Qsort quicksort in C USES instances

The brief introduction is as follows.

#include <stdio.h>
#include <string.h> //STRCMP function
#include <stdlib.h> //Tree function
#include <math.h> //fabs(double),abs(int)

int intcmp(const void*i1,const void *i2)
{
 return *(int*)i1-*(int*)i2;
}
int doublecmp(const void *d1,const void *d2)
{
 //return *(double*)d1 - *(double*)d2;// There is an error  double inaccurate 
 double tmp=*(double*)d1 - *(double*)d2;
 if(fabs(tmp) < 0.000001)
  return 0;
 else 
  return tmp>0 ? 1 : -1;
}
int stringcmp(const void *str1,const void *str2)
{
 return strcmp(*(char**)str1,*(char**)str2);
 
}
void main()
{
 printf("---------------------C In the qsort Method of use ( The default increment )----------------------n");

 int a[]={1,2,6,8,10,7,9,40,12,6,7};
 printf("-------int[] An array of qsort test -------nbefore sort:n");
 for(int i=0;i!=sizeof(a)/sizeof(int);i++)
  printf("%d ",a[i]);
 qsort(a,sizeof(a)/sizeof(int),sizeof(a[0]),intcmp);
 printf("nafter sort:n");
 for(int i=0;i!=sizeof(a)/sizeof(int);i++)
  printf("%d ",a[i]);
 printf("n");

 printf("-------double[] An array of qsort test -------nbefore sort:n");
 double d[]={1.12,1.1236,1.36,1.2456,2.48,2.24123,-2.3,0};
 for(int i=0;i!=sizeof(d)/sizeof(double);i++)
  printf("%f ",d[i]);
 qsort(d,sizeof(d)/sizeof(double),sizeof(d[0]),doublecmp);
 printf("nafter sort:n");
 for(int i=0;i!=sizeof(d)/sizeof(double);i++)
  printf("%f ",d[i]);
 printf("n");
 printf("-------string: char*[] An array of qsort test -------nbefore sort:n");
 char *str[]={"hello","hi","you","are","baby"};
 for(int i=0;i!=sizeof(str)/sizeof(str[0]);i++)
  printf("%s ",str[i]);
 printf("nafter sort:n");
 qsort(str,sizeof(str)/sizeof(str[0]),sizeof(str[0]),stringcmp);
 for(int i=0;i!=sizeof(str)/sizeof(str[0]);i++)
  printf("%s ",str[i]);
 printf("n");
}

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