Please help debug this C declaration [C]

[cdiem]

I have tried playing with some Project Euler problem (decided to do it in C), but was stopped in the beginning by the rather strange Segmentation Fault:

#include <stdio.h>

main()
{
    unsigned long int searched_prime, max, i, j;
    max = 1000000000L;
    unsigned long int nums[max];
    printf("%s", "success");
}

Can you please help me debug why this prints Segmentation Fault? Is there an error with the declarations? I have ran Valgrind on it, ran it through gdb, but wasn't able to find my error. I have tried compiling it with:
gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5)

[matt_symes]

Hi

max = 1000000000L;     unsigned long int nums[max];

Try creating your array on the heap, using malloc or some such, and then consider what you have done to your stack

Kind regards

[cdiem]

Thanks. I haven't used C so far, and it's a humbling experience after using other languages. Thanks for your help.

[jespdj]

I hope you realize how much memory this is going to take: you want to allocate a billion unsigned long values. An unsigned long is usually 64 bits (8 bytes) so that statement will allocate 8 GB memory. Do you have that much memory in your computer?

On a 32-bit system this will certainly fail (because a process is limited to a 4 GB address space), and even on a 64-bit system it is likely to fail unless you have a LOT of RAM (even if you have more than 8 GB RAM it might fail because there isn't a consecutive block of 8 GB free RAM).

[matt_symes]

Hi

I hope you realize how much memory this is going to take

I'm hoping the OP figured that out

Kind regards

[r-senior]

Ooh, Euler. Not done one of those for ages. I've done quite a few in C and had the odd foray deep into my swap partition.

Which problem is it?

[cdiem]

Ooh, Euler. Not done one of those for ages. I've done quite a few in C and had the odd foray deep into my swap partition.

Which problem is it?

It's the problem 7; my first try was an Euler sieve on a big array, for learning C better (hence the topic). My code was 30 lines long, but also nonworking I'll try to rework it to use less memory. Thanks!

[Barrucadu]

Why are you trying to allocate such a gigantic array?

[r-senior]

An Erasthones Sieve?

Why don't you try working your way up - find the 101st prime, then the 1001st prime and so on? You'll get a feel for whether you need (a) such a large sieve and (b) whether you need long integers.

It's a good learning exercise doing Euler in C. When you look at the solutions other people have done, especially in Python and J (which is a "program" of just 7 characters for this problem ) , you'll feel that you've had to work hard but it's good learning.

It's worth putting your prime number functions into a library and looking at ways to tune them. You'll use them a lot. You'll learn how big a sieve is worthwhile and how to go beyond the sieve. Functions to test whether a number is prime, generate sequences of primes, etc.

[cdiem]

Why are you trying to allocate such a gigantic array?

I was trying to follow the algorithm as said in Wikipedia:
1. Create a list of consecutive integers from two to n: (2, 3, 4, ..., n)
2. ...

But I set n too high ;]

Why don't you try working your way up - find the 101st prime, then the 1001st prime and so on? You'll get a feel for whether you need (a) such a large sieve and (b) whether you need long integers.

It's a good learning exercise doing Euler in C. When you look at the solutions other people have done, especially in Python and J (which is a "program" of just 7 characters for this problem ) , you'll feel that you've had to work hard but it's good learning.

It's worth putting your prime number functions into a library and looking at ways to tune them. You'll use them a lot. You'll learn how big a sieve is worthwhile and how to go beyond the sieve. Functions to test whether a number is prime, generate sequences of primes, etc.

Awesome! Thanks very much!