The largest size of 2-D array in C

Question

in my C program, when I am taking an array like this

int a[100000][100000];

I am getting segmentation fault. Now if I am using array of size less than 1000 * 1000 , like this

int a[1000][1000];

I am not getting any problem.In my program I need to use a 10^5 * 10^5 array. What should I do to fix it .

Dynamically allocate it with `malloc`. By declaring it statically you use the stack, which has a maximum size that the heap (used in dynamic allocations) does not. — AntonH, May 05 '14 at 12:50
Do you *have* that much memory in your system? Is it a 64-bit system? Do you declare the variable as a local variable (inside a function) or as a global variable? — Some programmer dude, May 05 '14 at 12:55
Do you really ***really*** need that many int at any given time? — Elias Van Ootegem, May 05 '14 at 12:56
You might also want to think about [memory mapped files](http://en.wikipedia.org/wiki/Memory-mapped_file), and only work on a portion of the data at a time. — Some programmer dude, May 05 '14 at 12:58
@JoachimPileborg Ij am using 64-bit system.I declare it as a local variable — jahan, May 05 '14 at 12:59
@jahan: you do realize that you're going to use `10000000000*sizeof(int)` bytes of memory with this, right? If you have that much memory to begin with, that's your business. But ask yourself if there isn't a better way — Elias Van Ootegem, May 05 '14 at 12:59
Can anyone tell me what should I do if my question is duplicate? — jahan, May 05 '14 at 13:03

score 1 · Accepted Answer · answered May 05 '14 at 12:53

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Dynamically allocate it with malloc. By declaring it statically you use the stack, which has a maximum size that the heap (used in dynamic allocations) does not.

int *pointer = malloc (sizeof (*pointer) * (100000*100000));

Then, to access it, use indices to represent the x and y coordinates.

answered May 05 '14 at 12:53

AntonH

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I need to use it as indices of array. How can I do that? – jahan May 05 '14 at 12:56
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@jahan: `pointer[0][0]` or `pointer[0]` will give you an int... – Elias Van Ootegem May 05 '14 at 12:57
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@jahan Have variables for the `x` and `y`. Access with `int valueToUse = *(pointer+(x*100000)+y)`. – AntonH May 05 '14 at 12:57
@jahan If you want e.g. `pointer[a][b]` then you can't use the technique in this answer. I'll add another method as answer. – Some programmer dude May 05 '14 at 12:59
@JoachimPileborg Yes I want to have a different answer. – jahan May 05 '14 at 13:01
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@jahan Can't add since the question is closed, but pointer to pointer to `int` might be the way to go. – Some programmer dude May 05 '14 at 13:03
@JoachimPileborg Since this thread is on hold, try a chat room instead. jahan seems to have enough rep for it. – AntonH May 05 '14 at 13:04
But what should I do in such cases. I think it is not fair at SO. We beginners are not duplicating anything. – jahan May 05 '14 at 13:05
@AntonH how can I start a chat room? – jahan May 05 '14 at 13:08
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@jahan You can't, you need at least 100 rep. but Joachim can, and then invite you. – AntonH May 05 '14 at 13:09

The largest size of 2-D array in C

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