#gpio - Raspberry-Pi GPIO over sysfs

import "gpio.j" as gpio;

General-purpose I/O pins on a Raspberry Pi (or any Linux single-board computer) through the sysfs interface - the physical-computing / IoT-teaching use case. /sys/class/gpio is plain files, so fs is the entire backend; there is no core change, no system library, and no build tag. Blink an LED from a five-line script:

import "gpio.j" as gpio;
gpio.setup(17, gpio.OUT);
gpio.write(17, 1);   # LED on
gpio.write(17, 0);   # LED off
gpio.release(17);

#Surface

Stateless and pin-keyed - sysfs derives every path from the pin number, so no handle is needed:

The direction is passed as one of two exported constants, gpio.IN / gpio.OUT (matching RPi.GPIO's GPIO.IN / GPIO.OUT). Their values are the sysfs strings "in" / "out", so a raw string still works, but the constant is the readable form. A bad direction (not gpio.IN / gpio.OUT) or a value other than 0 / 1 throws Error{kind: "gpio"}. When the sysfs GPIO tree is absent - not a GPIO-capable host, or sysfs GPIO disabled - every call throws a clear positioned Error{kind: "gpio"} ("base directory not found: ...") rather than crashing.

#The sysfs root

The root defaults to /sys/class/gpio. Set the JENNIFER_GPIO_BASE environment variable to point elsewhere - a differently mounted sysfs, or a mock tree under test:

use os;
os.setEnv("JENNIFER_GPIO_BASE", "/tmp/mock-gpio");   # e.g. in a test

That is how gpio_test.j drives the module against a temp-dir mock, and how gpio_demo.j runs on any machine (no hardware, no root) while making the same calls you'd run on a Pi.

#Why a module, not a system library

use / import are static - they resolve before execution, are uncatchable, and can't be conditional - so there is no "check the platform, then import." The portability seam is instead which module file is on the search path (Go uses build tags; Jennifer uses the module file). A program writes one uniform line, import "gpio.j" as gpio;, and the deployment supplies the right gpio.j: this sysfs module on a Pi, or an emulator that blinks a console cell on a laptop. A genuinely platform-bound capability being absent off its platform (so import fails at the top with a clear message) is the right shape

• distinct from a toolchain-bound one like net, which stubs on

jennifer-tiny because the same source must load in both binaries.

#sysfs, and the future

sysfs GPIO is deprecated in favour of the /dev/gpiochip character device and can be compiled out of a kernel. The bet is that it stays available on the hobbyist Pi kernels this targets. The API is kept deliberately stable, so if sysfs is ever removed the backend can be swapped for a /dev/gpiochip ioctl system library with no change to .j scripts - the pure-module form is the default because it costs the language nothing; the system library is future-proofing, taken only when forced.

That system library now exists as the gpio library (the /dev/gpiochipN GPIO v2 ioctls). It reuses this module's pin-keyed shape - setup / read / write / release plus the IN / OUT constants - so a script moves between the two unchanged; this sysfs module stays the portable default, and the library is the successor for targets that ship without sysfs GPIO.

#See also

• gpio library - the /dev/gpiochip character-device successor with the same pin-keyed surface (default jennifer binary, Linux).
• fs - the file I/O behind every call.
• os - os.setEnv to point JENNIFER_GPIO_BASE at a non-standard sysfs mount or a mock.