passing argument 4 of qsort from incompatible pointer type

Question

please help with this warning:

passing argument 4 of qsort from incompatible pointer type;

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BSIZE 200
int (*cmp)(void *, void *);
int main(int argc, char *argv[]){

  FILE *in;
  char buf[BSIZE];
  int i, nofl;
  char *p[20];



  if(argc<2){
    printf("too few parameters...\n");
    return 1;
}
  in=fopen(argv[1], "r+b");
  if(in == NULL){
    printf("can't open file: %s\n", argv[1]);
    return 0;
}

  while(fgets(buf,BSIZE+1,in)){
    p[i]=malloc(strlen(buf)+1);
    strcpy(p[i],buf);

    i++;
}

  nofl=i;

  qsort(p,nofl,sizeof(int), &cmp);

  for(i=0;i<nofl;i++)
    printf("%s\n",p[i]);

  return 0;
}

int (*cmp)(void *a, void *b){
  int n1 = atoi(*a);
  int n2 = atoi(*b);
  if (n1<n2)
    return -1;
  else if (n1 == n2)
    return 0;
  else
    return 1;
}

i think this c program must convert string to int and sort by asc. the problem is that with qsort strngs massive takes no effect, it stays the same as unsorted.

that's not how function pointers work. declare and define cmp as int cmp(const void *a, const void *b) and then pass &cmp (or simply cmp) to qsort(). Didn't check the code further... — user2371524
int (*cmp)(void *, void *); says that cmp is a function pointer, not a function. — mch
Change qsort(p,nofl,sizeof(int), &cmp); --> qsort(p,nofl,sizeof(int), cmp); and also change int (*cmp)(void *a, void *b) --> int cmp(void *a, void *b) — Weather Vane
void *a, void *b){ int n1 = atoi(*a); will not work as *a is de-referencing a void *. — chux - Reinstate Monica
@WeatherVane explicitly taking the address of a function isn't an error, just not necessary... — user2371524

R Sahu R Sahu · Accepted Answer · 2015-10-27T19:42:50

int (*cmp)(void *a, void *b){ ... }

is not legal code to define a function.

Instead of

int (*cmp)(void *, void *);

use

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

Update, in response to comment by OP

int cmp(const void *a, const void *b){

  // Here, a is really a pointer to a `char*`.

  const char* pa = *(const char**)a;
  const char* pb = *(const char**)b;

  int n1 = atoi(pa);
  int n2 = atoi(pb);

  // Simplify the return value.
  return ((n1 > n2) - (n1 < n2));

  /***
  if (n1<n2)
    return -1;
  else if (n1 == n2)
    return 0;
  else
    return 1;
  ***/

}

passing argument 4 of qsort from incompatible pointer type

1 Answers