Gpio from userspace in c++

GoogleGroups
1

I am trying to access gpio's from userspace in c++.
Toolchain = the one downloaded from narcissus.

Best i can come up with is

system("echo 133 > /sys/class/gpio/export");
SleepMS(5);
File* fdrdy = fopen ("/sys/class/gpio/gpio133");

int DRDY
for ( int i = 0 ; i < 10000 ; )
{
      DRDY = fgetc(fdrdy);
      fseek(fdrdy,0,SEEK_SET);

      if(DRDY == '0')
            {
               ... the variable i is incremented here..
            }
}

I just want to read the status of a gpio.
If its 0, run this code. Or keep polling. Until its zero.
Then run the code. That's all.

I used uboot to mux it. Set its direction as output.

Is there a better way to do this than the code above?

Read somewhere about gpiolib.
But i cant find linux/gpio.h in my toolchain's folder.
:s

Am i missing some basic thing here?

p.s. My beagle isn't handy atm. So this code might be wrong. Haven't
actually tested it. It should work i think...

2

I am trying to access gpio's from userspace in c++.
Toolchain = the one downloaded from narcissus.

Best i can come up with is

system("echo 133 > /sys/class/gpio/export");
SleepMS(5);
File* fdrdy = fopen ("/sys/class/gpio/gpio133");

int DRDY
for ( int i = 0 ; i < 10000 ; )
{
      DRDY = fgetc(fdrdy);
      fseek(fdrdy,0,SEEK_SET);

      if(DRDY == '0')
            {
               ... the variable i is incremented here..
            }
}

This code is okay and will work over a wide range of different
kernel-versions. If you can live with the fact that you can't wait for a
gpio-event without a active polling loop, then go for it.

There are other ways to do it in a more efficient way: Using
sys/class/gpio with events or binding your GPIO to the input system via
gpio_keys. These may be more efficient but the interface still changes
from kernel-version to kernel-version.

One note to your code: You may not need your fseek-call. It will most
likely work without it.

Read somewhere about gpiolib.
But i cant find linux/gpio.h in my toolchain's folder.
:s

gpiolib is the official kernel-interface to GPIO. It is not accessable
from user-space.

Am i missing some basic thing here?

No.
  Nils

3

Thank-you
:slight_smile:

4

I would recommend using standard file access over executing the echo command to read and write the gpio information. The one issue I ran into is simple access control to the part of the filesystem. If I run as root, everything works fine. If I run as a non-root user, I don’t have access. For my purposes, I decided to simply have my process run as root. I’m sure what I should do is figure out how to add my process owner to the correct system permissions group.

This example sets up a series of GPIO ports…

// Export the GPIO lines

FILE * fp = NULL;
for (int index = 131; index < 140; index++)
{
if ((fp = fopen("/sys/class/gpio/export", “ab”)) != NULL)
{
rewind(fp);
fprintf(fp,"%d",index);
fclose(fp);
//printf(“gpio%d exported\n”,index);
}
}

// Configure the GPIO lines

char gpio_file_name[256];
sprintf(gpio_file_name,"/sys/class/gpio/gpio%d/direction",130);
for (int index = 131; index < 140; index++)
{
sprintf(gpio_file_name,"/sys/class/gpio/gpio%d/direction",index);
if ((fp = fopen(gpio_file_name, “rb+”)) != NULL)
{
rewind(fp);
fprintf(fp,“out”);
fclose(fp);
//printf(“gpio%d configured for OUTPUT… “,index);
sprintf(gpio_file_name,”/sys/class/gpio/gpio%d/value”,index);
if ((fp = fopen(gpio_file_name, “rb+”)) != NULL)
{
fprintf(fp,“0”);
fclose(fp);
//printf(“value set to “0"”,index);
}
//printf(”\n");
}
}

// Do whatever you want with your GPIO files here…
// I use somethign like

if ((fp[7] = fopen("/sys/class/gpio/gpio137/value", “rb+”)) != NULL)

{
rewind(fp[7]);
// to open each of my individual GPIO files, and then write to it and close it again.

// UnExport your GPIO lines to clean things up.

for (int index = 131; index < 140; index++)
{
FILE * fp = NULL;
if ((fp = fopen("/sys/class/gpio/unexport", “ab”)) != NULL)
{
rewind(fp);
fprintf(fp,"%d",index);
fclose(fp);
//printf(“gpio%d unexported\n”,index);
}
}

5

Hi guys,

I am trying to access gpio's from userspace in c++.
Toolchain = the one downloaded from narcissus.

...snip...

Read somewhere about gpiolib.
But i cant find linux/gpio.h in my toolchain's folder.
:s

gpiolib is the official kernel-interface to GPIO. It is not accessable
from user-space.

I created a driver which reflects the gpiolib API into user space.

You can find it on my subversion repository here:
http://svn.hylands.org/linux/gpio/

There is a kernel driver, a user-mode library, and an example test app
which uses the library.

I originally did it for the gumstix, and I wrote up a wiki page here:
http://wiki.gumstix.org/index.php?title=User_GPIO_Driver

It should be generic and work on the beagleboard. There isn't anything
gumstix specific in it. I wrote it before the /sys interface existed.
I think it's slightly more efficient as there are fewer kernel to user
transitions, but that's just a gues, not based on any profiling.

6

I've been using these two links as reference to access gpios (even got
the GPT module working with it) :

http://linuxjunk.blogspot.com/2009/01/beagleboard-gpio-input-driverless.html
http://x4350.blogspot.com/2011/01/digital-io-on-beagleboard-using-gpio.html

They do not use drivers to access gpios, but gives you excellent
options.
Another reference needed will be the 'Technical reference manual' of
the processor (for config values).

7

The above links directly work with registers in the processor.
Userspace is not supposed to work there.

I don't need to use gpio's much so i'll skip on the driver.
Simple file i/o's is enough.

Although the concept of GPIOLib for userspace is excellent.
It should be included in the kernel.