#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define IN  0
#define OUT 1

#define LOW  0
#define HIGH 1

#define POUT            26   
#define BUFFER_MAX      3
#define DIRECTION_MAX   48

static int GPIOExport(int pin)
{
    char buffer[BUFFER_MAX];
    int len;
    int fd;

    fd = open("/sys/class/gpio/export", O_WRONLY);
    if (fd < 0) {
        fprintf(stderr, "Failed to open export for writing!\n");
        return(-1);
    }

    len = snprintf(buffer, BUFFER_MAX, "%d", pin);
    write(fd, buffer, len);
    
    close(fd);
    return(0);
}

static int GPIOUnexport(int pin)
{
    char buffer[BUFFER_MAX];
    int len;
    int fd;

    fd = open("/sys/class/gpio/unexport", O_WRONLY);
    if (fd < 0) {
        fprintf(stderr, "Failed to open unexport for writing!\n");
        return(-1);
    }

    len = snprintf(buffer, BUFFER_MAX, "%d", pin);
    write(fd, buffer, len);

    close(fd);
    return(0);
}

static int GPIODirection(int pin, int dir)
{
    static const char dir_str[]  = "in\0out";
    char path[DIRECTION_MAX];
    int fd;

    snprintf(path, DIRECTION_MAX, "/sys/class/gpio/gpio%d/direction", pin);
    fd = open(path, O_WRONLY);
    if (fd < 0) {
        fprintf(stderr, "failed to open gpio direction for writing!\n");
        return(-1);
    }

    if (write(fd, &dir_str[dir == IN ? 0 : 3], dir == IN ? 2 : 3) < 0) {
        fprintf(stderr, "failed to set direction!\n");
        return(-1);
    }

    close(fd);
    return(0);
}

static int GPIORead(int pin)
{
    char path[DIRECTION_MAX];
    char value_str[3];
    int fd;

    snprintf(path, DIRECTION_MAX, "/sys/class/gpio/gpio%d/value", pin);
    fd = open(path, O_RDONLY);
    if (fd < 0) {
        fprintf(stderr, "failed to open gpio value for reading!\n");
        return(-1);
    }

    if (read(fd, value_str, 3) < 0) {
        fprintf(stderr, "failed to read value!\n");
        return(-1);
    }

    close(fd);
    return(atoi(value_str));
}

static int GPIOWrite(int pin, int value)
{
    static const char s_values_str[] = "01";
    char path[DIRECTION_MAX];
    int fd;

    snprintf(path, DIRECTION_MAX, "/sys/class/gpio/gpio%d/value", pin);
    fd = open(path, O_WRONLY);
    if (fd < 0) {
        fprintf(stderr, "failed to open gpio value for writing!\n");
        return(-1);
    }

    if (write(fd, &s_values_str[value == LOW ? 0 : 1], 1) < 0) {
        fprintf(stderr, "failed to write value!\n");
        return(-1);
    }

    close(fd);
    return(0);
}

int main(int argc, char *argv[])
{
    int i = 0;

    GPIOExport(POUT);
    GPIODirection(POUT, OUT);

    for (i = 0; i < 20; i++) {
        GPIOWrite(POUT, i % 2);
        usleep(500 * 1000);
    }

    GPIOUnexport(POUT);
    return(0);
}