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#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <error.h>
#include <errno.h>
#include <limits.h>
#include <unistd.h>
#include <dirent.h>
#include <string.h>
#include <poll.h>
int main(void) {
DIR *gpio_dir;
const char *gpio_dir_path = "/sys/class/gpio";
const char *gpiochip_str = "gpiochip";
const char *gpio_label = "gpio@0x100005000";
size_t gpiochip_str_len = strlen(gpiochip_str);
int result;
struct dirent *dir_entry;
char path[PATH_MAX];
int path_length;
int file_fd;
char buffer[PATH_MAX+1] = {0};
char gpio_number_buffer[PATH_MAX+1] = {0};
char *str_result = NULL;
char *newline_ptr;
struct pollfd pollfd_struct;
// open the sysfs gpio directory
gpio_dir = opendir(gpio_dir_path);
if(gpio_dir == NULL)
error(1, errno, "could not open directory '%s'", gpio_dir_path);
// find the gpio controller for the KEY0 push button 'gpio@0x100005000'
while(1) {
// read the next directory entry
errno = 0;
dir_entry = readdir(gpio_dir);
if(dir_entry == NULL) {
if(errno != 0)
error(1, errno, "reading directory '%s'",
gpio_dir_path);
else
break;
}
// check if this is a gpio controller entry
result = strncmp(dir_entry->d_name, gpiochip_str,
gpiochip_str_len);
if(result != 0)
continue;
// open the gpio controller label file and read label value
path_length = snprintf(path, PATH_MAX, "%s/%s/label",
gpio_dir_path, dir_entry->d_name);
if(path_length < 0)
error(1, 0, "path output error");
if(path_length >= PATH_MAX)
error(1, 0, "path length overflow");
file_fd = open(path, O_RDONLY | O_SYNC);
if(file_fd < 0)
error(1, errno, "could not open file '%s'", path);
result = read(file_fd, buffer, PATH_MAX);
if(result < 0)
error(1, errno, "reading from '%s'", path);
if(result == PATH_MAX)
error(1, errno, "buffer overflow reading '%s'", path);
result = close(file_fd);
if(result < 0)
error(1, errno, "could not close file '%s'", path);
buffer[PATH_MAX] = 0;
// test the gpio controller label value for our gpio controller
str_result = strstr(buffer, gpio_label);
if(str_result != NULL)
break;
}
if(str_result == NULL)
error(1, 0, "unable to locate gpio controller");
// open the gpio controller base file and read base value
path_length = snprintf(path, PATH_MAX, "%s/%s/base",
gpio_dir_path, dir_entry->d_name);
if(path_length < 0)
error(1, 0, "path output error");
if(path_length >= PATH_MAX)
error(1, 0, "path length overflow");
file_fd = open(path, O_RDONLY | O_SYNC);
if(file_fd < 0)
error(1, errno, "could not open file '%s'", path);
result = read(file_fd, gpio_number_buffer, PATH_MAX);
if(result < 0)
error(1, errno, "reading from '%s'", path);
if(result == PATH_MAX)
error(1, errno, "buffer overflow reading '%s'", path);
result = close(file_fd);
if(result < 0)
error(1, errno, "could not close file '%s'", path);
gpio_number_buffer[PATH_MAX] = 0;
// remove the newline at the end of the string
newline_ptr = strchr(gpio_number_buffer,'\n');
if(newline_ptr != NULL)
memset(newline_ptr, '\0', 1);
// open the gpio export file and write our gpio number
path_length = snprintf(path, PATH_MAX, "%s/export",
gpio_dir_path);
if(path_length < 0)
error(1, 0, "path output error");
if(path_length >= PATH_MAX)
error(1, 0, "path length overflow");
file_fd = open(path, O_WRONLY | O_SYNC);
if(file_fd < 0)
error(1, errno, "could not open file '%s'", path);
result = write(file_fd, gpio_number_buffer, strlen(gpio_number_buffer));
// NOTE: we don't bother checking for errors here because if this gpio
// has already been exported this write will receive a device busy error
// which is perfectly normal.
result = close(file_fd);
if(result < 0)
error(1, errno, "could not close file '%s'", path);
// open the gpio edge file and write 'falling' to it
path_length = snprintf(path, PATH_MAX, "%s/gpio%s/edge",
gpio_dir_path, gpio_number_buffer);
if(path_length < 0)
error(1, 0, "path output error");
if(path_length >= PATH_MAX)
error(1, 0, "path length overflow");
file_fd = open(path, O_WRONLY | O_SYNC);
if(file_fd < 0)
error(1, errno, "could not open file '%s'", path);
result = write(file_fd, "falling", 7);
if(result < 0)
error(1, errno, "writing to '%s'", path);
if(result != 7)
error(1, errno, "buffer underflow writing '%s'", path);
result = close(file_fd);
if(result < 0)
error(1, errno, "could not close file '%s'", path);
// open the gpio value file and poll the file
path_length = snprintf(path, PATH_MAX, "%s/gpio%s/value",
gpio_dir_path, gpio_number_buffer);
if(path_length < 0)
error(1, 0, "path output error");
if(path_length >= PATH_MAX)
error(1, 0, "path length overflow");
file_fd = open(path, O_RDONLY | O_SYNC);
if(file_fd < 0)
error(1, errno, "could not open file '%s'", path);
// we first need to read the file before we can poll it, otherwise poll
// will not block
result = read(file_fd, buffer, PATH_MAX);
if(result < 0)
error(1, errno, "reading from '%s'", path);
if(result == PATH_MAX)
error(1, errno, "buffer overflow reading '%s'", path);
printf("Please press the KEY0 push button.\n");
buffer[PATH_MAX] = 0;
pollfd_struct.fd = file_fd;
pollfd_struct.events = POLLPRI | POLLERR;
pollfd_struct.revents = 0;
result = poll(&pollfd_struct, 1, -1);
if(result < 0)
error(1, errno, "poll returned error");
if(result == 1)
printf("KEY0 push button pressed.\n");
else
printf("Poll returned value other than 1.\n");
result = close(file_fd);
if(result < 0)
error(1, errno, "could not close file '%s'", path);
// open the gpio unexport file and write our gpio number
path_length = snprintf(path, PATH_MAX, "%s/unexport",
gpio_dir_path);
if(path_length < 0)
error(1, 0, "path output error");
if(path_length >= PATH_MAX)
error(1, 0, "path length overflow");
file_fd = open(path, O_WRONLY | O_SYNC);
if(file_fd < 0)
error(1, errno, "could not open file '%s'", path);
result = write(file_fd, gpio_number_buffer, strlen(gpio_number_buffer));
if(result < 0)
error(1, errno, "writing to '%s'", path);
if((size_t)(result) != strlen(gpio_number_buffer))
error(1, errno, "buffer underflow writing '%s'", path);
result = close(file_fd);
if(result < 0)
error(1, errno, "could not close file '%s'", path);
}
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