Pidfd

Since Linux 5.2, pidfds have more or less allowed one to refer to a process using a file descriptor, making it possible to eliminate a set of race conditions and ambiguities.

At the heart of the pidfd abstraction is the CLONE_PIDFD flag to the clone(2) system call (note that this recycles the deprecated CLONE_PID bit). It is not possible to use this flag with CLONE_THREAD, and thus the created process will always be a thread group leader. It cannot be used together with the (deprecated) CLONE_DETACHED flag. The resulting file descriptor is placed in parent_tid when used with clone(2) (and thus it cannot there be used together with CLONE_PARENT_SETTID) and cl_args.pidfd when used with clone3(2).