Here we explain how Linux handles threads and processes.
The following information explains in a simplistic way how Linux handles these components' ids.
This information was taken from
here.
A thread has a PID when viewed from above (from you as a user), but what you call a PID is not what the kernel (looking from below) calls a PID.
In the kernel, each thread has it's own ID, called a PID (although it would possibly make more sense to call this a TID) and they also have a TGID (thread group ID) which is the PID of the thread that started the whole process.
Simplistically, when a new process is created, it appears as a thread where both the PID and TGID are the same (new) number.
When a thread starts another thread, that started thread gets its own PID (so the scheduler can schedule it independently) but it inherits its TGID from the thread that created it.
That way, the kernel can happily schedule threads independent of what process they belong to, while processes (thread group IDs) are reported to you.
The following hierarchy of threads may help:
You can see that starting a new process gives you a new PID and a new TGID (both set to the same value), while starting a new thread gives you a new PID while maintaining the same TGID as the thread that started it.