How to write qsort() comparator for sentence and text structs?

Question

Suppose I have a struct Sentence and a struct Text. How do I implement qsort on capital_letters_quant? How can you write out the same compare function?

struct Sentence {
    wchar_t* sentence_text;
    size_t sentence_size;   
    bool is_sentence_anagram;
    int64_t capital_letters_quant;
};

struct Text {
    struct Sentence* full_text;
    size_t full_text_size;
};

So, I tried to change the arguments of the compare function to struct Sentence *, but the compiler gave warnings or errors, I would not want to receive them, to be honest...

This is my atmpt to do it

int second_compare(const void* first_current_sentence, const void* second_current_sentence) {
    return ((struct Sentence*)second_current_sentence -> capital_letters_quant) - ((struct Sentence*)first_current_sentence -> capital_letters_quant);
}

But gcc send me this

error: conflicting types for 'second_compare'; have 'int(const void *, const void *)'

note: previous declaration of 'second_compare' with type 'int(struct Sentence *, struct Sentence *)'

warning: dereferencing 'void *' pointer

error: request for member 'capital_letters_quant' in something not a structure or union

So, I tried to change the arguments of the compare function to struct Sentence *, but the compiler gave warnings or errors, I would not want to receive them, to be honest... — Amur Dzagkoev, Dec 21 '21 at 16:27
Please [edit] your question to add information, don't use comments for this purpose. Show your attempts to implement a comparison function and copy&paste the error messages. Does this help: https://stackoverflow.com/q/43100263/10622916 — Bodo, Dec 21 '21 at 16:29
K, it seems that I did everything, as I was told, did I formulate the question correctly? — Amur Dzagkoev, Dec 21 '21 at 16:36
Hey, Bodo, how can I make your answer correct? It really help me, thanks! — Amur Dzagkoev, Dec 21 '21 at 16:44
You are not showing the relevant code... that's obvious from errors — Support Ukraine, Dec 21 '21 at 17:38

score 0 · Answer 1 · answered Dec 21 '21 at 20:13

First, you should change your declaration of second_compare to int second_compare (const void *, const void *); because the types of arguments in declaration and in definition must be the same.
Second, the -> operator has greater priority relatively to cast operator (struct Sentence *), which means that compiler first tries to access field capital_letters_quant in the structure pointed to by the void pointer, thus it tries to dereference it. To fix this, you should first cast, then dereference: ... ((struct Sentence *) data) -> field ....
Third, your function takes two pointers, referencing constant values (as far as you marked them with const). But in the body of function you cast these values to non-constant values of type (struct Sentence *), whilst you should cast to constant values of type (const struct Sentence *).

How to write qsort() comparator for sentence and text structs?

1 Answers1