Re: [Qemu-devel] [PATCH] use a thread id variable

[Gilad Ben-Yossef <gilad@codefidence.com>]

Jamie Lokier wrote:
> Gilad Ben-Yossef wrote:
>> Glauber Costa wrote:
>>> This patch introduces a "thread_id" variable to CPUState.
>>> It's duty will be to hold the process, or more generally, thread
>>> id of the current executing cpu
>>>
>>>     env->nb_watchpoints = 0;
>>> +#ifdef __WIN32
>>> +    env->thread_id = GetCurrentProcessId();
>>> +#else
>>> +    env->thread_id = getpid();
>>> +#endif
>>>     *penv = env;
>> hmm... maybe I'm missing something, but in Linux at least I think you 
>> would prefer this to be gettid() rather then getpid as each CPU has it's 
>> own thread, not a different process.
> 
> On most platforms, getpid() returns the same value for all threads, so
> it's not useful as a thread id.

Of course it does - this what POSIX says it should do (Linux 2.4 in 
compliance not withstanding). Which is why I suggested to use on Linux 
the non standard gettid() rather then getpid
> 
> On Linux, it depends which version of threads.  The old package,
> LinuxThreads, has different getpid() for each thread.  The current one,
> NPTL, has them all the same.


LinuxThreads behavior is wrong according to the POSIX standard. Of 
course, nothing else was possible with 2.4 kernels, so this is not an 
error on LinuxThreads coders part.


> What you're supposed to do with pthreads in general is use pthread_self().

Unfortunately, AFAIK the opaque handle that pthread_self() returns is 
not  quite meaningless outside of the process whereas what the non 
standard gettid() returns can actually be used to identify a thread from 
"outside" the process, like the shell.


Gilad

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