Hi Ian, Thanks for your quick reply! Sorry, It's the first time for me to post to gcc mailing list. I have removed gcc@xxxxxxxxxxx now. Glad to get your confirmations. Further more, could you please give me some links that introduce ld internals? BTW, I guess gold means google ld. Regarding 1.1), I suppose gold will also work in similar way, since the obj/lib file formats are same. Best Regards, Ly ---------------------------------------- > Date: Fri, 27 Jul 2012 12:19:26 -0700 > Subject: Re: Shared librares dependency at compile time > From: iant@xxxxxxxxxx > To: xmlymt@xxxxxxxxxxx > CC: gcc@xxxxxxxxxxx; gcc-help@xxxxxxxxxxx > > On Fri, Jul 27, 2012 at 12:10 PM, LiLy <xmlymt@xxxxxxxxxxx> wrote: >> >> I have two existing shared libraries liba.so, libb.so. >> Liba.so depends on libb.so(liba.so is dynamically linked with libb.so >> ). Now I want to compile an application which uses both liba.so and >> libb.so. > > > Please never send e-mail to both gcc@xxxxxxxxxxx and > gcc-help@xxxxxxxxxxx. This message should only have gone to > gcc-help@xxxxxxxxxxx. Please take any followups to gcc-help only. > Thanks. > > >> 1. On Linux, following command can pass >> gcc -Wall -o app app.c -L.. -la >> Note: -lb is not specified in the command >> >> I have several questions: >> 1). How is libb.so also linked actually while dynamic linking liba.so? > > There are two answers, one using GNU ld, one using gold. > >> It seems the linker knows libb.so according to the NEEDED entry in >> dynamic section of liba.so(which contains the name of libb.so)? > > That is how GNU ld works, yes. > >> Dynamic >> symbol table is also used to resolved symbols during linkage stage of >> compile, right?(since the program still can compile if the shared >> libraries are stripped) > > Correct. > >> 2) With -la or without -la >> We have two choices: >> i) gcc -Wall -o app app.c -L.. -la >> ii) gcc -Wall -o app app.c -L.. -la -lb >> Which one is more preferred? > > If your application refers to symbols in libb, then the second one is > preferred. If your application only uses symbols in liba, then the > first one is preferred. In general you should link your application > against the libraries that your application refers to directly. > > >> 2. On Cygwin, while, the same command can not pass >> gcc -Wall -o app app.c -L.. -la >> (same as on Linux, no -lb ) >> The error report reads like this: undefined reference to xxx(symbols defined in libb.so) >> >> Questions: >> What's the difference between gcc's behaviors on Linux and Cygwin? > > The shared library implementations on GNU/Linux and cygwin are > completely different. > > Ian
