Undefined reference - despite lib being found by linker

Try:

gcc -I/usr/include/python2.7 p.c -lpython2.7 

the linker doesn't yet know that Py_Initialize is a required symbol when it loads libpython2.7.a, so it tosses it away. And then it gets to p.o and throws a fit about the missing symbol. Ordering it this way will let the linker look for the missing symbol in subsequent inputs.

See: http://gcc.gnu.org/onlinedocs/gcc/Link-Options.html

It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o' searches libraryz' after file foo.o but before bar.o. If bar.o refers to functions in `z', those functions may not be loaded.

I encountered the same linking problem as well. But in my case it was not enough to provide -lpython. Also -L was needed. I.e.

g++ -I/usr/include/python3.5 hw.cpp -L/usr/lib/python3.5/config-3.5m-x86_64-linux-gnu -lpython3.5

And yes. Order matters.