Question

I want to sort a list of string. To parameter of qsort() function is a list of c[i]’s defined as so :

    //N is the number of words
    int N;
    scanf("%d",&N);
    //each c[i] is a word 
    char **c;
    c = malloc(N*sizeof(char*));

    for(i = 0; i < N; i++)
    {
      char *temp[100];
      scanf("%s", &temp);
      c[i]=strdup(temp);
    }

But when I sort the list, the result is wrong.

Precisely to debug, I added this

for(i = 0; i < N; i++){
  j=compare (c[0],c[i]);
  printf("%d",j);
  printf("%s",c[i]);
}

My input is ‘4;a;b;c;d’ and I get ‘ 0b-1c-1d1a’

Can anybody help about this?

Answer 1

This is incorrect:

char *temp[100];
scanf("%s", &temp);

temp is an array of 100 uninitialised char* pointers, there is no storage for a char[]. Change to:

char temp[100];
scanf("%99s", temp); /* Added maximum number of chars to prevent overrun. */

From you comment:

int compare(const void* l, const void* r)
{
    return strcmp(l, r);
}

This should be:

int compare(const void* a1, const void* a2)
{
    const char** s1   = a1;
    const char** s2   = a2;
    return strcmp(*s1, *s2);
}

From the C99 standard section 7.20.5.2 The qsort function:

The contents of the array are sorted into ascending order according to a comparison
function pointed to by compar, which is called with two arguments that point to the
objects being compared. The function shall return an integer less than, equal to, or
greater than zero if the first argument is considered to be respectively less than, equal to, or greater than the second.

Which means that the type of the arguments to the compare() function are char**, and therefore need to be dereferenced before performing a string comparision.

See http://ideone.com/rUG89 for example.