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If you want a world-wide unique identifier for a process it makes sense to combine the pair of pid_t and pidfd inode number with the system's boot ID (i.e. /proc/sys/kernel/random/boot_id). This triplet is awesome, because for the first time we can uniquely identify a Linux process, globally in this universe.

In systemd we are making use of this heavily now: internally we always store a triplet of pid, pidfd, pidfd inode for referencing processes we manage and…