Thread: Using malloc() & qsort with a union

What do the structure declarations look like?

Also define an instance of the union. Haven't used unions before, but it would look like this for example:

Code:

union a_rec record; //a single union varable
union a_rec *record; //pointer to type union
union a_rec record[5]; //array of five union variables

Also your malloc call looks a little off. Should be:

Code:

union a_rec *record = (union a_rec *) malloc (sizeof(union a_rec)*Size);

If you give me the declarations, I'll enter some of this stuff in my compiler. I can probably figure out the qsort as well. Also is there a file? Just curious.

> Q2. The above asumes the record is (structure) 'old',
qsort will only work (easily) if the key for the sort is common to all the variants.

So your structure should look like this

Code:

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

#define MAX_RECORDS 10

struct a { int a; };
struct b { char a; };
struct c { float a; };

typedef struct {
   int type;   // is it old,delete,new
   int code;   // unique code ID for the record
   union {
      struct a add;
      struct b delete;
      struct c new;
   }v;
} rec_st;

int compare ( const void *a, const void *b ) {
    const rec_st *pa = a;
    const rec_st *pb = b;
    if ( pa->code < pb->code ) return -1;
    if ( pa->code > pb->code ) return +1;
    return 0;
}

int main ( ) {
    rec_st  records[10];
    qsort( records, MAX_RECORDS, sizeof(rec_st), compare );
    return 0;
}

If the key is in the variant part of the record for some reason, then the compare function has to use the record 'type' in both the records in order to copy out the key to two local variables (or write out the 9 permutations of if/else logic), then compare those two keys.