CODEWITHSUNDEEP
c++qsort
mistack305
mistack305

Reputation: 27

Writing access violation while using qsort()

I am using qsort c++ function. I am passing wordArr in my insert function. The objective is to sort the letters for each word belonging to wordArr.However, while using qsort, I get write access violation. Please let me know, where is my mistake.

char* wordArr[]={"cat","bat","dog"};

int tableClass::compChar(const void* a, const void* b)//in my header file this is static
{
    return *(const char*)a - *(const char*)b;
}
void tableClass::insert(char* wordArr[],const int size)
{
    for (int i = 0; i < size; i++)
    {
        qsort(wordArr[i], strlen(wordArr[i]), sizeof(char),tableClass::compChar);

    }   

}

Upvotes: 2

Views: 452

Answers (2)

Marco A.
Marco A.

Reputation: 43662

There are several problems with your code:

1) The parameters are wrong, look at the documentation

2) You're trying to modify read-only memory by reordering an array of pointers to strings allocated into a read-only area. That is not allowed and can result in undefined behavior, i.e. anything could happen.

I would suggest using std::sort for this kind of things and possibly use a stl vector

#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
using namespace std;

void sort_stuff(std::vector<std::string>& arr)
{

    sort(arr.begin(), arr.end());
}

bool myfunction (char *i,char *j) {

    int res = strcmp(i,j); // Needed to compare all characters

    return (res < 0) ? true : false; // Weak ordering criterion
}

int main() {
    std::vector<std::string> arr;
    arr.push_back("cat");
    arr.push_back("dog");
    arr.push_back("bat");

    sort_stuff(arr);

        // --- edit: if you really want to use a char array ---

    char cat[] = "cat";
    char bat[] = "bat";
    char dog[] = "dog";
    char* wordArr[]={cat, bat, dog};
    sort(wordArr, wordArr+sizeof(wordArr)/sizeof(char*), myfunction);
    cout << wordArr[0] << wordArr[1] << wordArr[2];

    return 0;
}

Try it live: http://ideone.com/ErEWnb

Edit: I'm not sure why you really want to use a char array but in case you really need it, you should

1) Put those into writeable memory (e.g. local memory, not string literals) 2) Specify the comparison function since if you pass a char pointers array straight away to sort you would compare the pointers instead of the data they point to. And make sure you compare all characters of the strings. strcmp does what you want

bool myfunction (char *i,char *j) {

    int res = strcmp(i,j); // Needed to compare all characters

    return (res < 0) ? true : false; // Weak ordering criterion
}

...

char cat[] = "cat";
char bat[] = "bat";
char dog[] = "dog";
char* wordArr[]={cat, bat, dog};
sort(wordArr, wordArr+sizeof(wordArr)/sizeof(char*), myfunction);
cout << wordArr[0] << wordArr[1] << wordArr[2];

Upvotes: 3

danielschemmel
danielschemmel

Reputation: 11126

Your code attempts to sort the characters of each string.

char* wordArr[] = { "cat", "bat", "dog" };

Is an array of pointers to string literals. No matter that you store the location of those character literals as char* and not as char const* (which is only possible due to the wish of the c++ standards committee to preserve backwards compatibility), changing a string literal causes undefined behavior.

In your case, the storage for the string literals will be allocated in read only memory, leading to an access violation when you attempt to write it.

By storing the string literals in memory under your own control, the access violation does not happen:

char cat[] = "cat";
char bat[] = "bat";
char dog[] = "dog";
char* wordArr[]={cat, bat, dog};

Upvotes: 3

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