Following the man pages: wait4's man page points to waitpid for details, and notes wait4 is deprecated in favor of waitpid. So see the linux notes of this: ht

Following the man pages: wait4's man page points to waitpid for details, and notes wait4 is deprecated in favor of waitpid. So see the linux notes of this: https://man7.org/linux/man-pages/man2/waitpid.2.html https://man7.org/linux/man-pages/man2/waitpid.2.html The following Linux-specific options [..] can also, since Linux 4.7, be used with waitid(): __WCLONE [...] This option is ignored if __WALL is also specified. __WALL So to trigger this: * You have to call a deprecated function * With a flag that was at that time illegal (linux < 4.7) * And a second illegal flag that is cancelled out by the first illegal flag. This is something any userspace process can do, but no sane process should ever do.