On Wednesday, July 1, 2015 at 3:19:27 PM UTC-6, William Hermans wrote:
>
> debian@beaglebone:~$ *cat /sys/class/gpio/export*
> cat: /sys/class/gpio/export: *Permission denied*
> debian@beaglebone:~$ sudo *cat /sys/class/gpio/export*
> cat: /sys/class/gpio/export: *Permission denied*
> debian@beaglebone:~$ *sudo su*
> root@beaglebone:/home/debian# *cat /sys/class/gpio/export*
> cat: /sys/class/gpio/export: *Permission denied*
> root@beaglebone:/home/debian# *ls -al /sys/class/gpio/*
> total 0
> drwxr-xr-x 2 root root 0 Dec 31 1999 .
> drwxr-xr-x 59 root root 0 Dec 31 1999 ..
> *--w------- *1 root root 4096 Dec 31 1999 export
> lrwxrwxrwx 1 root root 0 Dec 31 1999 gpiochip0 ->
> ../../devices/virtual/gpio/gpiochip0
> lrwxrwxrwx 1 root root 0 Dec 31 1999 gpiochip32 ->
> ../../devices/virtual/gpio/gpiochip32
> lrwxrwxrwx 1 root root 0 Dec 31 1999 gpiochip64 ->
> ../../devices/virtual/gpio/gpiochip64
> lrwxrwxrwx 1 root root 0 Dec 31 1999 gpiochip96 ->
> ../../devices/virtual/gpio/gpiochip96
> --w------- 1 root root 4096 Dec 31 1999 unexport
> root@beaglebone:/home/debian# *whoami*
> root
>
> read this post . .
>
>
> http://unix.stackexchange.com/questions/118716/unable-to-write-to-a-gpio-pin-despite-file-permissions-on-sys-class-gpio-gpio18
>
> 3rd post or second answer should fix you up. However do note that what
> you're trying to do is "wrong". Meaning: it is insecure. You ( and I too )
> need to read up on process forking. IN short, and perhaps somewhat
> incorrect( as I'm not 100% up to speed either ) is that you fork a process,
> running privileged commands, and when that command is done, the privileges
> are done too . . .
>
> Anyway, probably safer to add your regular user to a group that has
> limited access to that file.
>
The spawned process inherits the real UID (and effective and saved UIDs)
from the parent. But here he needs root to write to the GPIO file.
This is really only a security issue if there are ever other users on the
box. Important security practices for a multiuser machine, yes, but for a
BBB acting as (say) a media server with only a wired network connection,
not really that important.
>
>
> On Wed, Jul 1, 2015 at 8:02 AM, Brendan Merna <bmer...@gmail.com
> <javascript:>> wrote:
>
>>
>> I'm trying to manipulate my GPIOs using C code and running into
>> "Permission Denied" when running my code and opening the file
>> /sys/class/gpio/export. I'm using nano editor, compiling on the Beaglebone
>> with gcc, and I'm under root user.
>>
>> I would like to do this, so I can set directions and values for the GPIOs
>> with my code. I've heard this might be a problem with user and kernel space
>> conflicting. I know there are library calls in python and other languages
>> to do this that work. Does anyone know what this problem might be and our
>> their alternate calls I can do in C?
>>
>> I tried to just include the necessary parts of the code.
>> Code:
>> #include<fcntl.h>
>> static const char *GPIO_PATH = "/sys/class/gpio/export";
>> int main(){
>> int file;
>> if ((file = open(GPIO_PATH, O_RDWR))<0){
>> perror("GPIO: Can't open the device.");
>> return -1;
>> }
>> return 0;
>> }
>>
>> --
>> For more options, visit http://beagleboard.org/discuss
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>
>
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