Question

Lets take qsort()’s comparison callback function as an example

int (*compar)(const void *, const void *)

What happens when the result of the comparison function depends on the current value of a variable? It appears my only two options are to use a global var (yuck) or to wrap each element of the unsorted array in a struct that contains the additional information (double yuck).

With qsort() being a standard function, I’m quite surprised that it does not allow for additional information to be passed in; something along the lines of execv()‘s NULL-terminated char *const argv[] argument.

The same thing can be applied to other functions which define a callback that leave no headroom for additional parameters, ftw() & nftw() being two others I’ve had this problem with.

Am I just “doing it wrong” or is this a common problem and chalked up to an oversight with these types of callback function definitions?

EDIT

I’ve seen a few answers which say to create multiple callback functions and determine which one is appropriate to pass to qsort(). I understand this method in theory, but how would you apply it in practice if say I wanted the comparison callback function to sort an array of ints depending on how close the element is to a variable ‘x’?. It would appear that I would need one callback function for each possible value of ‘x’ which is a non-starter.

Here is a working example using the global variable ‘x’. How would you suggest I do this via multiple callback functions?

#include <stdint.h>
#include <stdio.h>
#include <math.h>

int bin_cmp(const void*, const void*);

int x;

int main(void)
{
    int i;
    int bins[6] = { 140, 100, 180, 80, 240, 120 };

    x = 150;

    qsort(bins, 6, sizeof(int), bin_cmp);

    for(i=0; i < 6; i++)
       printf("%d ", bins[i]);

    return 0;
}

int bin_cmp(const void* a, const void* b)
{
    int a_delta = abs(*(int*)a - x);
    int b_delta = abs(*(int*)b - x);

    if ( a_delta == b_delta )
        return 0;

    return a_delta < b_delta ? -1 : 1;
}

Output

140 180 120 100 80 240 

Answer 1

It sounds like you need the bsearch_s() and qsort_s() functions defined by TR 24731-1:

§6.6.3.1 The bsearch_s function

Synopsis

#define __STDC_WANT_LIB_EXT1__ 1
#include <stdlib.h>
void *bsearch_s(const void *key, const void *base,
                rsize_t nmemb, rsize_t size,
                int (*compar)(const void *k, const void *y, void *context),
                void *context);

§6.6.3.2 The qsort_s function

Synopsis

#define __STDC_WANT_LIB_EXT1__ 1
#include <stdlib.h>
errno_t qsort_s(void *base, rsize_t nmemb, rsize_t size,
                int (*compar)(const void *x, const void *y, void *context),
                void *context);

Note that the interface has the context that you need.

Something rather close to this should be available in the MS Visual Studio system.