Qsort an array of chars nested in a struct

**Seag** · 04-07-2017

Hello,

I've searched through many code examples and think that I have the correct compare function, but my problem is that when I run the code, I get an error EXC_BAD_ACCESS. XCode points to the cmpfunc function as the culprit. I know I have allocated memory to all my elements, as I can print the array before it gets passed to qsort. Could someone point me in the right direction for where I am having issues? Please and thank you very much!

Below are my structs so you can see how many levels down I am when I am qsorting:

Code:

typedef struct {
    unsigned int siteId;
    unsigned int tableTypeId;    
    unsigned int surMatId;
    unsigned int strucMatId;
    char *streetAve;
    unsigned int neighbourhoodId;
    char *neighbourhoodName;
    unsigned int ward;
    char *latitude;
    char *longitude;
} Entries;

typedef struct {
    int size;
    Entries **entry;
} PicnicTable;

typedef struct {
    Table *tableTypeTable;
    Table *surfaceMaterialTable;
    Table *structuralMaterialTable;
    NeighbourHoodTable *neighborhoodTable;
    PicnicTable *picnicTableTable;
} DataBase;

extern DataBase *DB;

Code:

int cmpfunc(const void *a, const void *b) {
     return strcmp((*(Entries**)a)->neighbourhoodName, (*(Entries**)b)->neighbourhoodName);
}

//    just ignore the below, I was experimenting
//    Entries left = *(Entries*)a;
//    Entries right = *(Entries*)b;
//    return (*(int*)left.neighbourhoodName - *(int*)right.neighbourhoodName);

//    return (*(int*)(*(Entries**)a)->neighbourhoodName - *(int*)(*(Entries**)b)->neighbourhoodName);
    
//    Entries *left = *(Entries**)a;
//    Entries *right = *(Entries**)b;
//    return (*(int*)left->neighbourhoodName - *(int*)right->neighbourhoodName);

Code:

Entries **ent = DB->picnicTableTable->entry;
qsort(ent,DB->picnicTableTable->size-1, sizeof(Entries*), cmpfunc);

**Salem** · 04-07-2017

It looks like you have too much dereferencing going on.

> qsort(ent,DB->picnicTableTable->size-1, sizeof(Entries*), cmpfunc);
The type you want to cast to in the cmpfunc is Entries**

If you're sorting an array of T, then the cmp function gets two T* pointers (as const void*).

So do this, and stop trying to compress everything into a single line.

Code:

int cmpfunc(const void *a, const void *b) {
    Entries **pa = (Entries**)a;
    Entries **pb = (Entries**)b;
    return strcmp(pa->neighbourhoodName, pb->neighbourhoodName);
}

**Seag** · 04-07-2017

Thank you for the suggestion, though casting it as you pointed out doesn't allow me access to the members of the struct. I would have to cast it like:

Code:

Entries *pa = (Entries*)a;

Either way, I still get the same access error.

**Salem** · 04-07-2017

How about this?

Code:

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

typedef struct {
    unsigned int siteId;
    unsigned int tableTypeId;    
    unsigned int surMatId;
    unsigned int strucMatId;
    char *streetAve;
    unsigned int neighbourhoodId;
    char *neighbourhoodName;
    unsigned int ward;
    char *latitude;
    char *longitude;
} Entries;

int cmpfunc(const void *a, const void *b) {
    Entries **pa = (Entries**)a;
    Entries **pb = (Entries**)b;
    return strcmp((*pa)->neighbourhoodName, (*pb)->neighbourhoodName);
}

int main( ) {
  Entries test[2] = {
    { .neighbourhoodName = "world" },
    { .neighbourhoodName = "hello" },
  };
  Entries *t[2] = {
    &test[0],
    &test[1]
  };
  Entries **ent = t;
  qsort(ent,2, sizeof(Entries*), cmpfunc);
  printf("%s %s\n", t[0]->neighbourhoodName, t[1]->neighbourhoodName);
  return 0;
}

**Seag** · 04-07-2017

That definitely runs with my code, I forgot to put brackets around pa and pb before. Geez

It doesn't fix my bad_access error, which makes me think that my real problem is how I'm allocating/accessing memory. I'm pretty sure the memory malloc'd before qsort isn't affected in terms of how much needs to be allocated? Weirdness. I think the dereferencing dance has me spinning in circles.

Either way, thank you for narrowing down my scope of tests! :-)

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