Go the pthread_cancel()/pthread_cleanup_pop() route. It is the easiest.
And use pthread_setcancelstate() etc. to control cancelability so that you only cancel at known points to make cleanup easier.

Look at the source code for iostat and mpstat in the SYSSTAT package. All the information you need is there.

The null pointer constant (NULL) is guaranteed (by the compiler) not to point to any real object. It never points to "location zero in memory". Likewise the constant NULL is guaranteed to be 0.