Use of triple pointer over double pointer when only reading value of double pointer?

Question

This code only looks like this as the use of structs and global variables is prohibited in the assignment.

It works either way, but I was wondering whether only using a double pointer would reallocate a temporary copy of the pointer.

#include<stdio.h>
#include<string.h>
#include<stdlib.h>

void allocateData(char ***dptrArrayData, int *intArraySize)
{
    *dptrArrayData = (char**)malloc(*intArraySize*sizeof(char*));
    for (int i = 0; i < *intArraySize; i++)
    {
        (*dptrArrayData)[i] = (char*)malloc(12*sizeof(char));
        (*dptrArrayData)[i] = "Lorem Ipsum";
    }
}

void printDataDoublePointer(char **dptrArrayData, int intArraySize)
{
    for (int i = 0; i < intArraySize; i++)
    {
        printf("Text:%s at index:%d\n",dptrArrayData[i],i);
    }
}

void printDataTriplePointer(char ***dptrArrayData, int *intArraySize)
{
    for (int i = 0; i < *intArraySize; i++)
    {
        printf("Text:%s at index:%d\n",(*dptrArrayData)[i],i);
    }
}
int main()
{
    char **dptrArrayData;
    int intArraySize = 5;
    allocateData(&dptrArrayData,&intArraySize);
    printDataDoublePointer(dptrArrayData,intArraySize);
    printDataTriplePointer(&dptrArrayData,&intArraySize);
    return 0;
}

The function parameter needs to have one more `*` than the variable that it's updating. — Barmar, Nov 02 '21 at 00:12
You could use a `char **` return value instead of `char ***` parameter. — Barmar, Nov 02 '21 at 00:13
The question isnt about updating, and sadly I cannot use a return value as I will return many arrays in one function, the question is whether using the double pointer to only read is the same as using a triple pointer. — , Nov 02 '21 at 00:17
What is it about? `allocateData()` needs to update the variable `dptrArrayData`, so it needs `char ***`. The other functions don't update anything, so they can use `char **`. — Barmar, Nov 02 '21 at 00:20
So when I pass in the double pointer only it doesnt create a temporary copy of the array to read from? Because if I update something in the double pointer one the value stays in scope. Edit: I am a noob at c, oop is all I know... — , Nov 02 '21 at 00:22
(*dptrArrayData)[i] = (char*)malloc(12*sizeof(char)); (*dptrArrayData)[i] = "Lorem Ipsum"; This is nonsense. — gnasher729, Nov 02 '21 at 00:29
I put it as an example, but this is how you allocate a dynamic 2D char array if you have a triple pointer passed, am I wrong? The assignment of the value is only as a test. — , Nov 02 '21 at 00:31

score 0 · Accepted Answer · answered Nov 02 '21 at 00:25

It works either way, but I was wondering whether only using a double pointer would reallocate a temporary copy of the pointer.

I take you to be asking about the difference between

void printDataDoublePointer(char **dptrArrayData, int intArraySize)

and

void printDataTriplePointer(char ***dptrArrayData, int *intArraySize)

. In each case the function receives copies of all argument values presented by the caller. This is the essence of pass-by-value semantics, which are the only function-call semantics C provides.

But there is no reallocation involved. When a pointer is passed as a function argument, only the pointer is copied, not the object to which it points. The function receives an independent pointer to the same object the original points to.

Thank you, I didnt know that, maybe they should tell us this at school lol — , Nov 02 '21 at 00:27
It's right there in the name: "pointer". It's just something that points to something else, not the thing itself. — Barmar, Nov 02 '21 at 00:29

Use of triple pointer over double pointer when only reading value of double pointer?

1 Answers1