GPIO state change tool for scripting.

README.md

GPIO interrupt polling

This useful little tool checks for changes in GPIO state without periodic polling by using the interrupt interface provided by Linux. It's useful in scripts to wait for a state change with Raspberry Pi, but works with any hardware with GPIOs.

Public domain.

To make this work, you need to export the pin and then enable interrupts. For example:

echo 24 >/sys/class/gpio/export
echo both >/sys/class/gpio/gpio24/edge

Then just run gpio_wait 24 0 to wait for the state to turn to LOW state (or 1 for HIGH).

Compiling

gcc -Wall -O2 -o gpio_wait gpio_wait.c

gpio_wait.c

#define _GNU_SOURCE
#include <stdio.h>
#include <poll.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdbool.h>
#include <stdlib.h>
#include <err.h>
#include <unistd.h>

int main(int argc, char **argv)
{
	if (argc != 3) {
		fprintf(stderr, "Usage: %s GPIO value\n", argv[0]);
		return 1;
	}

	struct pollfd pf;
	char target = *argv[2];
	
	{
		char *filename = NULL;
		asprintf(&filename, "/sys/class/gpio/gpio%s/value", argv[1]);
		int fd = open(filename, O_RDONLY);
		if (fd == -1) err(1, "Unable to open %s", filename);
		printf("Polling %s for value %c\n", filename, target);
		free(filename);

		// Populate struct
		pf.fd = fd;
		pf.events = POLLPRI;
	}
	
	while (true) {
		int ret = poll(&pf, 1, -1);
		if (ret < 1) err(2, "Unable to poll");
		// Wait once after interrupts occur, for debouncing
		if (usleep(10000) == -1) err(2, "Unable to sleep");

		if (!(pf.revents & POLLPRI)) continue;

		// Seeking to start and getting the current value
		if (lseek(pf.fd, 0, SEEK_SET) == -1) err(3, "Unable to seek");
		char state;
		if (read(pf.fd, &state, 1) == -1) err(3, "Unable to read");

		// Ending processing if we get end condition
		if (state == target) {
			printf("Reached state %c\n", state);
			return 0;
		}
	}
}