system call library functions

All,

I am writing some benchmarking code for OS X 10.3.5.  When using 
getpid() to measure the overhead of a system call, I find the overhead 
to be much less than expected (1-3 clicks on the MPC 7447a TBR).  When 
making the call using syscall(20), however, the measurement is as 
expected (~33 clicks on the TBR).  Is getpid actually using a library 
function which retrieves the PID from a user space data structure 
(without entry into the kernel?)  Any idea what / where this function 
is located and the data structure it's accessing?

Thanks,
Dow