Thread: beginner in C: void pointer refers to an array of double. How do I read the data?

  1. #1
    Registered User
    Join Date
    Apr 2012

    beginner in C: void pointer refers to an array of double. How do I read the data?


    I am using code from the hardware manufacturer of this chip in robotics I bought, but I'm a beginner in C.

    The code is event driven. After x millisecond an eventhandler is called. This event handler receives a "void *userptr" variable for the user to do whatever they want with it.

    In the main() I have declared "double dataset1[10]" and I pass the address on to the eventhandler.

    In the eventhandler I need to access the data in dataset1 but I can't get the code right. I believe a typecasting is required here but can't get it right. Can someone tell me how I can access the data in the dataset1 array using the void *usertpr?

    for example how do I retrieve the value of dataset1[1] ?


  2. #2
    Registered User
    Join Date
    Jun 2005
    In C, a void pointer is implicitly convertible to any other pointer type, so all you should need in the function is
      double *data = userptr;
         /*  data[i] can now be accessed for i = 0 to 9 */
    If you insist on an explicit type conversion (aka typecast) then the syntax is
       double *data = (double *)userptr;
    If you actually need such a explicit conversion to stop you compiler complaining, then you are actually using a C++ compiler. Note that such an explicit conversion is considered poor form in C.
    Right 98% of the time, and don't care about the other 3%.

    If I seem grumpy or unhelpful in reply to you, or tell you you need to demonstrate more effort before you can expect help, it is likely you deserve it. Suck it up, Buttercup, and read this, this, and this before posting again.

  3. #3
    Registered User
    Join Date
    Apr 2012
    works great. Thx.

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