On Wed, Jan 13, 2010 at 12:10:06PM -0500, Adam Jackson wrote: > On Wed, 2010-01-13 at 16:24 +0000, Nick Clifton wrote: > > >> $ g++ -o string string.o -ldb > > >> /usr/bin/ld.bfd: string.11980.test: undefined reference to symbol > > >> 'pthread_cancel@@GLIBC_2.2.5' > > >> /usr/bin/ld.bfd: note: 'pthread_cancel@@GLIBC_2.2.5' is defined in DSO > > >> /lib64/libpthread.so.0 so try adding it to the linker command line > > > > But, you have added an explicit dependency upon libdb to your executable > > by mentioning -ldb on the gcc command line. Therefore libdb will be > > loaded at execution start-up. But libdb has a dependency upon > > libpthread, so that library will also be loaded at execution start-up. > > Hence when you run 'string' the pthread_cancel symbol will be resolved > > and so 'string' really does now have a resolved reference to > > pthread_cancel. Hence the linker is correct in complaining that you > > have a reference to a symbol that is defined in a library which have not > > included on the linker command line. > > > > At least that is how I see it at the moment. > > This is the whole point of weak references though. They go to 0 with no > error if they are not defined in the runtime image. In other words, > they're explicitly _not_ required; the object with the weak reference is > saying "I know how to behave even if this symbol isn't defined". Yeah, string will work well even when -ldb stops being linked against -lpthread, therefore forcing it to require -lpthread is a bad idea. And it will even pessimize it. The whole point why gthr.h uses these weak references is that compiled code is thread safe when -lpthread is loaded, but does not require it and is actually quite cheaper if -lpthread is not loaded. Jakub -- devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxxx https://admin.fedoraproject.org/mailman/listinfo/devel
