Warning: unless you specificly requested to run on a restricted number of cpus, there is now guarantee at all you will always run on the same cpu. I looked for the call you mention on http://codesearch.google.com/
and no one seems to use it. Then, I'm pretty sure it doesn't exist. However, a call that looks pretty similar to what you want is getcpu.
Additionaly, you can force your process to run on a given cpu or on some of the cpus with sched_setaffinity. You can also know the set of cpus you can run on with sched_getaffinity.
The three above calls are defined in sched.h.