CODEWITHSUNDEEP
csegmentation-faultmalloc
Conor
Conor

Reputation: 45

segmentation fault / malloc error?

EDIT: PROBLEM NOW SOLVED, THANKS ALL

So I'm writing a code for my final piece of C coursework. I've written a function FastDFS which carries out a Fourier Transform. The following code is supposed to time this process for different input sizes. 

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

void FastDFS(complex double *, complex double *, complex double *, complex double *, int, int);
void print_complex_vector(complex double *, int);
void free_complex_vector(complex double *);
complex double *make_complex_vector(int);
complex double *MakeWpowers(int);

int main(void)
{
    int i,N, operations_required;
    int index_counter = 1;
    double initial_time, final_time, time_taken, mega_flops_rate;

    complex double *mega_flops_vector = make_complex_vector(21);

    for(N = 2; N <= pow(2,20); N = N * 2)
    {
        complex double *Wp = MakeWpowers(N);
        complex double *y = make_complex_vector(N);
        complex double *x = make_complex_vector(N);
        complex double *w = make_complex_vector(N);

        printf("N = %d\n",N);

        operations_required = 1024;

        for(i = 1; i <= N; i++)
        {
            y[i] = i;
        }

        initial_time = clock();
        FastDFS(x,y,w,Wp,N,1);
        final_time = clock();

        time_taken = (final_time - initial_time) / CLOCKS_PER_SEC;

        mega_flops_rate = operations_required / (time_taken * pow(10,6));

        free(Wp);
        free(y);
        free(x);
        free(w);
    }

    return(0);
}

The code has been simplified slightly from the original, but contains the same error. At some stage either the error "timings.out(49621,0x7fff7e684300) malloc: *** error for object 0x7f81ca80c208: incorrect checksum for freed object - object was probably modified after being freed." is returned or Segmentation Fault:11. Where the program crashes varies on different runs, but never gets part N = 8192.

I've written a main function to test the code and don't encounter these errors. The main uses the same functions (make_complex_vector etc). A difference I can see is that I don't free any pointers in the main as I only test it for one value of N at a time, but I don't see how this could cause it to crash? I can run the code for much greater values of N than this in the main.

Can anyone see something obviously wrong here? Many thanks

Upvotes: 2

Views: 117

Answers (1)

Daniel
Daniel

Reputation: 6745

You are iterating from 1 -> N and assigning to y. In C, arrays are 0-indexed. For example, if you create an array of size 5, the elements are numbered like so:

[a[0], a[1], a[2], a[3], a[4]] (5 elements, numbered 0-4)

So instead of

for(i = 1; i <= N; i++)

what you really want is

for(i = 0; i < N; i++)

Otherwise, you are going to write past the end of the allocated memory. If you want the first element of the vector to be 1, write something like this:

for(i = 0; i < N; i++)
{
    y[i] = i + 1;
}

(I am assuming that make_complex_vector(N) allocates an array of size N).

Upvotes: 1

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