: 0000000000000000 0 SECTION LOCAL DEFAULT 8
14: 0000000000000000 50 FUNC GLOBAL DEFAULT 1 _Z3foov
15: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND _GLOBAL_OFFSET_TABLE_
16: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND _ZSt4cerr
17: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND _ZStlsISt11char_traitsIcE
18: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND _ZSt4endlIcSt11char_trait
19: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND _ZNSolsEPFRSoS_E
20: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND _ZNSt8ios_base4InitC1Ev
21: 0000000000000000 0 NOTYPE GLOBAL HIDDEN UND __dso_handle
22: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND _ZNSt8ios_base4InitD1Ev
23: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND __cxa_atexit
(I'm working on Ubuntu, not OS X.)
Note that there is only one global function defined in this object file and
its name is _Z3foov.
That's the mangled name of the C++ function called foo in shared-lib.cpp. That's
the name the linker sees.
Now the symbol table of wrapper-lib.o:
Symbol table '.symtab' contains 11 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 FILE LOCAL DEFAULT ABS wrapper-lib.c
2: 0000000000000000 0 SECTION LOCAL DEFAULT 1
3: 0000000000000000 0 SECTION LOCAL DEFAULT 3
4: 0000000000000000 0 SECTION LOCAL DEFAULT 4
5: 0000000000000000 0 SECTION LOCAL DEFAULT 6
6: 0000000000000000 0 SECTION LOCAL DEFAULT 7
7: 0000000000000000 0 SECTION LOCAL DEFAULT 5
8: 0000000000000000 21 FUNC GLOBAL DEFAULT 1 wrapper
9: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND _GLOBAL_OFFSET_TABLE_
10: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UND foo
This object file makes an undefined reference to foo, because wrapper-lib.c
is a C source file and you compiled it as such. C does not mangle names. No definition
of foo is provided by any object file in your linkage, so it fails with that
symbol unresolved.
To avoid this and accomplish your linkage, you can direct the C++ compiler
not to mangle the name foo, when compiling shared-lib.cpp. You do so like:
shared-lib.cpp
#include <iostream>
using namespace std;
extern "C" {
void foo()
{
cerr << "Hi from the shared lib" << endl;
}
} //extern "C"
Enclosing the definition of foo in extern "C" {...} has no effect on
C++ compilation except the one you want: the symbol foo will be emitted
as a C symbol; not mangled.
Having done that, you must of course follow suit in shared-lib.h:
shared-lib.h
#ifndef SHARED_LIB_H
#define SHARED_LIB_H
#ifdef __cplusplus
extern "C" {
#endif
void foo();
#ifdef __cplusplus
}
#endif
#endif
With those corrections, let's try again:
$ g++-6 -c -Wall -fpic -std=c++11 shared-lib.cpp
and check the symbol table:
$ readelf -s shared-lib.o | grep foo
14: 0000000000000000 50 FUNC GLOBAL DEFAULT 1 foo
Now the one global function defined is foo, not _Z3foov, and your
linkage will succeed.
If you want to write a C++ library that exports a C++ API and not a C API to
the linker, then you cannot call its API from C except by discovering the
mangled names of the API (with readelf, nm, objdump) and explicitly
calling those mangled names from C. Thus without those extern "C" fixes,
your linkage would also succeed with:
wrapper-lib.c
extern void _Z3foov(void);
int wrapper()
{
_Z3foov();
return 123;
}