Red Hat Bugzilla – Bug 145941
pthread_atfork symbol weird behavior
Last modified: 2007-11-30 17:10:59 EST
From Bugzilla Helper: User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.3) Gecko/20041020 Epiphany/1.4.4 Description of problem: references to the pthread_atfork symbol seem to behave a bit differently from references to other pthread functions. It seems that pthread_atfork symbols are not correctly resolved upon link time. Version-Release number of selected component (if applicable): glibc-2.3.3-74 How reproducible: Always Steps to Reproduce: I build a shared library out of a set of PIC object files, some of which reference the pthread_atfork symbol. My shared library is built with the following command line: cc -lpthread -ldl -shared -o bin/libprofiler/libprofiler.so bin/libprofiler/manager.o bin/libprofiler/record.o -Lbin/libdebug -ldebug-pic Once it is built, I run readelf: [mlacage@chronos profiler]$ readelf -s bin/libprofiler/libprofiler.so|grep atfork 71: 00000000 0 NOTYPE GLOBAL DEFAULT UND pthread_atfork 142: 00000000 0 NOTYPE GLOBAL DEFAULT UND pthread_atfork At this point, I am a bit puzzled by the NOTYPE flag of pthread_atfork as compared by the output for pthread_create: [mlacage@chronos profiler]$ readelf -s bin/libprofiler/libprofiler.so|grep pthread_create 26: 00000000 387 FUNC GLOBAL DEFAULT UND pthread_create@GLIBC_2.1 (4) 97: 00000000 387 FUNC GLOBAL DEFAULT UND pthread_create@@GLIBC_2.1 Actual Results: Ok, so, once the shared dynamic library is built, I link a program which does not use pthreads with this library. The command-line I use is: [mlacage@chronos profiler]$ cc -lprofiler -Wl,-rpath,bin/libprofiler -Lbin/libprofiler -ldl bin/test/test-profiler.o -o bin/test/test-profiler bin/libprofiler/libprofiler.so: undefined reference to `pthread_atfork' collect2: ld returned 1 exit status Which obviously fails. Expected Results: Of course, at this point, I can easily add a -lpthread argument to the link line for my application but I get the feeling this should not be needed and the dependency of libprofiler.so towards libpthread.so should be enough. i.e., the link of my application should not fail without the -lpthread. Additional info:
You are linking the shared library incorrectly. cc -lpthread -ldl -shared -o bin/libprofiler/libprofiler.so bin/libprofiler/manager.o bin/libprofiler/record.o -Lbin/libdebug -ldebug-pic Both -lpthread and -ldl must come after the object files (or .a libraries) that use them on the link line. libpthread.so is a linker script that sources in a small .a library and a shared library. If doesn't come up after the objects that use the pthread_* etc. interfaces, although you link the shared library against libpthread.so.0, you don't link in libpthread_nonshared.a (unless you surround almost everything on the command line between -Wl,--start-group and -Wl,--end-group, so that .a libs are searched repeatedly). pthread_atfork is defined in libpthread_nonshared.a, so that's why things don't work as expected for you. Simply move -lpthread -ldl to the end of cmdline and it will DTRT.