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copied!<p>The four threads will have the same PID but only when viewed from <em>above.</em> What <em>you</em> (as a user) call a PID is not what the kernel (looking from below) calls a PID.</p> <p>In the <em>kernel,</em> each thread has it's own ID, called a PID (although it would possibly make more sense to call this a TID, or thread ID) and they also have a TGID (thread group ID) which is the PID of the thread that started the whole process.</p> <p>Simplistically, when a new <em>process</em> is created, it appears as a thread where both the PID and TGID are the same (new) number.</p> <p>When a thread starts another <em>thread,</em> that started thread gets its own PID (so the scheduler can schedule it independently) but it inherits the TGID from the original thread.</p> <p>That way, the kernel can happily schedule threads independent of what process they belong to, while processes (thread group IDs) are reported to you.</p> <p>The following hierarchy of threads may help<sup>(a)</sup>:</p> <pre><code> USER VIEW <-- PID 43 --> <----------------- PID 42 -----------------> +---------+ | process | _| pid=42 |_ _/ | tgid=42 | \_ (new thread) _ _ (fork) _/ +---------+ \ / +---------+ +---------+ | process | | process | | pid=44 | | pid=43 | | tgid=42 | | tgid=43 | +---------+ +---------+ <-- PID 43 --> <--------- PID 42 --------> <--- PID 44 ---> KERNEL VIEW </code></pre> <p>You can see that starting a new <em>process</em> (on the left) gives you a new PID <em>and</em> a new TGID (both set to the same value), while starting a new <em>thread</em> (on the right) gives you a new PID while maintaining the same TGID as the thread that started it.</p> <hr> <p><sup>(a)</sup> <em>Tremble in awe</em> at my impressive graphical skills :-)</p>

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