在Cygwin上链接Boost库

Question

I have been at this for several hours now so I have come here to ask for help. Pretty sure I almost have it figured out but I continue to have linker errors of undefined references to boost::system::generic_category and boost::system::system_category .

I have just one file I am trying to link to make an executable.

I began with compiling it to make an object file:

g++ -c main.cpp -IC:/boost/boost_1_61_0

This successfully creates main.o.

My next and final objective is to link it to an executable. I have tried different things from what I have read on other posts:

g++ main.o -LC:/boost/boost_1_61_0/stage/lib

g++ main.o -LC:/boost/boost_1_61_0/stage/lib/libboost_system.a

g++ main.o -lboost_system

The result either tells me it can't find the library or something like:

main.o:main.cpp:(.text+0x89): undefined reference to `boost::system::generic_category()'
main.o:main.cpp:(.text+0x89): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `boost::system::generic_category()'
main.o:main.cpp:(.text+0x95): undefined reference to `boost::system::generic_category()'
main.o:main.cpp:(.text+0x95): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `boost::system::generic_category()'
main.o:main.cpp:(.text+0xa1): undefined reference to `boost::system::system_category()'
main.o:main.cpp:(.text+0xa1): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `boost::system::system_category()'
main.o:main.cpp:(.text$_ZN5boost11this_thread9sleep_forERKNS_6chrono8durationIlNS_5ratioILl1ELl1000000000EEEEE[_ZN5boost11this_thread9sleep_forERKNS_6chrono8durationIlNS_5ratioILl1ELl1000000000EEEEE]+0x24): undefined reference to `boost::this_thread::hiden::sleep_for(timespec const&)'
main.o:main.cpp:(.text$_ZN5boost11this_thread9sleep_forERKNS_6chrono8durationIlNS_5ratioILl1ELl1000000000EEEEE[_ZN5boost11this_thread9sleep_forERKNS_6chrono8durationIlNS_5ratioILl1ELl1000000000EEEEE]+0x24): relocation truncated to fit: R_X86_64_PC32 against undefined symbol `boost::this_thread::hiden::sleep_for(timespec const&)'
collect2: error: ld returned 1 exit status

I know I built the boost libraries correctly as there is a libboost_system.a file along many other libraries in the stage/lib directory. Any ideas please?

Answer 1

Let's start by looking at the commands you've tried.

g++ main.o -L C:/boost/boost_1_61_0/stage/lib

This tells g++ to look for libraries in the C:/boost/boost_1_61_0/stage/lib directory. It doesn't say which libraries to pull in, but once you do, g++ will look there.

Since your code has references to things (like boost::system::generic_category ) found in boost_system and since you did not tell the linker to pull in that library, those references end up being undefined.

g++ main.o -L C:/boost/boost_1_61_0/stage/lib/libboost_system.a

This tells g++ to look for libraries in the C:/boost/boost_1_61_0/stage/lib/libboost_system.a directory. Since this is (presumably) not a directory, there is no real effect of the -L flag.

g++ main.o -lboost_system

This tells g++ to link in the boost_system library. While the linker knows how to convert a library name (eg boost_system ) to the corresponding file name (eg libboost_system.a ), there is no indication of where this file can be found. So the linker will look in the default directories it knows of. When the file is not found there, g++ complains about being unable to find the library.

---

At this point you should see the two pieces that need to be combined: tell the linker which library to pull in and where to find it.

g++ main.o -lboost_system -L C:/boost/boost_1_61_0/stage/lib