How to qsort an array of pointers to char in C?

user_257942 0
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Suppose I have an array of pointers to char in C:

char *data[5] = { "boda", "cydo", "washington", "dc", "obama" };

And I wish to sort this array using qsort:

qsort(data, 5, sizeof(char *), compare_function);

I am unable to come up with the compare function. For some reason this doesn't work:

int compare_function(const void *name1, const void *name2)
{
    const char *name1_ = (const char *)name1;
    const char *name2_ = (const char *)name2;
    return strcmp(name1_, name2_);
}

I did a lot of searching and found that I had to use ** inside of qsort:

int compare_function(const void *name1, const void *name2)
{
    const char *name1_ = *(const char **)name1;
    const char *name2_ = *(const char **)name2;
    return strcmp(name1_, name2_);
}

And this works.

Can anyone explain the use of *(const char **)name1 in this function? I don't understand it at all. Why the double pointer? Why didn't my original function work?

Thanks, Boda Cydo.

double-pointer qsort sorting c
8 Answers

Maybe it is easier to give you an code example from me. I am trying to sort an array of TreeNodes and the first few lines of my comparator looks like:

int compareTreeNode(const void* tt1, const void* tt2) {
   const TreeNode *t1, *t2;
   t1=*(const TreeNode**)tt1;
   t2=*(const TreeNode**)tt2;

After that you do your comparison using t1 and t2.

user_24073630
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