how is linking done in c++? - c++

Question

i am using proxygen library by facebook to build a simple client example . in a directory i have two object files how do i link them :
i am using:
g++ -std=c++11 -o my_echo CurlClientMain.o CurlClient.o -lproxygenhttpserver -lfolly -lglog -lgflags -pthread
i think i ma missing some linker flag like -lgflags in above example. maybe after including some -someflag will help out compilation . how do i know what all possilble library flags are posiible like the one they have used -lproxyhttpserver.
In short where is all these libs defined or located. i am using ubuntu.
Here is my error message
In function `main':
/home/kshitij/proxygen/httpclient/samples/curl/CurlClientMain.cpp:91: undefined reference to `proxygen::HTTPConnector::HTTPConnector(proxygen::HTTPConnector::Callback*, folly::HHWheelTimer*)'
/home/kshitij/proxygen/httpclient/samples/curl/CurlClientMain.cpp:102: undefined reference to `proxygen::HTTPConnector::connect(folly::EventBase*, folly::SocketAddress const&, std::chrono::duration<long, std::ratio<1l, 1000l> >, std::map<folly::AsyncSocket::OptionKey, int, std::less<folly::AsyncSocket::OptionKey>, std::allocator<std::pair<folly::AsyncSocket::OptionKey const, int> > > const&, folly::SocketAddress const&)'
/home/kshitij/proxygen/httpclient/samples/curl/CurlClientMain.cpp:91: undefined reference to `proxygen::HTTPConnector::~HTTPConnector()'
/home/kshitij/proxygen/httpclient/samples/curl/CurlClientMain.cpp:99: undefined reference to `proxygen::HTTPConnector::connectSSL(folly::EventBase*, folly::SocketAddress const&, std::shared_ptr<folly::SSLContext> const&, ssl_session_st*, std::chrono::duration<long, std::ratio<1l, 1000l> >, std::map<folly::AsyncSocket::OptionKey, int, std::less<folly::AsyncSocket::OptionKey>, std::allocator<std::pair<folly::AsyncSocket::OptionKey const, int> > > const&, folly::SocketAddress const&, std::string const&)'
/home/kshitij/proxygen/httpclient/samples/curl/CurlClientMain.cpp:91: undefined reference to `proxygen::HTTPConnector::~HTTPConnector()'
CurlClient.o: In function `CurlService::CurlClient::connectSuccess(proxygen::HTTPUpstreamSession*)':
/home/kshitij/proxygen/httpclient/samples/curl/CurlClient.cpp:69: undefined reference to `proxygen::HTTPUpstreamSession::newTransaction(proxygen::HTTPTransactionHandler*)'
collect2: error: ld returned 1 exit status
kshitij#forgetit:~/proxygen/httpclient/samples/curl$ g++ -std=c++11 -o my_echo CurlClientMain.o CurlClient.o -lproxygenhttpserver -lfolly -lglog -lgflags -pthread
CurlClientMain.o: In function `main':
/home/kshitij/proxygen/httpclient/samples/curl/CurlClientMain.cpp:91: undefined reference to `proxygen::HTTPConnector::HTTPConnector(proxygen::HTTPConnector::Callback*, folly::HHWheelTimer*)'
/home/kshitij/proxygen/httpclient/samples/curl/CurlClientMain.cpp:102: undefined reference to `proxygen::HTTPConnector::connect(folly::EventBase*, folly::SocketAddress const&, std::chrono::duration<long, std::ratio<1l, 1000l> >, std::map<folly::AsyncSocket::OptionKey, int, std::less<folly::AsyncSocket::OptionKey>, std::allocator<std::pair<folly::AsyncSocket::OptionKey const, int> > > const&, folly::SocketAddress const&)'
/home/kshitij/proxygen/httpclient/samples/curl/CurlClientMain.cpp:91: undefined reference to `proxygen::HTTPConnector::~HTTPConnector()'
/home/kshitij/proxygen/httpclient/samples/curl/CurlClientMain.cpp:99: undefined reference to `proxygen::HTTPConnector::connectSSL(folly::EventBase*, folly::SocketAddress const&, std::shared_ptr<folly::SSLContext> const&, ssl_session_st*, std::chrono::duration<long, std::ratio<1l, 1000l> >, std::map<folly::AsyncSocket::OptionKey, int, std::less<folly::AsyncSocket::OptionKey>, std::allocator<std::pair<folly::AsyncSocket::OptionKey const, int> > > const&, folly::SocketAddress const&, std::string const&)'
/home/kshitij/proxygen/httpclient/samples/curl/CurlClientMain.cpp:91: undefined reference to `proxygen::HTTPConnector::~HTTPConnector()'
CurlClient.o: In function `CurlService::CurlClient::connectSuccess(proxygen::HTTPUpstreamSession*)':
/home/kshitij/proxygen/httpclient/samples/curl/CurlClient.cpp:69: undefined reference to `proxygen::HTTPUpstreamSession::newTransaction(proxygen::HTTPTransactionHandler*)'
collect2: error: ld returned 1 exit status

Answer 1

how do i know what all possilble library flags are posiible like the one they have used -lproxyhttpserver.
It depends on your compilation environment.
You appear to use the GNU compiler in linux. There is a tool to list all installed shared libraries:
ldconfig -p
You will get a list of lines like this
libpthread.so.0 (libc6,x86-64, OS ABI: Linux 2.6.32) => /lib/x86_64-linux-gnu/libpthread.so.0
Remove the lib prefix and the extension .so.X and you get the name of the library. In this case it's pthread. To link with a libray use the option -lNAME. So, -lpthread in this example.
In short where is all these libs defined or located
The righthand part of the => is the full path to the library.