help undestanding what pointer is being passed

This is a discussion on help undestanding what pointer is being passed within the C Programming forums, part of the General Programming Boards category; Hello, I am refactoring some code that uses a non-GPL'ed FFT function (will replace with fftw), but I don't yet ...

  1. #1
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    help undestanding what pointer is being passed

    Hello,

    I am refactoring some code that uses a non-GPL'ed FFT function (will replace with fftw), but I don't yet understand what exactly is being passed. Specifically, I don't understand what is pointed to when you pass "*pointer -1", as shown below:

    My fft buffer is allocated with:
    pFFftBuffer = (float *) calloc(sizeFft+1, sizeof(float));

    and then passed to the fft function with:
    realft (pFFftBuffer-1, sizeMag, -1);

    realft is defined as:
    void realft(float *data, int n, int isign);

    If anyone could help me understand what data realft is receiving, I would be deeply thankful.

    regards,
    Rich

  2. #2
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    >Specifically, I don't understand what is pointed to when you pass "*pointer -1", as shown below:
    I'm guessing realft() never refers to location 0 of the array (data[0]). This function is probably written to start with an array based at location 1. This is why the calloc() allocates an extra array location (sizeFft+1), as location 1 of the array is never used.

    Strictly speaking, passing pFFftBuffer-1 to the function is not defined by the C Standard, so the code is not portable. However it's likely to work on most platforms.

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