Re: system call library functions

On Nov 29, 2004, at 13:06, Dow Street wrote:

> 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?

The first call to getpid() goes to the kernel; subsequent calls 
retrieve the pid cached in user space (since a process's PID doesn't 
change from moment to moment).  That's a bit of a simplification, but 
check the source.

Cf the file Libc/ppc/sys/getpid.s

Regards,

Justin

--
Justin C. Walker, Curmudgeon-At-Large  *
Institute for General Semantics        |   If you're not confused,
                                        |   You're not paying attention
*--------------------------------------*-------------------------------*