【发布时间】：2017-10-31 20:27:03
【问题描述】：
我必须使用 g++ 6.4.0 (Homebrew g++-6) 将 c++ 代码编译为静态库，然后将其包装到 C 静态库 (Homebrew gcc-6) 中并链接到macos sierra 上的 clang++ (clang 8.1.0) 应用程序。所以图片是：
c++ (gcc) wrapped in c (gcc) linked to clang app.
作为测试用例，我使用 shared-lib.cpp：
#include <iostream>
using namespace std;
void foo()
{
cerr << "Hi from the shared lib" << endl;
}
连同shared-lib.h
extern void foo();
和wrapper-lib.c
#include "shared-lib.h"
int wrapper()
{
foo();
return 123;
}
连同wrapper-lib.h
#ifdef __cplusplus
extern "C"
{
#endif
extern int wrapper();
#ifdef __cplusplus
}
#endif
使用所有库的main.cpp看起来像
#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;
}
我的测试构建脚本是
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
如果我只调用 gcc c++ 函数 foo() 那么一切正常。 如果我调用 C 包装函数 wrapper()，那么 clang 会得出：
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)
也许有人可以简单地发现，我的工作流程有什么问题？
注意，为了完整起见整个构建脚本输出
Note2，由于 gcc@6 工具链中的 ar 不起作用（缺少 liblto_plugin.so）我使用 clang 的 ar 工具.. .
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)
【问题讨论】：
-
糟糕。看起来，这不是 GCC - Clang 问题，因为如果我纯粹使用 clang++ / clang 尝试所有操作，也会发生链接器错误。所以现在的问题是**如何摆脱仍然存在的讨厌的链接器错误，如果我尝试将静态 c++ 库包装在普通 c 库中..