Linus Torvalds wrote:
> 
> On Sat, 5 Jun 2004, Davide Libenzi wrote:
> 
>> It is likely used by pthread_self(), that is pretty much
>> performance sensitive. I'd agree with Ulrich here.
> 
> 
> It _can't_ be used for pthread_self(), since the pid is the _same_
> across all threads in a pthread environment.
> 
> See?
> 
> I re-iterate:
> 
> - getpid() was used in some historical threading packages (maybe even
>  LinuxThreads) to get the thread ID thing.
> 
> But this particular usage requires the old Linux behaviour of
> returning separate threads ID's for separate threads. CLONE_THREAD
> does not do that, and in particular the glibc caching _breaks_ any
> such attempt even when you don't use CLONE_THREAD.
> 
> pthreads() in modern libc sets CLONE_THREAD and then uses the 
> per-thread segment thing to identify threads. No getpid() anywhere.
> 
> Ergo: getpid() caching may result in faster execution, but in this
> case it's actively _wrong_, and breaks the app.  It's easy to make
> things go fast if you don't actually care about whether the end
> result is correct or not.
> 
> - getpid() is used in some silly benchmarks.
> 
> But in this case caching getpid() just lies to the benchmark, and is 
> pointless.
> 
> Ergo: getpid() caching results in higher scores, but the scores are
> now meaningless, since they have nothing to do with system call speed
> any more.
> 
> So in both of these cases, the caching literally results in WRONG 
> behaviour.
> 
> Can somebody actually find an application that doesn't fall into
> either of the above two categories, and calls getpid() more than a
> handful of times?
> 
Well, not exactly sure about my reply, but let me try.

The other day I was debugging some config problems with my qmail instalation and I ended up doing:
# strace -p 4563 -f -F
...
[pid 13097] read(3, "\347\374\375TBH~\342\233\337\220\302l\220\317\237\37\25"..., 32) = 32
[pid 13097] close(3)                    = 0
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getuid32()                  = 89
[pid 13097] getpid()                    = 13097
[pid 13097] time(NULL)                  = 1086497450
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] time(NULL)                  = 1086497450
[pid 13097] getpid()                    = 13097
[pid 13097] getpid()                    = 13097
[pid 13097] write(0, "\26\3\1\0J\2\0\0F\3\1@\302\242\2527j\266m4{\323>6\232\243"..., 1127) = 1127
[pid 13097] read(0, "\26\3\1\1\6", 5)   = 5
...
where 4563 is a tcpserver process and 13097 is its child. The process was started as:
tcpserver -s -n ssmtp.crt -p -v -R -x /etc/tcprules.d/tcp.smtp.cdb -c 40 -u 89 -g 89 0.0.0.0 465 /var/qmail/bin/qmail-smtpd mail.tar.bz /var/vpopmail/bin/vchkpw /bin/true

No idea why are all those getpid() call there in the child... Looked at the code, but couldn't get it. Attaching tcpserver.c for reference.

Again, sorry if unrelated.

Kalin.

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