cfdprogrammer

Hi there,

I need to use a function that requires a 1D array in entry; to do so I created a 1D array from my original 2D array in order to pass it to the function (consider that I cannot modify the function because it's part of a library that must be kept as it is), but with the obvious waste of memory.

In fact, when I am treating very large matrices (extremely large), I get to a segmentation fault that is more than likely due to the size of the 2D array plus the new writing of the 1D vector.

I would like to pass directly the original 2D array to the function that requires a 1D array.

In this pseudo code probably my need is clearer:

This is what I am doing now before passing the required 1D array to the function; and this is where I get a segmentation fault when using extremely large arrays:


As said above, I would like to pass the 2D array as if its content were to be written in a 1D array

I hope someone can help. Please, ask me if something is not clear from my description

All the best
CFD

itCbitC

What do you think, can a pointer equal a double value??
Multi-dimensional arrays are laid out as a row of logically contiguous memory locations ie 1D.

cfdprogrammer

Hi there,

I was thinking the same thing, but I wouldn't know how to apply that concept when passing my 1D pointer to the calling function:

Would this be correct?

itCbitC

yep, that is correct.

cfdprogrammer

Thank you, Ill try it and let u know soon

cfdprogrammer

Hi again, the problem is that, given the following definitions:

and

How would I pass only one of the columns of **COORDS (let's say column 2) to "function"?
I cannot call it as:
because I get an error saying : error: incompatible type for argument 1 of 'function',

nor can I call it as:
which gives me a segmentation fault in execution

Sorry about the many problems
CFD

itCbitC

Dereferencing the pointer makes it a double instead of a pointer to double.
To pass column 2 (array element at index 2??) the function call would be:

itCbitC

An assumption was that enough memory has been allocated for each of the pointers of the array that COORDS points to; is that correct?

cfdprogrammer

Hello again; yes, your assumption is correct, that is why I am having all this doubts about the segmentation faults that I get with very large matrices.

However, when passing the 3rd column of the matrix as you suggested

apparently I am not passing what I mean to pass, because when I print it out to screen from inside the function to verify its correctness, I do not get either of the 3 columns of that 2D array, but a strange combination of values.

I am lost

thank you again
CFD

itCbitC

Hard to tell unless you post the portion of the code that's causing the problem.

brewbuck

If you are keeping data in format A, and you need to pass it somewhere that expects format B, then there is no way around the fact that you have to convert it, which will be inefficient. Unless format B is completely bad for your own purposes, it might be better to just represent the data as format B yourself, to avoid this overhead.

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cfdprogrammer

Thank you very much for replying.
I wanted to avoid to write another array because they are extremely big ones, and I believe that is what is causing my system to crash (segmentation fault if big matrices are used).

I see what I can do in a different way

I appreciate

CFD

itCbitC

How big is the size of your array? Given that each double is 8 bytes, sizeof COORDS will be 8 x rows x cols bytes.

cfdprogrammer

Hi,

The arrays Im using can be as big as 10000 elements; I changed all my doubles into floats to start "saving"

itCbitC

With 10000 elements, 8x10000 is approx. 80Kb, hardly enough to fill up the heap. Perhaps storage hasn't been allocated for first-level pointers to double.