OS_Q_Create Documentation

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  • OS_Q_Create Documentation

    Hello,

    I got a question regarding the documentation for OS_Q_Create (manual 3.82f).
    For the parameter pData it says: "Pointer to a memory area used as data buffer for the queue."
    But the example is as this:

    C Source Code

    1. #define MEMORY_QSIZE 10000;
    2. static OS_Q _MemoryQ;
    3. static char _acMemQBuffer[MEMORY_QSIZE];
    4. void MEMORY_Init(void) {
    5. OS_Q_Create(&_MemoryQ, &_acMemQBuffer, sizeof(_acMemQBuffer));


    The '&' in front of '_acMemQBuffer' (line 5) puzzles me: that would be a pointer to a pointer to a memory area. Or am I wrong?

    Thanx, Arne

  • Hello Arne,

    you are right, there is a mistake in the sample code.
    The corrected code is:

    C Source Code

    1. OS_Q_Create(&_MemoryQ, _acMemQBuffer, sizeof(_acMemQBuffer));


    I will correct the embOS manual, thanks for the hint.

    Regards,
    Til
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  • Arne wrote:

    The '&' in front of '_acMemQBuffer' (line 5) puzzles me: that would be a pointer to a pointer to a memory area. Or am I wrong?
    An array name often (nearly always) evaluates (or 'decays') to a pointer to the first element of the array, but the array is not itself a pointer. Taking the address of the array (&_acMemQBuffer) also results in a pointer to the array (the pointer is of a different type, though), not a pointer to a pointer. Both expressions will be pointers with the same value, just somewhat different types.

    The expression _acMemQBuffer is a pointer with type char*.

    The expression &_acMemQBuffer is a pointer with type char(*)[10000] (ie., pointer to an array of 10000 char).

    Both pointers have the same 'numeric' value (they both point to the same address).

    Since the 2nd parameter of OS_Q_Create() is a void* there's no complaint about the pointer's type by the compiler (any pointer can convert implicitly to a void*).

    I think there's no error in the original example.