Linker Error when compiling with static library [duplicate] - c++

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Why does the order in which libraries are linked sometimes cause errors in GCC?
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I have a project with several .cpp and .h files, and I am trying to split the source into core / generic stuff, and application specific code. Based on that, I separated the code files into a Common folder and compiled it as a library. Below is the Makefile for that lib:
CFLAGS=-std=c++11 -g -c -pedantic -Wall -Wextra -I../boost_1_57_0 -L../boost_1_57_0/stage/lib
SOURCES = $(wildcard *.cpp)
OBJECTS=$(SOURCES:.cpp=.o)
OUTPUTFILE=libcommon.a
all: $(OUTPUTFILE)
$(OUTPUTFILE): $(OBJECTS)
ar rcs $(OUTPUTFILE) $(OBJECTS)
.cpp.o:
$(CC) $(CPPFLAGS) $(CFLAGS) $< -o $# $(GPROF)
clean:
rm -f $(OBJECTS) $(OUTPUTFILE)
Then I created a sample main file just to try out the compilation of an app using that library.
Here is the code of the main source file:
#include "../Common/Functions.h"
#include "../Common/Logger.h"
int main() {
Logger::Init(false, false);
Logger::Debug("Test");
string path = Functions::GetAppPath();
Logger::Debug("App Path: ", path.c_str());
return 0;
}
The functions file that is being referenced uses boost, and here is the makefile for this app that uses the library:
CC=g++
CFLAGS=-Wl,--verbose -std=c++11 -g -c -pedantic -Wall -Wextra -I../boost_1_57_0 -L../boost_1_57_0/stage/lib
LDFLAGS=-I../boost_1_57_0 -L../boost_1_57_0/stage/lib -L../boost_libs/lib -L/usr/lib64 -L/usr/kerberos/lib -L/usr/lib -L../Common
LIBS=-lz -lkrb5 -lk5crypto -lcom_err -lresolv -lm -lpthread -lrt -lboost_system -lboost_filesystem -lboost_thread -lboost_date_time -rdynamic -lcurl -lcommon
SOURCES = $(wildcard *.cpp)
OBJECTS=$(SOURCES:.cpp=.o)
EXECUTABLE=testApp
GPROF=-pg
all: $(SOURCES) $(EXECUTABLE)
$(EXECUTABLE): $(OBJECTS)
$(CC) $(LDFLAGS) $(OBJECTS) -o $# $(LIBS) $(GPROF)
.cpp.o:
$(CC) $(CPPFLAGS) $(CFLAGS) $< -o $# $(GPROF)
clean:
rm -rf *o $(OBJECTS)
When I try to compile, I get the following errors, complaining about undefined references to boost:
g++ -Wl,--verbose -std=c++11 -g -c -pedantic -Wall -Wextra -I../boost_1_57_0 -L../boost_1_57_0/stage/lib main.cpp -o main.o -pg
g++ -I../boost_1_57_0 -L../boost_1_57_0/stage/lib -L../boost_libs/lib -L/usr/lib64 -L/usr/kerberos/lib -L/usr/lib -L../Common main.o -o testApp -lz -lkrb5 -lk5crypto -lcom_err -lresolv -lm -lpthread -lrt -lboost_system -lboost_filesystem -lboost_thread -lboost_date_time -rdynamic -lcurl -lcommon -pg
../Common/libcommon.a(Functions.o): In function `Functions::GetAppPath()':
/media/software/Robots/Common/Functions.cpp:43: undefined reference to `boost::filesystem::path::parent_path() const'
../Common/libcommon.a(Functions.o): In function `boost::filesystem::initial_path()':
/media/software/Robots/Common/../boost_1_57_0/boost/filesystem/operations.hpp:583: undefined reference to `boost::filesystem::detail::initial_path(boost::system::error_code*)'
../Common/libcommon.a(Functions.o): In function `boost::filesystem::system_complete(boost::filesystem::path const&)':
/media/software/Robots/Common/../boost_1_57_0/boost/filesystem/operations.hpp:655: undefined reference to `boost::filesystem::detail::system_complete(boost::filesystem::path const&, boost::system::error_code*)'
../Common/libcommon.a(Functions.o): In function `unsigned short boost::date_time::month_str_to_ushort<boost::gregorian::greg_month>(std::string const&)':
/media/software/Robots/Common/../boost_1_57_0/boost/date_time/date_parsing.hpp:67: undefined reference to `boost::gregorian::greg_month::get_month_map_ptr()'
../Common/libcommon.a(Logger.o): In function `Logger::Cleanup()':
/media/software/Robots/Common/Logger.cpp:84: undefined reference to `boost::thread::interrupt()'
../Common/libcommon.a(Logger.o): In function `Logger::InitFile(std::string, bool, std::_Ios_Openmode)':
/media/software/Robots/Common/Logger.cpp:186: undefined reference to `boost::filesystem::path::parent_path() const'
/media/software/Robots/Common/Logger.cpp:187: undefined reference to `boost::filesystem::path::parent_path() const'
../Common/libcommon.a(Logger.o): In function `Logger::Process()':
/media/software/Robots/Common/Logger.cpp:241: undefined reference to `boost::this_thread::interruption_point()'
../Common/libcommon.a(Logger.o): In function `boost::detail::thread_data_base::thread_data_base()':
/media/software/Robots/Common/../boost_1_57_0/boost/thread/pthread/thread_data.hpp:143: undefined reference to `vtable for boost::detail::thread_data_base'
../Common/libcommon.a(Logger.o): In function `boost::thread::start_thread()':
/media/software/Robots/Common/../boost_1_57_0/boost/thread/detail/thread.hpp:179: undefined reference to `boost::thread::start_thread_noexcept()'
../Common/libcommon.a(Logger.o): In function `boost::thread::~thread()':
/media/software/Robots/Common/../boost_1_57_0/boost/thread/detail/thread.hpp:254: undefined reference to `boost::thread::detach()'
../Common/libcommon.a(Logger.o): In function `boost::thread::get_id() const':
/media/software/Robots/Common/../boost_1_57_0/boost/thread/detail/thread.hpp:741: undefined reference to `boost::thread::native_handle()'
../Common/libcommon.a(Logger.o): In function `boost::thread::join()':
/media/software/Robots/Common/../boost_1_57_0/boost/thread/detail/thread.hpp:767: undefined reference to `boost::thread::join_noexcept()'
../Common/libcommon.a(Logger.o): In function `boost::filesystem::exists(boost::filesystem::path const&)':
/media/software/Robots/Common/../boost_1_57_0/boost/filesystem/operations.hpp:404: undefined reference to `boost::filesystem::detail::status(boost::filesystem::path const&, boost::system::error_code*)'
../Common/libcommon.a(Logger.o): In function `boost::filesystem::create_directories(boost::filesystem::path const&)':
/media/software/Robots/Common/../boost_1_57_0/boost/filesystem/operations.hpp:523: undefined reference to `boost::filesystem::detail::create_directories(boost::filesystem::path const&, boost::system::error_code*)'
../Common/libcommon.a(Logger.o): In function `boost::detail::thread_data<void (*)()>::~thread_data()':
/media/software/Robots/Common/../boost_1_57_0/boost/thread/detail/thread.hpp:90: undefined reference to `boost::detail::thread_data_base::~thread_data_base()'
../Common/libcommon.a(Logger.o):(.rodata._ZTIN5boost6detail11thread_dataIPFvvEEE[_ZTIN5boost6detail11thread_dataIPFvvEEE]+0x10): undefined reference to `typeinfo for boost::detail::thread_data_base'
collect2: error: ld returned 1 exit status
Needless to say that the project as a whole compiles with no issue.
This is my first attempt at building a c++ linux library and using it in another application, so I might be doing a rookie mistake, so bear that in mind.
Help is very much appreciated.
Thank you.

Answer 1

The order of libraries on the command line is significant to the linker. It processes libraries from left to right, resolving as-yet unresolved references already known to it against any functions provided by the current library.
Moreover, and particularly relevant here, if a given library contains references to functions that it does not itself provide then the linker does not resolve those against libraries it has already processed. That's why you have unresolved references: you have -lcommon last among your libraries, so function references therein will be resolved against only the C standard library.
It is valid to list libraries more than once in the link command, and sometimes that is needed, but in this case I think it makes sense simply to put -lcommon as the first library instead of the last. That will resolve the issue. You can even conceptualize this as putting the library most closely related to the main program closest to it in the link command.