[beagleboard] C Code to Access GPIOs-Getting permission denied when using open() to access /sys/class/gpio/export

[Brendan Merna]

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;
}

[William Hermans]

***@beaglebone:~$ *cat /sys/class/gpio/export*
cat: /sys/class/gpio/export: *Permission denied*
***@beaglebone:~$ sudo *cat /sys/class/gpio/export*
cat: /sys/class/gpio/export: *Permission denied*
***@beaglebone:~$ *sudo su*
***@beaglebone:/home/debian# *cat /sys/class/gpio/export*
cat: /sys/class/gpio/export: *Permission denied*
***@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
***@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.

Post by Brendan Merna
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.
#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;
}
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[William Hermans]

Also, as I'm unsure what exporting a pin config will do while the system is
running . . . you should make 100% absolutely sure you know what you're
doing. So you do not fry your board.

Post by William Hermans
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
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
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.

Post by Brendan Merna
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.
#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;
}
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[Brendan Merna]

Thanks! That makes a lot of sense now. I figured out I have to open export
as write only which agrees with the permissions you showed. I've moved
GPIOs to export while the board was running using the command line, so I
think its okay.

So changing my code subsituting O_WRONLY for O_RDWR and now I'm getting the
error "Writing Error: Device or Resource is busy." It doesn't seem like I
can use C code to bring a GPIO to the user space. Is there a way around it?

I'll look up the process forking too.

Post by William Hermans
Also, as I'm unsure what exporting a pin config will do while the system
is running . . . you should make 100% absolutely sure you know what you're
doing. So you do not fry your board.

Post by William Hermans
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
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
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.

Post by Brendan Merna
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.
#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;
}
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[William Hermans]

***@beaglebone:/home/debian# *echo 0 > /sys/class/gpio/export*
***@beaglebone:/home/debian# l*s /sys/class/gpio/*
export gpio0 gpiochip0 gpiochip32 gpiochip64 gpiochip96 unexport
***@beaglebone:/home/debian# *echo 0 > /sys/class/gpio/unexport*
***@beaglebone:/home/debian# *ls /sys/class/gpio/*
export gpiochip0 gpiochip32 gpiochip64 gpiochip96 unexport


So . . .write only.Here is a decent read on the subject.
http://falsinsoft.blogspot.com/2012/11/access-gpio-from-linux-user-space.html

Post by Brendan Merna
Thanks! That makes a lot of sense now. I figured out I have to open export
as write only which agrees with the permissions you showed. I've moved
GPIOs to export while the board was running using the command line, so I
think its okay.
So changing my code subsituting O_WRONLY for O_RDWR and now I'm getting
the error "Writing Error: Device or Resource is busy." It doesn't seem like
I can use C code to bring a GPIO to the user space. Is there a way around
it?
I'll look up the process forking too.

Post by William Hermans
Also, as I'm unsure what exporting a pin config will do while the system
is running . . . you should make 100% absolutely sure you know what you're
doing. So you do not fry your board.

Post by William Hermans
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
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
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.

Post by Brendan Merna
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.
#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;
}
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[William Hermans]

DO also keep in mind that /sys/ is a pseudo file system. SO does not always
play by the same rules as disk based file systems.

Post by William Hermans
export gpio0 gpiochip0 gpiochip32 gpiochip64 gpiochip96 unexport
export gpiochip0 gpiochip32 gpiochip64 gpiochip96 unexport
So . . .write only.Here is a decent read on the subject.
http://falsinsoft.blogspot.com/2012/11/access-gpio-from-linux-user-space.html

Post by Brendan Merna
Thanks! That makes a lot of sense now. I figured out I have to open
export as write only which agrees with the permissions you showed. I've
moved GPIOs to export while the board was running using the command line,
so I think its okay.
So changing my code subsituting O_WRONLY for O_RDWR and now I'm getting
the error "Writing Error: Device or Resource is busy." It doesn't seem like
I can use C code to bring a GPIO to the user space. Is there a way around
it?
I'll look up the process forking too.

Post by William Hermans
Also, as I'm unsure what exporting a pin config will do while the system
is running . . . you should make 100% absolutely sure you know what you're
doing. So you do not fry your board.

Post by William Hermans
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
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
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.

Post by Brendan Merna
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.
#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;
}
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[William Hermans]

Any forward traction ?

Post by William Hermans
DO also keep in mind that /sys/ is a pseudo file system. SO does not
always play by the same rules as disk based file systems.

Post by William Hermans
export gpio0 gpiochip0 gpiochip32 gpiochip64 gpiochip96 unexport
export gpiochip0 gpiochip32 gpiochip64 gpiochip96 unexport
So . . .write only.Here is a decent read on the subject.
http://falsinsoft.blogspot.com/2012/11/access-gpio-from-linux-user-space.html

Post by Brendan Merna
Thanks! That makes a lot of sense now. I figured out I have to open
export as write only which agrees with the permissions you showed. I've
moved GPIOs to export while the board was running using the command line,
so I think its okay.
So changing my code subsituting O_WRONLY for O_RDWR and now I'm getting
the error "Writing Error: Device or Resource is busy." It doesn't seem like
I can use C code to bring a GPIO to the user space. Is there a way around
it?
I'll look up the process forking too.

Post by William Hermans
Also, as I'm unsure what exporting a pin config will do while the
system is running . . . you should make 100% absolutely sure you know what
you're doing. So you do not fry your board.

Post by William Hermans
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
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
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.

Post by Brendan Merna
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.
#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;
}
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[j***@colorado.edu]

Post by William Hermans
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
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
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.

Post by William Hermans

Post by Brendan Merna
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.
#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;
}
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...

[William Hermans]

*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.*

Fair enough. Though I tend to err on the side of caution, since *sometimes*
you never know what can come back and bite you.

By the way, that was my own file system / commands above. No idea what his
actual file system groups / permissions looks like.

Since you seem to be more knowledgeable on the subject. What would you
suggest that one do in this situation. Assuming the system could be multi
user, and internet facing ?

Post by William Hermans
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
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
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.

Post by William Hermans

Post by Brendan Merna
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.
#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;
}
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[j***@colorado.edu]

*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.*

Fair enough. Though I tend to err on the side of caution, since
*sometimes* you never know what can come back and bite you.
By the way, that was my own file system / commands above. No idea what his
actual file system groups / permissions looks like.
Since you seem to be more knowledgeable on the subject. What would you
suggest that one do in this situation. Assuming the system could be multi
user, and internet facing ?

...

This is a big topic... securing an Internet-facing server requires a lot of
training on how to write secure software, and it's easy to get it wrong
(there are daily reports of breaches when people blow it). I would say you
install only well-known and well-trusted software, go for servers that are
known to be more secure (eg, qmail instead of sendmail and djbdns instead
of bind). And if you're writing web apps, take a course in webapp security
(OWASP is a good place to start).

Unrelated topic: I've tried to post my own question about the BBB here
twice now and it says "your post will appear after it's been approved" but
this never happens. Is the group moderated? When do the mods come by?
(Sorry to derail the thread!!)

Post by William Hermans
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
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
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.

Post by William Hermans

Post by Brendan Merna
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.
#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;
}
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...

[William Hermans]

Gerald and maybe Jason have the power to approve new users to post. No idea
when they'll get on, but do also keep in mind it's the 4th of July weekend(
possibly already started for some ) So you may have to wait until Monday.

Post by j***@colorado.edu

*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.*

Fair enough. Though I tend to err on the side of caution, since
*sometimes* you never know what can come back and bite you.
By the way, that was my own file system / commands above. No idea what
his actual file system groups / permissions looks like.
Since you seem to be more knowledgeable on the subject. What would you
suggest that one do in this situation. Assuming the system could be multi
user, and internet facing ?

This is a big topic... securing an Internet-facing server requires a lot
of training on how to write secure software, and it's easy to get it wrong
(there are daily reports of breaches when people blow it). I would say you
install only well-known and well-trusted software, go for servers that are
known to be more secure (eg, qmail instead of sendmail and djbdns instead
of bind). And if you're writing web apps, take a course in webapp security
(OWASP is a good place to start).
Unrelated topic: I've tried to post my own question about the BBB here
twice now and it says "your post will appear after it's been approved" but
this never happens. Is the group moderated? When do the mods come by?
(Sorry to derail the thread!!)

Post by William Hermans
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
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
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.

Post by William Hermans

Post by Brendan Merna
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.
#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;
}
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...

[William Hermans]

jrblack, also, you can mail me personally, and I'll forward, or just make a
new post on your behalf. Assuming I can not answer the question myself.

Post by William Hermans
Gerald and maybe Jason have the power to approve new users to post. No
idea when they'll get on, but do also keep in mind it's the 4th of July
weekend( possibly already started for some ) So you may have to wait until
Monday.

Post by j***@colorado.edu

*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.*

Fair enough. Though I tend to err on the side of caution, since
*sometimes* you never know what can come back and bite you.
By the way, that was my own file system / commands above. No idea what
his actual file system groups / permissions looks like.
Since you seem to be more knowledgeable on the subject. What would you
suggest that one do in this situation. Assuming the system could be multi
user, and internet facing ?

This is a big topic... securing an Internet-facing server requires a lot
of training on how to write secure software, and it's easy to get it wrong
(there are daily reports of breaches when people blow it). I would say you
install only well-known and well-trusted software, go for servers that are
known to be more secure (eg, qmail instead of sendmail and djbdns instead
of bind). And if you're writing web apps, take a course in webapp security
(OWASP is a good place to start).
Unrelated topic: I've tried to post my own question about the BBB here
twice now and it says "your post will appear after it's been approved" but
this never happens. Is the group moderated? When do the mods come by?
(Sorry to derail the thread!!)

Post by William Hermans
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
cat: /sys/class/gpio/export: *Permission denied*
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
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.

Post by William Hermans

Post by Brendan Merna
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.
#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;
}
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...