MinGW + Boost：对“ WSAStartup @ 8”的未定义引用

Question

below is what I execute

D:\\Just4Fun\\software\\>make -f Makefile.hands stest
g++.exe -g -D_WIN32_WINNT=0x0601 -ID:\\Boost\\boost_1_51_0 -LD:\\Boost\\boost_1_51_0\\stage\\lib -LD:\\MinGW\\lib -lboost_system-mgw46-d-1_51 -lboost_filesystem-mgw46-d-1_51 -lboost_iostreams-mgw46-d-1_51 -lws2_32 -lwsock32 -o TestSerial.exe TestSerial.cpp

below is the result of that

C:\\Users\\oleg\\AppData\\Local\\Temp\\ccXpC7RG.o: In function 'ZN5boost4asio6detail17winsock_init_base7startupERNS2_4dataEhh': D:/Boost/boost_1_51_0/boost/asio/detail/impl/winsock_init.ipp:39: undefined reference to 'WSAStartup@8'C:\\Users\\oleg\\AppData\\Local\\Temp\\ccXpC7RG.o: In function 'ZN5boost4asio6detail17winsock_init_base7cleanupERNS2_4dataE': D:/Boost/boost_1_51_0/boost/asio/detail/impl/winsock_init.ipp:48: undefined reference to 'WSACleanup@0' collect2: ld returned 1 exit status make: *** [stest] Error 1

Below is result of checking libraries regarding to mentioned symobls (I failed to format it in appropriate way)

`d:\\MinGW\\lib>nm -C libws2_32.a | grep WSAStartup
00000000 T WSAStartup@8
00000000 I imp _WSAStartup@8

d:\\MinGW\\lib>nm -C libws2_32.a | grep WSACleanup
00000000 T WSACleanup@0
00000000 I imp _WSACleanup@0

d:\\MinGW\\lib>nm -C libwsock32.a | grep WSACleanup
00000000 T WSACleanup@0
00000000 I imp _WSACleanup@0

d:\\MinGW\\lib>nm -C libwsock32.a | grep WSAStartup
00000000 T WSAStartup@8
00000000 I imp _WSAStartup@8`

As wee see symbols are existing in the libraries and libraries are on the place referred by the corresponding -L command line options.
So, why "undefined refernce"????

Answer 1

It's very common error, with a very simple solution: Place the libraries you link with after the source and object files.

---

If an object (or source) file A depends on a library B, then A must come before B on the command line when linking. This is because of how the GNU (and possibly other) linkers solve symbol dependencies.

If doing it the opposite (and wrong) way

g++ -lB A.cpp

then when the linker loads the library B, there are no dependencies on any of the functions in the library, so the linker just discards that library.

So the right way is to put the library last

g++ A.cpp -lB

This is the reason many examples will always put libraries last on the command line.

So for the command line of the question, it should instead be

g++.exe -g -D_WIN32_WINNT=0x0601 -ID:\Boost\boost_1_51_0 \
    -LD:\Boost\boost_1_51_0\stage\lib -LD:\MinGW\lib \
    -o TestSerial.exe TestSerial.cpp \
    -lboost_system-mgw46-d-1_51 -lboost_filesystem-mgw46-d-1_51 -lboost_iostreams-mgw46-d-1_51 -lws2_32 -lwsock32

This order is also important for inter-library dependencies. If library L1 depends on library L2, then L1 must be before L2 on the command line.