why my code shows segmentation fault.. for n = 1000000000

Question

#include<stdio.h>
#include<stdlib.h>
#include<math.h>

#define mod 1000000007

int main()
{

    unsigned long long int n,i,j,k;
    unsigned long long int *power = (unsigned long long int *)malloc(sizeof(unsigned long long int) * (1000000000LL));
    power[0] = 1;
    for(i = 1;i <= 1000000000LL;i++){
        power[i] = (power[i-1] * 2 ) % mod;
    }

    int t;
    scanf("%d",&t);
    while(t--){

        scanf("%lld",&n);
        unsigned long long int f = 2;
        for(i=2; i<=n;i++){
            f = (f + power[i/2]) % mod;
        }

        printf("%llu\n",f);

    }
    return 0;
}

When `malloc` is unable to allocate the memory for you, it may return a null pointer. Dereferencing one would cause undefined behaviour. You should always check the return value before using is. — eq-, Sep 03 '12 at 14:25
There's no need for the `LL`. An integral literal always has a type in which it fits, if such a type exists. — Kerrek SB, Sep 03 '12 at 14:29
@KerrekSB I believe that's a pretty recent behavior though, else why do we have the type suffixes? — unwind, Sep 03 '12 at 14:39
@unwind: Well, you can say `printf("%lu", 1UL)`, but you cannot say `printf("%lu", 1)`... — Kerrek SB, Sep 03 '12 at 14:42
[Do not cast the return of malloc](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — Eregrith, Sep 03 '12 at 14:42
Welcome to StackOverflow. Please provide a simple explanation of what you're trying to achieve with your code. Also, try reducing your code to an [SSCCE](http://sscce.org/) - often you will discover the problem during this process. See the [FAQ](http://meta.stackexchange.com/questions/7931/faq-for-stack-exchange-sites) for more details. — Joseph Mansfield, Sep 27 '12 at 11:46

score 6 · Answer 1 · answered Sep 03 '12 at 14:25

6

Surely power[1000000000] is out of bounds! Your array doesn't have that many elements. You need <, not <= in the loop.

answered Sep 03 '12 at 14:25

Kerrek SB

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score 4 · Answer 2 · answered Sep 03 '12 at 14:25

4

You should check if malloc properly allocated power.

answered Sep 03 '12 at 14:25

Alexander

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score 2 · Answer 3 · answered Sep 03 '12 at 14:25

2

Because you did not test the return value of malloc. It fails by giving NULL (e.g. when out of memory).

answered Sep 03 '12 at 14:25

Basile Starynkevitch

223,805
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score 0 · Answer 4 · answered Sep 03 '12 at 14:26

This is not a good idea

for(i = 1;i <= 1000000000LL;i++){

indices are zero based anyway but you should replace the <= with a < to prevent from reading past the end of the array

Also given the size of the array you are trying to allocate you should check your power pointer is valid.

why my code shows segmentation fault.. for n = 1000000000

4 Answers4