Qsort ordering in the wrong way

Question

My activity is to read a sentence and ordered in alphabetical order, but is giving error

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

int ordena(const void * a, const void * b)
{
   return strcmp((char *)a, (char *)b); 
}

int main(int argc, char** argv)
{
  char **nomes, frase[900], *ponteiro;
  nomes = malloc(sizeof(char*) * 3000);
  int indice, cont = 0, i;
  for(indice = 0; indice < 3000; indice++)
  {
    nomes[indice] = malloc(sizeof(char) * 100);
  }

  scanf("%[^\n]", frase);

  ponteiro = strtok(frase, " ");
  while(ponteiro != NULL)
  {
     strcpy(nomes[cont], ponteiro);
     cont++;
     ponteiro = strtok(NULL, " ");
  }
  qsort((void*)nomes, cont, sizeof(nomes[0]), ordena);
  for(i = 0; i < cont; i++)
  {
    printf("%s\n", nomes[i]);
  }

   return 0;
}

Example of orange meat starter juice orange pickles orange pickles

Example of orange beef output pickles pickles juice

Answer 1

Your problem is actually quite typical in the qsort, which is to forget that the comparison function receives pointers to the array values. So if you have an array of ints the comparison function receives two int*. In your case you have an array of char* the function will receive two char**, what makes this:

int ordena(const void * a, const void * b)
{
   return strcmp((char *)a, (char *)b);
                     ^----------^ 
}

Is not correct.

Instead you have to interpret each parameter as one char** and access the target value char* who intends:

int ordena(const void * a, const void * b)
{
    char **palavra1 = (char**) a; //interpretar como char**
    char **palavra2 = (char**) b; //interpretar como char**
    return strcmp(*palavra1, *palavra2); //comparar o apontado de cada um
}

With this change the output gives:

carne
laranja
laranja
picles
picles
suco

View execution in Ideone

As a note, the cast for void* in the qsort is not necessary, ending up being simpler and readable without it:

qsort((void*)nomes, cont, sizeof(nomes[0]), ordena);
        ^---este

Answer 2

Pay close attention to RAM memory misalignment after use! free in them!

See if this resolves:

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

char** palavras;

int comparador(const void* item1, const void* item2)
{
    char* elemento1 = *(char**)item1;
    char* elemento2 = *(char**)item2;

    return strcmp(elemento1, elemento2);
}

int main()
{
    char palavra[80];
    int qtdPalavras;
    int tamanhoPalavra;
    int i;

    do
    {
        palavras = NULL;

        fflush(stdin);
        printf("Digite a frase (8 para sair): ");

        qtdPalavras = 0;
        do {
            scanf("%s", palavra);
            tamanhoPalavra = strlen(palavra);
            if (*palavra != '8')
            {
                palavras = (char**)realloc(palavras, (qtdPalavras + 1) * sizeof(char*));
                palavras[qtdPalavras] = (char*)malloc((tamanhoPalavra + 1) * sizeof(char));
                strcpy(palavras[qtdPalavras], palavra);
                ++qtdPalavras;
            }
        } while (*palavra != '8');

        if (qtdPalavras)
        {
            printf("Ordenando a lista de palavras...\n");
            qsort(palavras, qtdPalavras, sizeof(char*), comparador);
            printf("Lista ordenada:\n");
            for (i = 0; i < qtdPalavras; ++i)
            {
                printf("%s ", palavras[i]);
            }
            printf("\n");
        }

        for (i = 0; i < qtdPalavras; ++i)
        {
            free(palavras[i]);
        }

        free(palavras);
    } while (qtdPalavras);

    return 0;
}