macOS memcpy.

[farmerdoug]

As I understand memcpy takes from one memory location and copies it to another. Therefore it needs the address of both locations.

float *A,  *B;

 if ((A = (float *)calloc(reduced_image ,sizeof(float))) == NULL)
        {
            printf("no cys1 memory\n");
            exit(0);
        }

    if ((B = (float *)calloc(slices*reduced_image,sizeof(float))) == NULL)
        {
            printf("no cys1 memory\n");
            exit(0);
        }

memcpy(B + slice*reduced_image, A, sizeof(float)* reduced_image);

Now I want to copy a part of a large array B into a part of a large array C defined as a **:

float *B,  **C;

 if ((B = (float *)calloc(slice* reduced_image ,sizeof(float))) == NULL)
        {
            printf("no cys1 memory\n");
            exit(0);
        }

 C = calloc(slice, sizeof(float*));
    for (i = 0; i < slice; i ++)
        C[i] = calloc(imagesize,sizeof(float));
  

memcpy(?,B + slice*reduced_image, sizeof(float)* reduced_image);

So what replaces the question mark? I would have thought that C[slice] would work but neither that nor &C[slice] works nor does &C + slice*imagesize but I get a can't access memory at that line.

 

[iSee]

I don't understand what you are trying to accomplish, but I have one questiona and noticed one problem:

* When you say it doesn't work, you mean it crashes? (vs. e.g., doesn't complile, etc.)

C is an array of float pointers (float*). The size of the array is slice. That is, you have slice pointers, where the index of the first one is 0 and the index of the last one is slice-1.

So C[slice] is not going to be a valid pointer because you're accessing a pointer out of bounds of the array you allocated. But I can't suggest how to fix it since I don't know what you're trying to do.

 

[ChrisA]

What are you trying to do? If is never a good idea to put "alloc" inside a loop and allocate hundreds of tiny pieces. It is FAR better to allocate the entire array at once.

Assuming two large arrays..

memcpy ( &(B), &(A[j]), n*sizeof(float))

this should move n elements from a[j] to b.