Question

There are lots of questions on stackoverflow regarding how to sort an array of structure pointers. I looked through them all, to no avail. I want to sort an array of pointers to an array of structures. I first allocate storage for the pointer array, then for the structures themselves. All that seems fine, but I can’t get them sorted. I’m sure the problem is in the compare function. I’ve copied a few of them from stackoverflow, and they are listed below. But none of them work…

typedef struct s_stream{
int amc;
char *name;
} dataStream;

void abc(void)
{
       int count = 100;

    dataStream *_UniqueStreamBuild  = calloc(count, sizeof(dataStream ));
    dataStream **UniqueStreamBuild =  calloc(count, sizeof(dataStream *));
    for ( int i = 0; i < count; ++i) UniqueStreamBuild[i] = _UniqueStreamBuild + i; 

//**Edit: ******** **
        // here I call a cascade of functions that assign values to amc; those
        // functions are correct: they produce an unsorted array of amc values;
        // the output I am getting is an array of structures seemingly in random order.

    qsort(UniqueStreamBuild, count, sizeof(dataStream *), compare);  
}

int compare (const void * a, const void * b)
{
    const dataStream *x = a;
    const dataStream *y = b;

    if (x->amc > x->amc)
      return(1);

  if (x->amc < x->amc)
      return(-1);

  return(0);   
}


int compare( const void *a, const void *b )
{
  dataStream *m1 = *(dataStream **)a;
  dataStream *m2 = *(dataStream **)b;

  if (m1->amc > m2->amc)
      return(1);

  if (m1->amc < m2->amc)
      return(-1);

  return(0);
}

Answer 1

Your second possible compare() function should work unless there’s some difference I didn’t notice between it and this version just below. When sorting an array of pointers, the comparison function is passed two pointers to a dataStream *, hence the comparator should be very similar to this:

int compare (const void *a, const void *b)
{
    const dataStream *x = *(const dataStream **)a;
    const dataStream *y = *(const dataStream **)b;

    if (x->amc > y->amc)
        return(1);
    else if (x->amc < y->amc)
        return(-1);
    else
        return(0);   
}

Also, as originally written, one of your functions always returns 0 because x->amc == x->amc (you dereference x twice, instead of x and y).

Your test code does not fully initialize the data structures – it uses calloc() so the strings and pointers in the structures are all zeroed, so sorting doesn’t do much.

---

This code works for me…how about you?

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

typedef struct s_stream
{
    int   amc;
    char *name;
} dataStream;

static int compare(const void *a, const void *b)
{
    const dataStream *x = *(const dataStream **)a;
    const dataStream *y = *(const dataStream **)b;

    if (x->amc > y->amc)
        return(1);
    else if (x->amc < y->amc)
        return(-1);
    else
        return(0);
}

static void dump(FILE *fp, const char *tag, dataStream * const * const data, int num)
{
    const char *pad = "";
    fprintf(fp, "Stream Dump (%s): (%d items)\n", tag, num);
    for (int i = 0; i < num; i++)
    {
        fprintf(fp, "%s%d", pad, data[i]->amc);
        if (i % 10 == 9)
        {
            putc('\n', fp);
            pad = "";
        }
        else
            pad = ", ";
    }
    putc('\n', fp);
}

static void abc(void)
{
    int count = 100;

    dataStream *_UniqueStreamBuild  = calloc(count, sizeof(dataStream ));
    dataStream **UniqueStreamBuild =  calloc(count, sizeof(dataStream *));
    for ( int i = 0; i < count; ++i)
    {
        UniqueStreamBuild[i] = _UniqueStreamBuild + i;
        UniqueStreamBuild[i]->amc = (7 * i + 3) % count + 1;
    }

    dump(stdout, "Before", UniqueStreamBuild, count);
    qsort(UniqueStreamBuild, count, sizeof(dataStream *), compare);
    dump(stdout, "After", UniqueStreamBuild, count);

    free(_UniqueStreamBuild);
    free(UniqueStreamBuild);
}

int main(void)
{
    abc();
    return 0;
}