Can the thread ID of a multithreaded process be the same as the process ID of another running process?

Question

I'm trying to find a way to uniquely identify threads in a multi-process environment. I have a server that keeps track of the different processes connecting to it, some of which are multi-threaded and some of which are not. To identify the threads from multi-threaded connections I'm using the thread ID as a unique identifier (there will be a maximum of 1 multi-threaded process connected at any given time). My question is: is it possible the thread ID of one of these threads could be the same as the process ID of another processes running on the system?

Thanks in advance for the help!

Answer 1

The TID (as returned by the sys_gettid() system call) is unique across all threads on the system¹, and for a single-threaded process the PID and TID are equal. This means that a TID will never clash with a PID from another process.

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^{1. With the caveat that if PID namespaces are in use, TIDs and PIDs are only unique within the same PID namespace.}