why and when is a double-pointer required?

Question

I have been a sysadmin most of my life, but recently decided to practice some my dev knowledge and attempt a devops position. I have as such been practicing some C and Pytho

Answer 1

In C, arguments to function are passed by values. Even pointers are passed by values.

For example:

#include
#include
int allocatingMemory(int* ptr)
{
    ptr = malloc(sizeof(int)); 
    if(ptr==NULL)
        return -1;
    else 
        return 0;
}// We are not returning the pointer to allocated memory

int main(void)
{
    int* ptr;
    int allocated = allocatingMemory(ptr);
    if(allocated == 0)
    {
        *ptr = 999;// Boom!!! 
        free(ptr);
    }

    return 0;
}

To overcome this issue, we use

int allocatingMemory(int** ptr)
{
    *ptr = malloc(sizeof(int));
    if(*ptr == NULL)
        return -1;
    else 
        return 0;
}

int main(void)
{
    int* ptr;
    int isAllocated = allocatingMemory(&ptr);
    if(isAllocated == 0)
    {
        *ptr = 999;
        free(ptr);
    }

    return 0;
}

If you are working with linked lists and say for example, you want to modify the head. You will pass a pointer to pointer (Note that, it is not called as double pointer) to head node.