Using the state of a gpio in an if condition

Question

I have a function and inside that function I am using usleep(). However, I would only like to use usleep() under the condition that a certain gpio has a value of zero. Here is the code I have so far:

const char *const amplifierGPIO = "/sys/class/gpio/gpio107/value";
const char *const hardwareID = "/sys/class/gpio/gpiox/value";

    bool isWM8750()
    {
      std::ifstream id(hardwareID);
      if (id.is_open())
      {
        const char *const value;
        id >> value;

        if (value == "0")
        {
          return true;
        }
      }
      return false;
    }

    void amplifierUnmute()
    {
      std::ofstream amp(amplifierGPIO);
      if (amp.is_open())
      {
        amp << "1";
        amp.close();
      }

      if(isWM8750())
      {
        usleep(50000);
      }
    }

I am getting an error I am not sure how to resolve:

sound_p51.cpp:38: error: no match for 'operator>>' in 'id >> value'
sound_p51.cpp:40: warning: comparison with string literal results in unspecified behaviour

Answer 1

You are trying to put data into a const char* const variable. A const char* const is a pointer to a string where the pointer can not change, and the string data that is pointed to can not change, hence the const's.

The warning is because there is no overloaded == operator for const char*. For this type of comparison generally you will use strcmp().

However, since you are using c++, you probably want to use a std::string which should solve both of the referenced compiler messages like so:

#include <string>
// ...
bool isWM8750()
    {
      std::ifstream id(hardwareID);
      if (id.is_open())
      {
        std::string value;
        id >> value;
        id.close();

        if (value == "0")
        {
          return true;
        }
      }
      return false;
    }

Some more examples with raspberry pi gpios here: http://www.hertaville.com/introduction-to-accessing-the-raspberry-pis-gpio-in-c.html