SIGINFO: Everything Is a Punch Card

It’s commonly said that in UNIX, “everything is a file.” The meaning of this catchy aphorism is that most UNIX resources can be accessed using names in the file system with a small, consistent set of function calls. So not only can we open(), read(), and eventually close() regular files like /etc/motd and /bin/ls, we can read the contents of the hard drive (if you have suitable permissions) by opening /dev/disk0, the first physical disk on a Macintosh, and even /dev/mem, the Linux “device” that lets user processes read system memory.