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About c : Getting-a-stack-overflow-exception-when-declaring-a-large-array

Question Detail

The following code is generating a stack overflow error for me

int main(int argc, char* argv[])
    int sieve[2000000];
    return 0;

How do I get around this? I am using Turbo C++ but would like to keep my code in C


Thanks for the advice. The code above was only for example, I actually declare the array in a function and not in sub main. Also, I needed the array to be initialized to zeros, so when I googled malloc, I discovered that calloc was perfect for my purposes.

Malloc/calloc also has the advantage over allocating on the stack of allowing me to declare the size using a variable.

Question Answer

Your array is way too big to fit into the stack, consider using the heap:

int *sieve = malloc(2000000 * sizeof(*sieve));

If you really want to change the stack size, take a look at this document.

Tip: – Don’t forget to free your dynamically allocated memory when it’s no-longer needed.

There are 3 ways:

  1. Allocate array on heap – use malloc(), as other posters suggested. Do not forget to free() it (although for main() it is not that important – OS will clean up memory for you on program termination).
  2. Declare the array on unit level – it will be allocated in data segment and visible for everybody (adding static to declaration will limit the visibility to unit).
  3. Declare your array as static – in this case it will be allocated in data segment, but visible only in main().

That’s about 7MB of stack space. In visual studio you would use /STACK:###,### to reflect the size you want. If you truely want a huge stack (could be a good reason, using LISP or something :), even the heap is limited to small’sh allocations before forcing you to use VirtualAlloc), you may also want to set your PE to build with /LARGEADDRESSAAWARE (Visual Studio’s linker again), but this configure’s your PE header to allow your compiled binary to address the full 4GB of 32’bit address space (if running in a WOW64). If building truely massive binaries, you would also typically need to configure /bigobj as an additional linker paramerter.

And if you still need more space, you can radically violate convention by using something simular to (again MSVC’s link) /merge:, which will allow you to pack one section into another, so you can use every single byte for a single shared code/data section. Naturally you would also need to configure the SECTIONS permissions in a def file or with #pgrama.

Use malloc. All check the return type is not null, if it is null then your system simply doesn’t have enought memory to fit that many values.

You would be better off allocating it on the heap, not the stack. something like

int main(int argc, char* argv[])
    int * sieve;
    sieve = malloc(20000);
    return 0;

Your array is huge.

It’s possible that your machine or OS don’t have or want to allocate so much memory.

If you absolutely need an enormous array, you can try to allocate it dynamically (using malloc(...)), but then you’re at risk of leaking memory. Don’t forget to free the memory.

The advantage of malloc is that it tries to allocate memory on the heap instead of the stack (therefore you won’t get a stack overflow).

You can check the value that malloc returns to see if the allocation succeeded or failed.
If it fails, just try to malloc a smaller array.

Another option would be to use a different data structure that can be resized on the fly (like a linked list). Wether this option is good depends on what you are going to do with the data.

Yet another option would be to store things in a file, streaming data on the fly. This approach is the slowest.

If you go for storage on the hard drive, you might as well use an existing library (for databases)

As Turbo C/C++ is 16 bit compiler int datatype consumes about 2 bytes.
2bytes*2000000=40,00,000 bytes=3.8147MB space.

The auto variables of a function is stored in stack and it caused the overflow of the stack memory. Instead use the data memory [using static or global variable] or the dynamic heap memory [using the malloc/calloc] for creating the required memory as per the availability of the processor memory mapping.

Is there some reason why you can’t use alloca() to allocate the space you need on the stack frame based on how big the object really needs to be?

If you do that, and still bust the stack, put it in allocated heap. I highly recommend NOT declaring it as static in main() and putting it in the data segment.

If it really has to be that big and your program can’t allocate it on the heap, your program really has no business running on that type of machine to begin with.

What (exactly) are you trying to accomplish?

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