clang: error: linker command failed with exit code 1 (use -v to see invocation)

[Zanghu-ze]

I meet some compile problems, Can you solve it?

Undefined symbols for architecture x86_64:
  "_frame_counter", referenced from:
      Tracking::Track() in Tracking.cpp.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)