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    <p>If you are talking about the <code>getpid()</code> system call then yes, PID is unique per process. <em>Except</em>, that is, if you are using threads on older versions of the Linux kernel. Then each thread may have its own process-id.</p> <p>To quote from <a href="http://www.linuxquestions.org/questions/programming-9/pthread-problem-65271/" rel="nofollow">this discussion</a>:</p> <blockquote> <p>Kernel 2.4.20 uses <a href="http://en.wikipedia.org/wiki/Native_POSIX_Thread_Library" rel="nofollow">NPTL (Native posix thread library)</a> and this is the kernel shipped with RH9. RH8 uses Kernel 2.4.18 which doesn't implement NPTL (Meaning each thread gets its own PID and therefore a good description of it's status in /proc). NPTL is a "real" implementation of POSIX threads meaning that the threads share alot more including the PID. It is more efficient way of running threads for a couple of reasons, however, I don't know of any easy tricks to debug these kind of threads. How do you know when your thread is sleeping versus waiting on a semaphore, or which threads have died in a process with lots of threads, etc. </p> </blockquote> <p>From the <a href="http://en.wikipedia.org/wiki/Native_POSIX_Thread_Library" rel="nofollow">wikipedia link on NPTL</a>:</p> <blockquote> <p>NPTL has been part of Red Hat Enterprise Linux since version 3, and in the Linux kernel since version 2.6. It is now a fully integrated part of the GNU C Library.<a href="http://en.wikipedia.org/wiki/Native_POSIX_Thread_Library" rel="nofollow">2</a></p> </blockquote> <p>Under the covers even the 2.6.X kernels have a virtual-process for threads. You can see the thread process-ids with <code>ps auxf</code>:</p> <pre><code>root 2501 0.0 0.3 244448 25576 ? Ss Jul03 0:11 /usr/sbin/httpd apache 2716 0.0 0.5 384776 46696 ? S Oct14 0:17 \_ /usr/sbin/httpd apache 2717 0.0 0.5 382208 44304 ? S Oct14 0:11 \_ /usr/sbin/httpd </code></pre> <p>The following program spits out the same pid for both main and thread under Linux kernel 2.6.18. The self id returned from <code>pthread_self()</code> identifies the thread uniquely.</p> <pre><code>#include &lt;pthread.h&gt; void foo() { printf("thread: pid = %d, self = %ld\n", getpid(), pthread_self()); } main() { pthread_t thread; printf("main: pid = %d, self = %ld\n", getpid(), pthread_self()); pthread_create(&amp;thread, 0L, foo, 0L); pthread_join(thread, 0L); } </code></pre> <p>The output is:</p> <pre><code>main: pid = 13246, self = 46912496175248 thread: pid = 13246, self = 1084229952 </code></pre>
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    1. POIs pid really unique
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    1. COSo recent versions of Linux would have the same PID for processes and their threads? What would be the unique identifier for a thread in this case? T[G]ID?
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    2. COI'd use `pthread_self();` @Buffalo. I've changed my answer to show its output.
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