error: ‘getpid’ was not declared in this scope

This is expected you are not including unistd.h in your source which is required if you want to use getpid.  It was a bug before GCC 4.7 that some of the C++ headers included that header file.

As documented at http://gcc.gnu.org/gcc-4.7/changes.html

"Avoid polluting the global namespace by inclusion of ."