CODEWITHSUNDEEP
c++linkercmakempiundefined-reference
Blackclaws
Blackclaws

Reputation: 446

Why do I get undefined reference with the same link line?

I am currently compiling two programs.

The same link line produces no error on one, and a whole lot of undefined references on the other one.

The link line is:

/usr/bin/c++   -Wall -pedantic -std=c++11   -fopenmp -O3 -DNDEBUG  
CMakeFiles/program.dir/src/main.cpp.o  -o program -rdynamic 
/home/felix/Development/local/lib/volop/libvolop.so 
/home/felix/Development/local/lib/ghost/libghost.so 
/home/felix/Development/local/lib/libhwloc.so 
/usr/local/lib/libmpicxx.so 
/usr/local/lib/libmpi.so -lrt -lpthread -L/usr/lib -lgsl -lgslcblas -lm 
/home/felix/Development/PRIMME/libprimme.a 
-L/usr/lib -lgsl -lgslcblas -lm -llapack 
-L/usr/lib -lgsl -lgslcblas -lm /home/felix/libblas/lib/libopenblas.so 
/home/felix/Development/PRIMME/libprimme.a 
-llapack 
/home/felix/libblas/lib/libopenblas.so 
-Wl,-rpath,/home/felix/Development/local/lib/volop:/home/felix/Development/local/lib/ghost:/home/felix/Development/local/lib:/usr/local/lib:/home/felix/libblas/lib

And the associated error:

/home/felix/Development/local/lib/ghost/libghost.so: undefined reference to `MPI_Allgather'
/home/felix/Development/local/lib/ghost/libghost.so: undefined reference to `MPI_Init_thread'

and so on, all MPI Linker errors. However as you can see from the link line mpi is linked in. And in the other program it works just fine.

This is on the same computer with the same compiler. The link line is generated by cmake. The CMakeLists.txt is identical for both programs.

I stop getting linker errors once I start using MPI Methods explicitely in the program which produces the errors. Why?

EDIT: As requested in the comments

ldd /home/felix/Development/local/lib/ghost/libghost.so
linux-vdso.so.1 =>  (0x00007fff6dff8000)
libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f8d70837000)
libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f8d70531000)
libgomp.so.1 => /usr/lib/x86_64-linux-gnu/libgomp.so.1 (0x00007f8d70321000)
libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f8d7010b000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f8d6feed000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f8d6fb26000)
/lib64/ld-linux-x86-64.so.2 (0x00007f8d70f81000)

Upvotes: 0

Views: 1269

Answers (2)

Wesley Bland
Wesley Bland

Reputation: 9062

Usually when someone gets weird linker errors with MPI, it's because they're not using the compiler wrappers that do the work for them.

When you (or someone else on your behalf) installed MPI on your machine, some wrappers were created to add all of the necessary flags to the compiler line just like the launcher mpiexec was also installed. Those wrappers are called mpicc, mpic++, mpicxx, mpifc, etc. You can use these just like you'd use any other compiler. You're welcome to throw in your own compiler flags if you need to add your own libraries, etc.

Upvotes: 2

Svalorzen
Svalorzen

Reputation: 5608

I can't be 100% sure, but this happened to me lots of times. The reason is the linking order: it DOES matter.

You need to link libraries from the "top" of the hierarchy to the "bottom"; as in, first you put the most complex libraries, and after you put the libraries on which other libraries depend on.

Try putting libmpi.so as last, and see if it works (or shuffle other things around if you think it matters).

Upvotes: 0

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