ctrtool pidfd_ctl is a wrapper for the pidfd_open and pidfd_getfd system calls. Although originally designed for use with PID file descriptors, it can also perform a number of operations not related to PID file descriptors (as the three-register machine was found to be quite versatile).

pidfd_ctl is implemented as a three-"register" machine, which allows for a variety of common and compound PID file descriptor operations.

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