Using qsort on string array.

[Violet_Shift]

Hi, I'm a bit of a novice C programmer and I'd like help with what I'm trying to do. I've got a rather large array of strings implemented as a char** and I'm trying to sort it for later processing. If anyone could tell me where and what I'm doing wrong, it'd be greatly appreciated. At the moment it's not altering the ordering of the array at all. I tried to post more details but it seems the forum's cutting me off.

[Adak]

We have first timer's post code all the time, so I'm not sure what's up.

Post what you're working with. That's a requirement of the board, actually. qsort() has some tricky casts that need to be made.

[Violet_Shift]

Not sure what was happening. It was just cutting my post off halfway through my first snippet when I tried. Anyway, here's the qsort() call and the comparator I give it:

void sort_dict(){
     qsort(words, dicSize, sizeof(char)*30, cstring_cmp);
}

int cstring_cmp(const void *a, const void *b) { 
    const char **ia = (const char **)a;
    const char **ib = (const char **)b;
    return strcmp(*ia, *ib);
}

And the array declaration and allocation:

char **words;

   words = (char**)malloc(sizeof(char*)*dicSize);
   for(i = 0; i<dicSize; i++){
      words[i] = (char*)malloc(sizeof(char)*30);
   }

[phantomotap]

sizeof(char)*30

O_o

What did you get this? That parameter marks the size of each element of the things to be sorted. You are not sorting an array of things that is 30 `char' in size. You are sorting an array of pointers; things that are `sizeof(char *)' in size. You even said so in a different bit of code. The `qsort' functions doesn't care what the things it sorts are underneath. Only the comparison function cares about what a thing really is underneath.

Soma

[Violet_Shift]

Well, that was exceptionally stupid of me - I'm still not that great with pointers. I changed the call to:

 qsort(words, wordCount, sizeof(char*), cstring_cmp);

And it's working now. It's always some stupid mistake, it seems. Thanks for the help.

[iMalc]

The thing you want to sort is an array of char*'s hence sizeof(char*) is appropriate for the third parameter.
The length of what each pointer is doesn't come into it as strcmp just goes up until the null-terminator.

[Violet_Shift]

I've run into further problems with the same code, when I've tried to move to significantly larger examples.

On test input which dealt with arrays of length less than 100, this worked fine, however for the purpose I need it needs to deal with a string array of length 250000. (I'm trying to sort a dictionary that my parser has generated).

Unfortunately, when I try this, the program crashes on the qsort statement. Am I simply giving it too big an input, or is there something else going on?

[ZuK]

We would have to see how you're defined that stringarray. Also what compiler you use, your operating system and also the range of an int on you system could be smaller then 250000.
Kurt

[iMalc]

Possibly some of your malloc calls failed. I don't know what you're doing with regards to putting data into each string, but the compare function as you have it now, would definitely crash on the NULL pointers.
Tell us more about the crash. Usually they say something about reading or writing, or invalid instruction etc, and possibly some hexadecimal number. Please copy and paste the message you get here.