Segmantation fault when adding elements to a structure

Question

Why do I get segmentation fault in this function:

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

vec_t mtrx_multiple (sparse_mat_t a, vec_t c) {

    vec_t result;
    int i;

    result.n = a.n;
    printf("result.n: %d\n", result.n);


    result.vec = malloc(a.n * sizeof *result.vec);
    for(i=0; i<a.n; i++) 
        result.vec[i] = c.vec[i] * a.a[a.ja[i]];


    return result;
}

The structure is:

typedef struct {
    int n;
    int *vec;
} vec_t;

typedef struct {
    int *a;
    int *ia;
    int *ja;
    int n;
} sparse_mat_t;

Thanks for help

Answer 1

I suspect the problem is with a.a[a.ja[i]], you should try verifying the values a.ja[i] before using them to index a.a.

It would be useful to know how a is initialised, and also on which line the segfault occurs.

Answer 2

I suspect this is the line where the trouble is:

    result.vec = malloc(a.n * sizeof *result.vec);
    for(i=0; i<a.n; i++) 
        result.vec[i] = c.vec[i] * a.a[a.ja[i]];

The reason is that you are not mallocing for each result.vec[i]..

Can you confirm this?

Edit: Thanks Alok and Devel for informing me about my error... What does sizeof *result.vec return? Admittedly it looks confusing as if the precedence between sizeof gets mixed with the *...

Hope this helps, Best regards, Tom.

Answer 3

Malloc could be failing and returning null. a.ja[i] might not be between 0 and n. What is the ja array supposed to represent, anyway?

Our speculating isn't going to produce the answer. Running your program under a debugger will.