CODEWITHSUNDEEP
cpointers
Sam
Sam

Reputation: 51

passing argument 4 of 'qsort' from incompatible pointer type

I'm having some trouble with a function we are supposed to be writing. Supposedly, this is how it should work, but it's giving me the incompatible pointer type error and I'm not sure how to fix it.

The issue is in the qsort referencing the compare_last function.

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

#define MAX_PERSONS 100

//person structure definition goes here
typedef struct{
    char last[32];
    char first[32];
    int year;
}Person;

//function declarations go here
int compare_last(Person * ptr1, Person * ptr2);

void main(void){//main function goes here
    char *infilename[20];
    char *outfilename[20];
    FILE * fptrin;
    FILE * fptrout;
    int i, j;
    Person musicians[MAX_PERSONS];
    printf("Enter input file name: ");
    scanf("%s", infilename);
    printf("Enter output file name: ");
    scanf("%s", outfilename);
    strcat(*outfilename, ".htm");
    fptrin = fopen(*infilename, "r");
    fptrout = fopen(*outfilename, "w");

    for(i = 0; i < MAX_PERSONS; i++)
    {
        fscanf(fptrin, "%s %s %i", musicians[i].last, musicians[i].first, &musicians[i].year);
    }

    qsort(musicians, i, sizeof(musicians[0]), compare_last);

    fprintf(fptrout, "<html>\n<body>\n<title>LAB14</title>\n");

    for(j = 0; j < i; j++)
    {
        fprintf(fptrout, "%s %s %i <br>", musicians[j].last, musicians[j].first, musicians[j].year);
    }

    fprintf(fptrout, "</body>\n</html>\n");
    fclose(fptrin);
    fclose(fptrout);

}//end main

//function definitions go here

int compare_last(Person * ptr1, Person * ptr2)
{
    int result = strcmp(ptr1 -> last, ptr2 -> last);
    if(result != 0)
        return result;
    else
        return strcmp(ptr1 -> first, ptr2 -> first); 
}

Upvotes: 5

Views: 11499

Answers (2)

unwind
unwind

Reputation: 400129

The prototype for qsort() is:

void qsort(void *base, size_t nmemb, size_t size,
           int(*compar)(const void *, const void *));

Thus, your sorting function needs to either match int(*compar)(const void *, const void *), or you need to cast when calling qsort().

If you want to sort Person*, the easiest way is to declare your sorting function as required, then cast inside the function:

static int compare_last(const void *ptr1, const void *ptr2)
{
    const Person *p1 = ptr1, *p2 = ptr2;
    int result = strcmp(p1 -> last, p2 -> last);
    ...
}

there's no need at all to cast then, since const void * converts just fine to const Person *.

Upvotes: 3

Erik
Erik

Reputation: 91320

int compare_last(Person * ptr1, Person * ptr2);

should be

int compare_last(void * ptr1, void * ptr2);

Then you need to cast within compare_last

Upvotes: 4

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