#include <stdio.h>
#include <unistd.h>
int main(){
printf("LED Flash Start\n");
FILE *LEDHandle = NULL;
const char
*LEDBrightness="/sys/class/leds/beaglebone:green:usr0/brightness";
int i;
for(i=0; i<10; i++){
if((LEDHandle = fopen(LEDBrightness, "r+")) != NULL){
fwrite("1", sizeof(char), 1, LEDHandle);
fclose(LEDHandle);
}
usleep(1000000);
if((LEDHandle = fopen(LEDBrightness, "r+")) != NULL){
fwrite("0", sizeof(char), 1, LEDHandle);
fclose(LEDHandle);
}
usleep(1000000);
}
printf("LED Flash End\n");
}
$ gcc --version
gcc (Debian 4.6.3-14) 4.6.3
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ gcc test.c -o test
$ sudo ./test
LED Flash Start
LED Flash End
First, this works as expected. So a couple of notes. First, the iteration
variable must be declared outside of the for loop as demonstrated in the
code above. This is a C11 standard "feature" I believe. Second, this code
will appear to work at first glance, if run without elevated permissions,
but really, accessing */sys/class/leds/beaglebone:green:usr0/brightness* in
the code will fail. So, you have a few options:
1. Run the code as root
2. Run the code with sudo
3. Change the GUID for the
*/sys/class/leds/beaglebone:green:usr0/brightness* file.
The last option here I believe would have to be done every system up(boot),
or you would need to create a udev rule. Essentially though, you need your
regular user to be part of a group that has permissions to write to this
file. Well actually since fopen() uses the mode r+, you probably need read
permissions as well. r+ meaning read / update.
On Mon, Nov 30, 2015 at 9:49 PM, William Hermans <[email protected]> wrote:
> This should compile using gcc, if you're not interested in the C++ warts
> in that code . . . Although for a modern version of gcc, you may have to
> explicitly include another header or two.
>
> #include <stdio.h>
> #include <unistd.h>
>
>
> int main(){
> printf("LED Flash Start\n");
> FILE *LEDHandle = NULL;
> const char *LEDBrightness="/sys/class/leds/beaglebone:green:usr0/brightness";
>
> for(int i=0; i<10; i++){
> if((LEDHandle = fopen(LEDBrightness, "r+")) != NULL){
> fwrite("1", sizeof(char), 1, LEDHandle);
> fclose(LEDHandle);
> }
> usleep(1000000);
>
> if((LEDHandle = fopen(LEDBrightness, "r+")) != NULL){
> fwrite("0", sizeof(char), 1, LEDHandle);
> fclose(LEDHandle);
> }
> usleep(1000000);
> }
> printf("LED Flash End\n");
> }
>
>
>
> On Mon, Nov 30, 2015 at 9:42 PM, William Hermans <[email protected]>
> wrote:
>
>> By the way, no idea why that code is compiled in g++, or uses the using
>> keyword. Because it's all C. Well, it does use cout for stdout too I
>> suppose, but the code uses C style strings, instead of the C++ string
>> classes . . .
>>
>> On Mon, Nov 30, 2015 at 9:39 PM, William Hermans <[email protected]>
>> wrote:
>>
>>> Probably because the OS think it still has control of that LED - And
>>> rightly so(because it does ). So disable the heartbeat pattern on USR0
>>> *first* then try again. Also, if you're not running that app as root, or
>>> sudo. It could fail silently, or possibly seg fault. The following disables
>>> the heartbeat pattern:
>>>
>>> $ ls /sys/class/leds/beaglebone:green:usr0
>>> brightness device max_brightness power subsystem trigger uevent
>>> $ sudo su
>>> # echo "none" > /sys/class/leds/beaglebone:green:usr0/trigger
>>>
>>>
>>> On Mon, Nov 30, 2015 at 8:09 PM, <[email protected]> wrote:
>>>
>>>> Hello,
>>>>
>>>> I have been going through these steps:
>>>> http://elinux.org/Beagleboard:C/C%2B%2B_Programming
>>>>
>>>> I have successfully ssh'd into my BeagleBone Black using MobaXterm and
>>>> typed the code in the above link into a file (I have tried this with geddit
>>>> and nano now and get the same results with both). The code is supposed to
>>>> write "LED Flash Start" to the terminal, flash USR0 on the board 10 times,
>>>> and then write "LED Flash Finish". The code compiles fine and when it runs
>>>> it displays "LED Flash Start", waits for awhile, and then displays "LED
>>>> Flash Finish" but the USR0 never changes from it's usual heartbeat flashing
>>>> pattern. Any ideas on why the USR0 LED doesn't work as it should?
>>>>
>>>> Thanks in advance.
>>>>
>>>> --
>>>> For more options, visit http://beagleboard.org/discuss
>>>> ---
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>>>>
>>>
>>>
>>
>
--
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