История изменений

       CLONE_THREAD (since Linux 2.4.0-test8)
              If CLONE_THREAD is set, the child is placed in the  same  thread
              group as the calling process.  To make the remainder of the dis‐
              cussion of CLONE_THREAD more readable, the term "thread" is used
              to refer to the processes within a thread group.

              Thread  groups  were a feature added in Linux 2.4 to support the
              POSIX threads notion of a set of threads  that  share  a  single
              PID.   Internally, this shared PID is the so-called thread group
              identifier (TGID) for the thread group.  Since Linux 2.4,  calls
              to getpid(2) return the TGID of the caller.