qsort gives [Error] : invalid conversion from `int (*)(cricketer*, cricketer*)' to `int (*)(const void*, const void*)'

Question

This is the code, it sorts the data of cricketers by avg runs. The qsort function is showing errors:

[Error] C:\Users\Encoder\Documents\C-Free\Temp\Untitled3.cpp:29: error: invalid conversion from int (*)(cricketer*, cricketer*) to int (*)(const void*, const void*)

[Error] C:\Users\Encoder\Documents\C-Free\Temp\Untitled3.cpp:29: error: initializing argument 4 of `void qsort(void*, size_t, size_t, int ()(const void, const void*))'

include

#include<stdlib.h>
struct cricketer  //structure for details of cricketer
{
    int avg_run;
    char name[20];
    int age;
    int match_no;
} c[4];
int sort(struct cricketer *a, struct cricketer *b);   //pre-defining sort function 
int main()     //main function
{
    int i, s;
    for (i = 0; i < 3; i++)    //enumerating structure records.
    {
        printf("enter the name of cricketer ");
        fflush(stdin);
        gets(c[i].name);
        printf("enter his age,his no of matches and total average runs ");
        scanf("%d%d%d",&c[i].age, &c[i].match_no, &c[i].avg_run);
        }
    printf("records before sorting");  
    for (i = 0; i < 3; i++)
    {
        printf("\n\nname ");
        puts(c[i].name);
        printf("age %d\nno of matches %d\naverage runs %d\n",c[i].age, c[i].match_no, c[i].avg_run);
    }
    qsort(c, 3, sizeof(c[0]), sort); //sorting using qsort
    printf("\nrecords after sorting");
    for (i = 0; i < 3; i++)
    {
        printf("\n\nname ");
        puts(c[i].name);
        printf("age %d\nno of matches %d\naverage runs %d\n",c[i].age, c[i].match_no, c[i].avg_run);
    }
}
int sort(struct cricketer *a, struct cricketer *b)  //sort function definition
{
    if (a->avg_run == b->avg_run)
        return 0;
    else 
        if ( a->avg_run > b->avg_run)
            return 1;
        else 
            return -1;
}

Answer 1

The function whose pointer you pass to qsort must be

int sort(const void* va, const void* vb);

Because that's what qsort expects. Then within that function you have to do at the beginning

const struct cricketer *a = (struct cricketer*) va;
const struct cricketer *b = (struct cricketer*) vb;

Or if you prefer accessing with dots . instead of arrows ->

const struct cricketer a = *(struct cricketer*) va;
const struct cricketer b = *(struct cricketer*) vb;

See an example at this reference

Regarding the error message, this int (*)(cricketer*, cricketer*) is a pointer to a function that gets 2 pointers to cricketer as arguments. The compiler expects such a function pointer int (*)(const void, const void*) and it's telling you it cannot convert the former to the latter. Also note how you need pointer to const data as sort is not supposed to modify the data.