qsort() fails on an array of char* strings

[fp_unit]

I'm trying to sort an array of strings. I wrote my comparison function and tested it with single char* strings, but when I try and loop over an array of strings it fails, my code crashes internally in the qsort() function and I dont understand why.

/* sample.cpp */
#include <iostream>
using namespace std;

const int NUM_STRINGS = 10;
int compare(const void* p1, const void* p2);	// for qsort()

int main()
{
    char* arr_strs[] = { "test", "hi", "hello", "debug" };
    char buf[80];

    /* this part fails on the first iteration */
    for(int i = 0; i < NUM_STRINGS; i++)
        qsort(arr_strs[i], strlen(arr_strs[i]), sizeof(char), compare);

    /* but this would work */
    strcpy(buf, arr_strs[0]);
    qsort(buf, strlen(buf), sizeof(char), compare);

    /* I'm not sure if this would work, I havent tested it */
    {
        char* test = "helloworld";
        qsort(test, strlen(test), sizeof(char), compare);
    }
}

int compare(const void* p1, const void* p2)
{
    return (*(char*)p1 - *(char*)p2);
}

[Valmont]

Is this a nice hint?
(observe the details)

#include <iostream>
#include <cstdlib>

using namespace std;

static const int NUM_STRINGS = 4;
int compare(const void* p1, const void* p2);

int main()
{
  char* arr_strs[] = { "test", "hi", "hello", "debug" };

  qsort(arr_strs, 4 , sizeof(char*), compare);

  for(unsigned i = 0; i < NUM_STRINGS; ++i)
  {
    std::cout<<arr_strs[i]<<" "<<std::endl;
  }

  return 0;
}

int compare(const void* p1, const void* p2)
{
  return strcmp(*(char **)p1, *(char **)p2);
}