qsort on dynamic array of dynamic structs [duplicate]

Question

I have the following struct

struct FOO {
    int x;
    double y;
}

and the array

FOO **manyfoos;
manyfoos = malloc( 10 * sizeof(FOO *) );

After this manyfoos is filled with dynamically allocated FOO * elements

manyfoos[i] = malloc( sizeof(FOO) );
manyfoos[i]->x = x;
manyfoos[i]->y = y;

I now want to sort manyfoos with qsort() and the following compare function

int foocmp(const void * p, const void * q) {
    const FOO * pp = (const FOO *)p;
    const FOO * qq = (const FOO *)q;

    return qq->x - pp->x;
}

Unfortunately the following command is giving unexpected results (qq->x and pp->x are random weird numbers).

qsort(manyfoos, 10, sizeof(FOO *), foocmp);

How can I make this work as expected?

Your comparator is given const void * values that are 'really' const FOO ** values, not const FOO * values. If you sort an array of int, the comparator is given int * (cast to const void *). You're sorting an array of FOO *; therefore, you are given const FOO ** values cast to const void *. — Jonathan Leffler
– Jonathan Leffler, Commented Jun 6, 2018 at 23:40
See also My qsort comparison function causes strange stuff in memory. — Jonathan Leffler
– Jonathan Leffler, Commented Jun 6, 2018 at 23:45

Deduplicator · Accepted Answer · 2018-06-06 23:45:53Z

0

Well, your comparison-function is off.

The arguments it gets are pointers to the elements in the array qsort() tries to sort, so pointers to FOO*.

The fixed comparison-function is:

int foocmp(const void * p, const void * q) {
    const FOO* const * pp = p;
    const FOO* const * qq = q;

    return (*qq)->x - (*pp)->x; // Beware of overflow / underflow
}

I also left out the superfluous casts.

As an aside, avoid sizeof(TYPE). Use sizeof expr instead, so you don't repeat the type scattered all over your code.

answered Jun 6, 2018 at 23:45

Deduplicator

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