How to set Raspberry Pi GPIO pins to input or output using clean C?

Question

How can access the GPIO pins in C without writing to the sys/class/gpio directory, in a way that would allow me to create my own interface?

I would like to end up able to do something like this:

// Declaring `porta` at some memory address.
unsigned char porta @port 0x7;
// Setting `porta` as input.
porta = 1; 

I've seen similar syntax in microcontroller programming.

So is it possible to declare my pins like that, then set them as input or output?

Reference: http://www.atmel.com/images/avr_3_04.pdf

Answer 1

TRISA would have to be quite a complicated C macro for that to work on a Pi.

Have a look at http://abyz.me.uk/rpi/pigpio/examples.html#Misc_tiny_gpio which shows possibly the simplest way to access the Pi GPIO via C.

In particular it shows how to implement the following functions.

gpioSetMode       /* set GPIO mode */
gpioGetMode       /* get GPIO mode */
gpioSetPullUpDown /* set internal GPIO pull-up/down */
gpioRead          /* read GPIO */
gpioWrite         /* write GPIO */
gpioReadBank1     /* read levels of GPIO in bank 1 */
gpioReadBank2     /* read levels of GPIO in bank 2 */
gpioClearBank1    /* clear selected GPIO in bank 1 */
gpioClearBank2    /* clear selected GPIO in bank 2 */
gpioSetBank1      /* set selected GPIO in bank 1 */
gpioSetBank2      /* set selected GPIO in bank 2 */

Answer 2

First we need to declare our address of the BCM2708. Then we add the starting address of the GPIO pins to the BCM2708 address. We need those addresses so we can map them, using the mmap function, which will "expose" them and let us modify them and work with them.

Here is how we can do this:

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mmap.h>

#define BCM2708_PERI_BASE (0x3F000000)//decimal 1056964608
#define GPIO_BASE (BCM2708_PERI_BASE + 0x200000)//decimal 1059061760
#define BLOCK_SIZE (4096)

struct bcm2835_peripheral
{
    int mem_fd;
    void *map;
    volatile unsigned int *addr;//address of mapped area
};
//mmap function
int map_peripheral(struct bcm2835_peripheral *p)
{
    //open the /dev/mem folder with read/write
    if((p->mem_fd = open("/dev/mem", O_RDWR|O_SYNC)) < 0)
    {
        printf("Failed to open /dem/mem, did you sudo?\n");
        return -1;
    }
    p->map = mmap(NULL,
                  BLOCK_SIZE,
                  PROT_READ|PROT_WRITE,
                  MAP_SHARED,
                  p>mem_fd,
                  GPIO_BASE);

    if(p->map == MAP_FAILED)
    {
        printf("mmap failed, MAP_FAILED\n");
        return -1;
    }
    close(p->mem_fd);
    p->addr = (volatile unsigned int *)p->map;

    return 0;
}
//unmap the same memory block
void unmap_peripheral(struct bcm2835_peripheral *p)
{
    munmap(p->map, BLOCK_SIZE);
    close(p->mem_fd);
}


//here is the GPIO manipulation in CLEAN C
//setting the pin as input
void input_GPIO(int gpio_numb)
{
    *(gpio.addr + ((gpio_numb)/10)) &= ~(7<<(((gpio_numb)%10)*3));
}
//setting the pin as output
void output_GPIO(int gpio_numb)
{
    input_GPIO(gpio_numb);
    *(gpio.addr + ((gpio_numb)/10)) |=  (1<<(((gpio_numb)%10)*3));
}
//setting the pin to HIGH
void high_GPIO(int gpio_numb)
{
    *(gpio.addr + 7) = 1 << gpio_numb;
}
//setting the pin to LOW
void low_GPIO(int gpio_numb)
{
    *(gpio.addr + 10) = 1 << gpio_numb;
}