Simple program for switching relays via GPIO on Linux with FastCGI HTTP-based API and web interface.

• Raspberry Pi 3 Model B running Debian 10
• Orange Pi Lite (Allwinner H3) running Armbian 21.02.2 Buster (make CFLAGS="-DSET_DIRECTION")
• Orange Pi i96 (RDA8810) running Raspbian 8 on kernel 3.10.62

• Install GNU C99-compliant compiler (gcc or clang) and FastCGI library with development headers (libfcgi-dev package in Debian)
• Edit include/relay-conrol.h
• Run make install
• Install any HTTP server with FastCGI support. I am using lighttpd. There is a part of possible content in /etc/lighttpd/lighttpd.conf:

...
server.document-root = "/var/www/html"
server.username = "www-data"
server.groupname = "www-data"
...
server.modules = (
	...
        "mod_fastcgi",
	"mod_staticfile"
)
...
include_shell "/usr/share/lighttpd/create-mime.conf.pl" # for correct MIME type for html file
...
fastcgi.server = (
  "/relay" => ((
    "socket" => "/run/relay-control.sock",
    "check-local" => "disable"
  ))
)

• Run make install
• Add /usr/local/bin/relay-control to autostart. If you are using systemd simply copy relay-control.service to /etc/systemd/system and run systemctl enable --now relay-control.

• To get relay info in CSV format request /relay?status=csv. Typical response:

relay,active,name
474,0,Relay A
475,0,Relay B

• To set LOW or HIGH relay GPIO pin make POST request to /relay with body <"off" for LOW, "on" for HIGH>=<pin id>. Example request: on=475. Response: ok with code 200 or error with code 500.