[Resolved] How use select()/poll() with GPIO?

Modest Polykarpovich

Hello all! I'm using libugpio library (libugpio.so) and it has one special function (see listing of ugpio.h file below):

#!c++

/**
 * Return the GPIO context's value file descriptor.
 *
 * Using this function the application can feed the file descriptor into it's
 * select/poll... loop to monitor it for events.
 *
 * @param ctx a GPIO context
 * @return the file descriptor of the opened value file of the GPIO
 */
int ugpio_fd(ugpio_t *ctx);

But I can't find any example to how to use it.

Actually I need make pseudo-interrupts (special waitable pthread) because our libraries has no hardware IRQs and user-space library for it. For this we only can use select() or poll() system functions.

I tried code below, but select function returns with timeout, poll function returns immediate. Other the code of mine works correct - I can read real pin state.

#!c++

#include <iostream>
#include <ugpio.h>
#include <sys/select.h>
#include <poll.h>

using namespace std;


#define DEFAULT_GPIO_TEST_PIN 11

static int test_ugpio_pin(unsigned int pinNum) {

	int result;

	// Для функции select()
	fd_set fd_gpio_in;
	FD_ZERO(&fd_gpio_in);
	struct timeval select_timeout;

	// Переменные для функции poll()
	struct pollfd fds[1];
	fds[0].events = POLLIN;

	result = gpio_is_requested(pinNum);
	if (result < 0) {
		cerr << "Error: Pin #" << pinNum << " is already busy" << endl;
		return result;
	}

	ugpio_t *pMyPin = ugpio_request_one(pinNum, GPIOF_DIR_IN, "myTestPin");
	if (pMyPin == NULL) {
		cerr << "Error: Pin #" << pinNum << " cannot be requested." << endl;
		return -1;
	}

	result = ugpio_open(pMyPin);
	if (result < 0) {
		cerr << "Error open file pin in sysfs" << endl;
		goto err_exit;
	}

	result = ugpio_get_activelow(pMyPin);
	if (result < 0) {
		cerr << "Error: Pin #" << pinNum << " cannot get active level." << endl;
		goto err_exit2;
	}
	if (result) {
		cout << "Active level Pin#" << pinNum << "is HIGH" << endl;
	}
	else {
		cout << "Active level Pin#" << pinNum << "is LOW" << endl;
	}

	// Проверка работы псевдопрерывания
	// Using select()
	select_timeout = { 10, 0 };
	FD_SET(ugpio_fd(pMyPin), &fd_gpio_in);
	result = select(1, &fd_gpio_in, NULL, NULL, &select_timeout);
	if (result) {
		cout << "Pseudo-IRQ: Pin has been changed!" << endl;
	}
	else {
		cout << "Timeout: The pin doesn't changed in 10 secs." << endl;
	}

	// Using poll()
	fds[0].fd = ugpio_fd(pMyPin); fds[0].revents = 0;
	result = poll(fds, 1, 10000);
	if (result < 0) {
		cerr << "Error poll() calling!" << endl;
	}
	else if (result == 0) {
		cout << "Timeout: The pin doesn't changed in 10 secs." << endl;
	}
	else {
		cout << "Pseudo-IRQ: It works! The pin has been changed!" << endl;
		fds[0].revents = 0;
	}

	// Чтение состояния
	result = ugpio_get_value(pMyPin);
	if (result < 0) {
		cerr << "Error: Can't read value Pin #" << pinNum << "." << endl;
		result = -1;
		goto err_exit2;
	}
	else if (result) {
		cout << "Pin #" << pinNum << " read current state is HIGH" << endl;
	}
	else {
		cout << "Pin #" << pinNum << " read current state is LOW" << endl;
	}

	result = 0;

err_exit2:
	ugpio_close(pMyPin);

err_exit:
	ugpio_free(pMyPin);
	return result;
}

Both function normally works with stdint file descriptor number 0; May be libugpio with bags?
function ugpio_fd(pMyPin) return value 3 of file descriptor.

luz

Hi @Modest-Polykarpovich,
I am not familiar with libugpio but I am using GPIO interrupts a lot in my own C++ code by directly using the /sys/class/gpio interface.

So I can't be sure, but It seems to me that your code misses the step to actually enable GPIO IRQs. To do that, you need to write rising, falling or both to the file /sys/class/gpio/gpioX/edge (with X being the number of your GPIO).
Maybe there is a function in libugpio to do that...

Other than that, your code looks entirely fine to me.

See here for documentation on the sysfs GPIO API

Modest Polykarpovich

@luz said in How use select()/poll() with GPIO?:

Maybe there is a function in libugpio to do that...

Other than that, your code looks entirely fine to me.

This library is very simple, But lib-function gpio_set_edge() always return error and value in the file /sys/class/gpio/gpio[N]/edge is "none".

#!bash

# cat /sys/class/gpio/gpio11/edge
none

But I follow example and manually set value to "rising", but after that it's not works just as well.

@luz said in How use select()/poll() with GPIO?:

So I can't be sure, but It seems to me that your code misses the step to actually enable GPIO IRQs. To do that, you need to write rising, falling or both to the file /sys/class/gpio/gpioX/edge (with X being the number of your GPIO).
...
See here for documentation on the sysfs GPIO API

I saw this document and I can't find my error. I found one example, but this code after compile doesn't work, may be it need special GCC keys - it is doesn't matter. I took this code in my test application and it doesn't works too.

@luz said in How use select()/poll() with GPIO?:

Hi @Modest-Polykarpovich,
I am not familiar with libugpio but I am using GPIO interrupts a lot in my own C++ code by directly using the /sys/class/gpio interface.

Could you please send me 100% work your code?

Modest Polykarpovich

I fixed it! The library libugpio with bugs. Below is 100% work code GPIO (sysfs) preudo-IRQ + libgpio. See and analyze.

!#c++

#include <iostream>
#include <ugpio.h>

// For gpio (sysfs) test pseudo-IRQs control only
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <poll.h>


using namespace std;


#define DEFAULT_GPIO_TEST_PIN 11
#define GPIO_EDGE_RISING "rising"
#define GPIO_EDGE_FALLING "falling"
#define GPIO_EDGE_BOTH "both"


/*
 * The function below copied from project https://github.com/OnionIoT/gpioIrqTrigger/blob/master/gpioIrqTrigger.c
 * It's correct setting edge in "gpio - sysfs".
 * WARNING! The libugpio's function equivalent "ugpio_set_edge()" doesn't work (Bug)!
*/
static int my_gpio_set_edge(const unsigned int gpio, const char *edge)
{
	int fd, len;
	char buf[255];
	memset(buf, 0, sizeof(buf));

	len = snprintf(buf, sizeof(buf), "/sys/class/gpio/gpio%d/edge", gpio);

	fd = open(buf, O_WRONLY | O_NONBLOCK);
	if (fd < 0) {
		cerr << "Error open file file " << buf << " result code #" << fd << endl;
		return fd;
	}

	write(fd, edge, strlen(edge) + 1);
	close(fd);
	return 0;
}


static int test_ugpioIrq_pin(unsigned int pinNum, const char * edge) {

	int result;

	// Initializations for poll()'function using
	struct pollfd fds[1];
	int length;
	char buf[255];
	char d;

	/* Запрашиваем номер вывода */
	ugpio_t *pMyPin = ugpio_request_one(pinNum, GPIOF_DIR_IN, "MyTestPin");
	if (pMyPin == NULL) {
		cerr << "Error: Pin #" << pinNum << " cannot be requested." << endl;
		return -1;
	}

	/* 
	* Устанавливаем направление - вход 
	* ATTENTION! We are using low-level API of libugpio, because function "ugpio_direction_input()" doesn't work (Bug) and already returns value -1.
	*/
	result = gpio_direction_input(pinNum);
	if (result < 0) {
		cerr << "Error: Cannot assign direction, function ugpio_direction_input()." << endl;
		goto err_exit;
	}

	/* Устанавливаем триггер срабатывания [ "rising" | "falling" | "both" ] */
	if (result = my_gpio_set_edge(pinNum, edge) < 0) { 
		cerr << "Error: set edge fail" << endl;
		goto err_exit;
	}

	/* Открытие вывода для дальнейших операций */
	result = ugpio_open(pMyPin);
	if (result < 0) {
		cerr << "Error open file pin in sysfs" << endl;
		goto err_exit;
	}

	// Пятикратный опрос poll() 
	for (int i = 0; i < 5; i++) {

		memset((void*)fds, 0, sizeof(fds));
		fds[0].fd = ugpio_fd(pMyPin);			// File descriptor number of opened file: /sysfs/class/gpio/gpio[N]/value
		fds[0].events = POLLPRI;				// By official manual from https://www.kernel.org/doc/Documentation/gpio/sysfs.txt

		if (result = poll(fds, (sizeof(fds) / sizeof(struct pollfd)), 10000) < 0) {   // Wait change pin state while 10 seconds.
			cerr << "Error: Poll function call failed" << endl;
			goto err_exit2;
		}

		/* Event analyze block --begin */
		if (fds[0].revents & POLLPRI) {

			/*
			* [options] Чтение значения состояния пина
			*/
			memset(buf, 0, sizeof(buf));				// Очистка буфера для стабильности

			lseek(fds[0].fd, 0, SEEK_SET);				// Становка указателя в начало
			length = read(fds[0].fd, buf, sizeof(buf));	// Читаем значение файлового дескриптора

			cout << "GPIO current state is " << buf << endl;

		}
		/* Event analyze block --end */
	}

	result = 0;

err_exit2:
	ugpio_close(pMyPin);

err_exit:
	ugpio_free(pMyPin);
	return result;

}

luz

@Modest-Polykarpovich said in [Resolved] How use select()/poll() with GPIO?:

[...] Could you please send me 100% work your code?

I see that you solved the problem already - cool!

Bus just in case - my implementation is this one, which is part of a larger set of utilities handling other peripherals such as i2c, spi, pwm and a lot of other stuff, based on a common mainloop for timers and I/O polling.