tgid

This field contains the thread group id. For easy understanding, let's say when a new process is created, its PID and TGID are the same, as the process happens to be the only thread. When the process spawns a new thread, the new child gets a unique PID but inherits the TGID from the parent, as it belongs to the same thread group. The TGID is primarily used to support multi-threaded process. We will delve into further details in the threads section of this chapter.