Big array gives segmentation error in C

Question

I am really new to C, so I am sorry if this is a absolute beginner question, but I am getting a segmentation error when I am building large array, relevant bits of what I am doing is:

unsigned long long ust_limit;
unsigned long long arr_size;

/* ust_limit gets value around here ... */

arr_size = ((ust_limit + 1) / 2) - 1;
unsigned long long numbs[(int)arr_size];

This works for some values of ust_limit, but when it gets above approximately 4.000.000 a segmentation fault occurs. What I want is to detect a possible segfault and fail gracefully. How can I know which values would cause a segmentation fault. And, is this something platform dependant?

sth · Accepted Answer · 2011-10-06 18:38:31Z

6

You are most likely getting a stack overflow, since you are creating a very large array on the stack. To avoid this, allocate the memory dynamically:

unsigned long long *numbs = malloc(arr_size * sizeof(unsigned long long));

Later, when you are finished with the array, free it again:

free(numbs);

answered Oct 6, 2011 at 18:38

sth

231k56 gold badges288 silver badges370 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

yasar Over a year ago

Can I use numbs like how would I use an array after doing this? For example, can I access indexes?

sth Over a year ago

Yes, you can use it the same way. For example indexing works the same on a pointer to some memory as it works on an array.

lostyzd · Accepted Answer · 2011-10-06 18:37:06Z

1

You array stores on stack frame, which has a limit to its size, use malloc instead.

unsigned long long *numbs = malloc(arr_size * sizeof(long long));
// don't forget to free after use
free(numbs)

answered Oct 6, 2011 at 18:37

lostyzd

4,5233 gold badges22 silver badges35 bronze badges

1 Comment

yasar Over a year ago

I am not sure about how to do that, I am really beginner at this, an example maybe?

Mat · Accepted Answer · 2011-10-06 18:37:49Z

1

You're consuming too much stack. The limit is platform dependent.

The exact limit depends on the OS. These limits can be changed to some extent on some operating systems .

For large amounts of memory, you should be using the head with malloc and/or calloc (and free).

answered Oct 6, 2011 at 18:37

Mat

208k41 gold badges407 silver badges423 bronze badges

Comments

Keith Thompson · Accepted Answer · 2011-10-06 18:38:19Z

numbs is a Variable Length Array (VLA).

VLAs can be created only in block scope (i.e., inside a function). They're allocated like any other local variable, typically on the stack.

Unfortunately, the language doesn't provide a way of detecting or handling allocation failures for local variables. If you allocate too much memory, your program will crash if you're lucky.

Large objects whose size isn't known at compile time should be allocated via malloc() (which means you need to keep track of the allocation and release them with free()).

Incidentally, there's no need to cast arr_size to int. And both ust_limit and arr_size should be of type size_t (defined in <stddef.h>).

Example:

unsigned long long numbs = malloc(arr_size * sizeof *numbs);
/* And later, when you're done with it */
free(numbs);

Collectives™ on Stack Overflow

Big array gives segmentation error in C

4 Answers 4

2 Comments

1 Comment

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

2 Comments

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related