why am i getting segmentation fault when i run this code?

Question

#include <stdio.h>

void avg_sum(double a[], int n, double *avg, double *sum)
{
    int i;
    sum = 0;
    printf("%f", *sum);
    for(i=0; i<n; i++)
        *sum += a[i];
    *avg = *sum/n;
}


int main()
{
    double arr[2] = {0.0,1.0};
    double *sum;
    double *avg;
    int n = 2;
    avg_sum(arr, n, avg, sum);
    printf("...Done...\n");
    
    return 0;
}

Tried using both GCC(https://www.tutorialspoint.com/compile_c_online.php) and clang(from repl.it) online compilers

Pointers are variables that point to other variables. Which variable does sum point to? — user253751, Aug 26 '20 at 16:10

score 1 · Answer 1 · answered Aug 26 '20 at 15:56

1

Easy. In line 6 you assign 0 to sum but sum is not the actual sum but a pointer to it. When you try to print it you access invalid memory.

Edit: BTW if you try compling with -fanalyzer you will get a warning and an explanation. https://godbolt.org/z/W6ehh8

answered Aug 26 '20 at 15:56

Aleksander Bobiński

315
3
11

1

i changed it to *sum = 0; and changed the main () as told by @paxdiablo, now it works :), Thanks – Plutonian Fairy Aug 26 '20 at 16:08
@PlutonianFairy As a sign of gratitude you could accept paxdiablo's answer. :) – Aleksander Bobiński Aug 26 '20 at 16:10

paxdiablo · Accepted Answer · 2020-08-26T16:03:11.297

double *sum;

This creates a pointer to a double but it has an arbitrary value and therefore points at no dedicated memory.

In addition, in the called function, you set the sum pointer to zero (the null pointer) then try to use that pointer to dereference memory - that's a big non-no.

I'd also be wary of for(i=0; i<n-2; i++) for summing the values in the array. It's not going to include the final two which, since n is two, means it won't accumulate any of them.

The correct way to do this would be with:

void avg_sum(double a[], int n, double *avg, double *sum) {
    int i;
    *sum = 0;               // set content, not pointer.
    for(i=0; i<n; i++)      // do all elements.
        *sum += *(a+i);
    *avg = *sum/n;
}

int main(void) {
    double arr[2] = {0.0,1.0};
    double sum;                  // ensure actual storage
    double avg;                  // and here
    int n = 2;
    avg_sum(arr, n, &avg, &sum); // then pass pointers to actual storage

    printf("Sum=%f, Avg=%f\n", sum, avg);
    
    return 0;
}

This gives you, as expected:

Sum=1.000000, Avg=0.500000

@PlutonianFairy: then you haven't made all the corrections I suggested. The new code works fine, that final output line is from an actual run of it. — paxdiablo, Aug 26 '20 at 16:07
worked when also changed sum = 0; to *sum = 0; after changing the main loop, Thanks — Plutonian Fairy, Aug 26 '20 at 16:09

why am i getting segmentation fault when i run this code?

2 Answers2