Anyway, this is how I probably would have done things . . .
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h> /* Needed for O_WRONLY*/
int main(){
int fd = open("/sys/class/leds/beaglebone:green:usr0/brightness",
O_WRONLY);
if(fd < 0)
printf("Unable to open file, check permissions.\n");
return 1;
int i;
while(i++ < 10){
write(fd, "1", 1);
usleep(500000);
write(fd, "0", 1);
usleep(250000);
}
close(fd);
}
On Mon, Nov 30, 2015 at 11:24 PM, William Hermans <[email protected]> wrote:
> #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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>>>>> Groups "BeagleBoard" group.
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>>>>> an email to [email protected].
>>>>> For more options, visit https://groups.google.com/d/optout.
>>>>>
>>>>
>>>>
>>>
>>
>
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