g++ linking to external library creates undefined references

Question

I'm trying to build a simple program to test the use of an external library but am having trouble linking it with g++. See command/results:

user@user-Nuvo-2510VTC:~/Desktop/WDT_DIO/linux/test$ g++ -o main main.o -lwdt_dio /usr/lib/gcc/x86_64-linux-gnu/5/../../../../lib/libwdt_dio.so: undefined reference to `sem_unlink' /usr/lib/gcc/x86_64-linux-gnu/5/../../../../lib/libwdt_dio.so: undefined reference to `pthread_mutexattr_settype' /usr/lib/gcc/x86_64-linux-gnu/5/../../../../lib/libwdt_dio.so: undefined reference to `sem_close' /usr/lib/gcc/x86_64-linux-gnu/5/../../../../lib/libwdt_dio.so: undefined reference to `pthread_spin_lock' /usr/lib/gcc/x86_64-linux-gnu/5/../../../../lib/libwdt_dio.so: undefined reference to `pthread_spin_unlock' /usr/lib/gcc/x86_64-linux-gnu/5/../../../../lib/libwdt_dio.so: undefined reference to `pthread_create' /usr/lib/gcc/x86_64-linux-gnu/5/../../../../lib/libwdt_dio.so: undefined reference to `pthread_spin_init' /usr/lib/gcc/x86_64-linux-gnu/5/../../../../lib/libwdt_dio.so: undefined reference to `pthread_mutexattr_init' /usr/lib/gcc/x86_64-linux-gnu/5/../../../../lib/libwdt_dio.so: undefined reference to `pthread_spin_destroy' /usr/lib/gcc/x86_64-linux-gnu/5/../../../../lib/libwdt_dio.so: undefined reference to `sem_post' /usr/lib/gcc/x86_64-linux-gnu/5/../../../../lib/libwdt_dio.so: undefined reference to `sem_open' /usr/lib/gcc/x86_64-linux-gnu/5/../../../../lib/libwdt_dio.so: undefined reference to `sem_getvalue' /usr/lib/gcc/x86_64-linux-gnu/5/../../../../lib/libwdt_dio.so: undefined reference to `sem_wait' /usr/lib/gcc/x86_64-linux-gnu/5/../../../../lib/libwdt_dio.so: undefined reference to `pthread_join' collect2: error: ld returned 1 exit status

It looks like it isn't linking to some basic libraries that are part of the OS?

Details:

gcc --version --> 5.4.0 20160609 ldd --version --> 2.23 OS --> Ubuntu 16.04 x64, 4.8.0-36-generic kernel

Likely a duplicate of Using pthread in c++ and Compile/link error using pthread — jww
– jww, Commented Oct 3, 2017 at 18:32

Daniel Trugman · Accepted Answer · 2017-10-03 15:36:15Z

2

You seem to be missing a link to the pthread library. Add -pthread to your compilation command:

g++ -o main main.o -pthread -lwdt_dio

Checking the man page for sem_wait(3) shows:

Link with -pthread.

Note: sem_wait was selected randomly, all of them should specify that

edited Oct 3, 2017 at 15:36

answered Oct 3, 2017 at 15:23

Daniel Trugman

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3 Comments

DrTarr Over a year ago

Can't accept either yours or Petar's comments yet, but you were correct. How'd you guys know what library was missing? pthread was obvious but I didn't immediately recognize the semaphore functions were part of the same library

Daniel Trugman Over a year ago

@NateGreco, I am familiar with these methods, but just in case I also added a straightforward explanation...

DrTarr Over a year ago

Perfect, thakns

Petar Velev · Accepted Answer · 2017-10-03 15:36:27Z

If you read the errors closely it says that the undefined comes from libwdt_dio.so. It also says to which function you have an undefined reference.

In this particular case, it is the pthread library. It should probably be mentioned as a dependency in WDT lib documentation.

You can add -pthread to your g++ command if you're compiling through the command line.

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