Question

I try to sort a struct below, given an intention to sort their error rate, while retaining the information of sid and did. While there is no compilation error, I get a seg fault in runtime. I wonder what has gone wrong….

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

struct linkdata {
    int sid;
    int did;
    double err;
};
typedef struct linkdata LD;
typedef int (*qsort_func_t)(const void *, const void *);

static int compareByErr (const void * a, const void * b)
{
    fprintf(stderr, "aerr=%.3f, berr=%.3f\n", (*(LD**)a)->err, (*(LD**)b)->err);
    int aerr = (*(LD**)a)->err;
    int berr = (*(LD**)b)->err;

    return aerr - berr;
}

int main() {

    int idx;
    int numnode;
    struct linkdata* perr;
    qsort_func_t qsort_func = compareByErr;

    numnode = 3;
    perr = (LD*) malloc (numnode*numnode*sizeof(LD));

    perr[0].sid = 0; perr[0].did = 1; perr[0].err = 0.642; 
    perr[1].sid = 0; perr[1].did = 2; perr[1].err = 0.236; 
    perr[2].sid = 0; perr[2].did = 3; perr[2].err = 0.946;
    idx = 3;

    qsort(perr, idx, sizeof(perr), compareByErr);

    int i;
    for (i=0; i<idx; i++){
       fprintf(stderr,"err[%d][%d] = %.3f\n", perr[i].sid, perr[i].did, perr[i].err);            
    }

    free(perr); 
}

Answer 1

There are many errors in the code.

1. compareByErr

The a and b parameters of the compareByErr function are objects of LD*, not LD**. You did an unnecessary dereferencing. Try to change that function to:

static int compareByErr (const void * a, const void * b)
{
    fprintf(stderr, "aerr=%.3f, berr=%.3f\n", ((LD*)a)->err, ((LD*)b)->err);
    int aerr = ((LD*)a)->err;
    int berr = ((LD*)b)->err;

    return aerr - berr;
}

2. compareByErr

There is another problem, that you implicitly convert the double into int. Since all those “errors” are 0.???, they will all be truncated to 0. Making the whole array unsorted. Change it to:

    double aerr = ((LD*)a)->err;
    double berr = ((LD*)b)->err;

    return aerr < berr ? -1 : aerr > berr ? 1 : 0;

3. malloc

You are allocating for 3² nodes, but only 3 are needed. Change that to

perr = (LD*) malloc (numnode * sizeof(LD));

4. qsort

The 3rd argument is the size of each element of the array, not sizeof(perr) which is just the size of a pointer (4 bytes). Change that line to:

qsort(perr, idx, sizeof(*perr), compareByErr);
//                      ^

to actually get the element size.

The idx seems unnecessary. You could just use numnode here.