How To Properly Use realloc On A 2D Array In C-CodePudding

I am trying to use take realloc a 2D array but I keep getting a Segmentation Fault and it seems to almost positively be stemming from how I am reallocating my 2D array. The array is initially specified to be a size of 3 x 26. My goal is to simple reallocate the first array (3) and just multiply it by 2 while keeping the other arrays all a size of 26.

//Array creation
 int ** array_creation (){
    int ** arr = (int **) malloc(buffer_size * sizeof(int *));
        if(arr==NULL){
            cout<<"malloc fail"<<endl;
        }
        for(int i=0;i<buffer_size;i  ){
            arr[i] = (int *)malloc(26 * sizeof(int));
            if(arr[i]==NULL){
                cout<<"malloc fail"<<endl;
            }
        }
    return arr;
}
//Variables
buffer_size = 3;
buffer_count = 0;
 if(buffer_count >= buffer_size){
        cout<<"Doubling the size of dynamic arrays!"<<endl;
        buffer_size = buffer_size * 2;
        // arr = (int **) realloc(arr, buffer_size * sizeof(int));
}

So once the buffer_size is equal to the buffer_count I want to just double the first array that would start at 3, so make the 2D array go from 3 array of 26 integers to 6 arrays of 26 integers.

Thank you for any help.

CodePudding user response：

Look at your malloc

int ** arr = (int **) malloc(buffer_size * sizeof(int *));

Now look at your realloc

arr = (int **) realloc(arr, buffer_size * sizeof(int));

See the difference? It should be sizeof(int *) for both cases.

CodePudding user response：

Correct usage of realloc (in general) looks as follows; note that you need a temporary variable to hold the result of the reallocation because you'd lose the old pointer on failure otherwise:

auto tmp = realloc(thePointer, newSize);
if(tmp)
{
    // cast here or earlier, doesn't matter
    thePointer = static_cast<decltype(thePointer)>(tmp);
}
else
{
    // appropriate error handling; note how `thePointer` remainded
    // unchanged and you still have access to the previous array,
    // so you might e.g. free it to avoid memory leaks
}

In your specific case the pointer type is int**, the new size is buffer_size * sizeof(int*) (note that you missed an asterisk there in your code!).

However usage of malloc/realloc is discouraged in C . You avoid all this memory management hassle if you use std::vector instead of raw arrays.