My son is implementing a server on a Raspberry Pi that allows control of the GPIO pins via a network connection. He has discovered some strange behaviour, which at first seemed like a bug (but see answer below).
First, the OS being used is Raspbian, a version of Debian Linux. He is using the standard system file to control the GPIO ports.
We start with a GPIO pin, e.g. pin 17, in a non-exported state. For example,
echo "17" > /sys/class/gpio/unexport
Now, if the server is asked to turn on pin 17, it does the following:
- Opens the
/sys/class/gpio/export, writes "17" to it, and closes the export file - Open the
/sys/class/gpio/gpio17/directionfile for read, examines it to see if it is set as input or output. Closes the file. Then, if necessary, re-opens the file for write and writes "out" to the file, to set the pin as an output pin, and closes the direction file.
At this point, we should be able to open /sys/class/gpio/gpio17/value for write, and write a "1" to it.
However, the permissions on the /sys/class/gpio/gpio17/value file exists but the group permissions is read-only. If we put in a "sleep" in order to wait for a fraction of a second, the permissions change so the group permission has write permissions.
I would have expected that the OS should not return from the write to the direction file until it had set the permissions on the value file correctly.
Why is this happening? It seems like a bug. Where should I report this (with more detail...)? See answer below.
What follows are the relevant bits of code. The code has been edited and paraphrased a bit, but it is essentially what is being used. (Keep in mind it's the code of a grade 12 student trying to learn C++ and Unix concepts):
class GpioFileOut
{
private:
const string m_fName;
fstream m_fs;
public:
GpioFileOut(const string& sName)
: m_fName(("/sys/class/gpio/" + sName).c_str())
{
m_fs.open(m_fName.c_str());
if (m_fs.fail())
{
cout<<"ERROR: attempted to open " << m_fName << " but failed" << endl << endl;
}
else
{
cout << m_fName << " opened" << endl;
}
}
~GpioFileOut()
{
m_fs.close();
cout << m_fName << " closed" << endl << endl;
}
void reOpen()
{
m_fs.close();
m_fs.open(m_fName);
if (m_fs.fail())
{
cout<<"ERROR: attempted to re-open " << m_fName << " but failed" << endl << endl;
}
else
{
cout << m_fName << " re-opened" << endl;
}
}
GpioFileOut& operator<<(const string &s)
{
m_fs << s << endl;
cout << s << " sent to " << m_fName << endl;
return *this;
}
GpioFileOut& operator<<(int n)
{
return *this << to_string(n); //ostringstream
}
bool fail()
{
return m_fs.fail();
}
};
class GpioFileIn
{
private:
ifstream m_fs;
string m_fName;
public:
GpioFileIn(const string& sName)
: m_fs( ("/sys/class/gpio/" + sName).c_str())
, m_fName(("/sys/class/gpio/" + sName).c_str())
{
if (m_fs <= 0 || m_fs.fail())
{
cout<<"ERROR: attempted to open " << m_fName << " but failed" << endl;
}
else
{
cout << m_fName << " opened" << endl;
}
}
~GpioFileIn()
{
m_fs.close();
cout << m_fName << " closed" << endl << endl;
}
void reOpen()
{
m_fs.close();
m_fs.open(m_fName);
if (m_fs <= 0 || m_fs.fail())
{
cout<<"ERROR: attempted to re-open " << m_fName << " but failed" << endl;
}
else
{
cout << m_fName << " re-opened" << endl;
}
}
GpioFileIn& operator>>(string &s)
{
m_fs >> s;
cout << s << " read from " << m_fName << endl;
return *this;
}
bool fail()
{
return m_fs.fail();
}
};
class Gpio
{
public:
static const bool OUT = true;
static const bool IN = false;
static const bool ON = true;
static const bool OFF = false;
static bool setPinDirection(const int pinId, const bool direction)
{
GpioFileOut dirFOut(("gpio" + to_string(pinId) + "/direction").c_str());
if (dirFOut.fail())
{
if (!openPin(pinId))
{
cout << "ERROR! Pin direction not set: Failed to export pin" << endl;
return false;
}
dirFOut.reOpen();
}
dirFOut << (direction == OUT ? "out" : "in");
}
static bool setPinValue(const int pinId, const bool pinValue)
{
string s;
{
GpioFileIn dirFIn(("gpio" + to_string(pinId) + "/direction").c_str());
if (dirFIn.fail())
{
if (!openPin(pinId))
{
cout << "ERROR! Pin not set: Failed to export pin"<<endl;
return false;
}
dirFIn.reOpen();
}
dirFIn >> s;
}
if (strncmp(s.c_str(), "out", 3) == 0)
{
struct stat _stat;
int nTries = 0;
string fname("/sys/class/gpio/gpio"+to_string(pinId)+"/value");
for(;;)
{
if (stat(fname.c_str(), &_stat) == 0)
{
cout << _stat.st_mode << endl;
if (_stat.st_mode & 020 )
break;
}
else
{
cout << "stat failed. (Did the pin get exported successfully?)" << endl;
}
cout << "sleeping until value file appears with correct permissions." << endl;
if (++nTries > 10)
{
cout << "giving up!";
return false;
}
usleep(100*1000);
};
GpioFileOut(("gpio" + to_string(pinId) + "/value").c_str()) << pinValue;
return true;
}
return false;
}
static bool openPin(const int pinId)
{
GpioFileOut fOut("export");
if (fOut.fail())
return false;
fOut << to_string(pinId);
return true;
}
}
int main()
{
Gpio::openPin(17);
Gpio::setPinDirection(17, Gpio::OUT)
Gpio::setPinValue(17, Gpio::ON);
}
The key point is this: without the for(;;) loop that stat's the file, the execution fails, and we can see the permissions change on the file within 100ms.
