double pointer memory allocation - c++

Question

I am currently trying to allocate the same amount of memory for a double pointer. I take in a char** and want to use a bubble sort on that char** . So I create a temp char** and now I'm wondering how to correctly allocate enough memory so that I can return that temp char** to another method.
I know the way I'm allocating right now doesn't look right and it certainly doesn't work...otherwise I wouldn't be asking this question. If someone could respond with some helpful advice, I would greatly appreciate it!
char** bubble_sort(char **filenames, int n)
{
int i;
char **new_list;
new_list = malloc(sizeof(filenames));
for (i = 0; i < n; i++)
{
// malloc(file_list.size * sizeof(int));
new_list[i] = filenames[i];
}
for (i = 0; i < n; i++)
{
printf("%d: %s\n", i, new_list[i]);
}
int x;
int y;
for(x=0; x<n; x++)
{
for(y=0; y<n-1; y++)
{
if(new_list[y]>new_list[y+1])
{
char *temp = new_list[y+1];
new_list[y+1] = new_list[y];
new_list[y] = temp;
}
}
}
for (i = 0; i < n; i++)
{
printf("%d: %s\n", i, new_list[i]);
}
return new_list;
}

Answer 1

char** bubble_sort(char **filenames, int n)
{
int i;
char **new_list;
new_list = malloc(sizeof(filenames));
This code allocates enough space to store a single pointer (sizeof(filenames) is mostly likely 4), and gives the address of that pointer to new_list. If you want to access what new_list points to as an array (and I know you do, because you tried to do just that below), you'll need to allocate enough space for its elements.

Answer 2

Here is the working copy of the program:
#include <cstdio>
#include <cstdlib>
#include <cstring>
char** bubble_sort(const char **filenames, int n)
{
int i;
char **new_list;
new_list = (char**) malloc(sizeof(*new_list) * n);
for (i = 0; i < n; i++)
{
new_list[i] = (char*) filenames[i];
}
printf("Initial list:\n");
for (i = 0; i < n; i++)
{
printf("%d: %s\n", i, new_list[i]);
}
int x;
int y;
printf("List is sorted:\n");
for(x=0; x<n; x++)
{
for(y=0; y<n-1; y++)
{
if(strcmp(new_list[y],new_list[y+1])>0)
{
char *temp = new_list[y+1];
new_list[y+1] = new_list[y];
new_list[y] = temp;
}
}
}
for (i = 0; i < n; i++)
{
printf("%d: %s\n", i, new_list[i]);
}
return new_list;
}
int main(){
const char *ar[5]={
"eee", "aaa", "bbb", "ccc", "ddd",
};
bubble_sort(ar, 5);
return (0);
}
Still, keep in mind that your programming style resembles more to C than C++ (which is not always a bad thing).
If you want to allocate new strings for your array elements, you should change the first for like this:
for (i = 0; i < n; i++)
{
//new_list[i] = (char*) filenames[i];
new_list[i] = (char*) malloc(sizeof(**new_list) * (strlen(filenames[i]) + 1));
strcpy(new_list[i], filenames[i]);
}
And this is the C version (first one was the C++ version). Note that the string array has all its elements newly allocated, and is not using the initial strings from the input parameter.:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char** bubble_sort(char **filenames, int n)
{
int i;
char **new_list;
new_list = malloc(sizeof(*new_list) * n);
for (i = 0; i < n; i++)
{
//new_list[i] = (char*) filenames[i];
new_list[i] = malloc(sizeof(**new_list) * (strlen(filenames[i]) + 1));
strcpy(new_list[i], filenames[i]);
}
printf("Initial list:\n");
for (i = 0; i < n; i++)
{
printf("%d: %s\n", i, new_list[i]);
}
int x;
int y;
printf("List is sorted:\n");
for(x=0; x<n; x++)
{
for(y=0; y<n-1; y++)
{
if(strcmp(new_list[y],new_list[y+1])>0)
{
char *temp = new_list[y+1];
new_list[y+1] = new_list[y];
new_list[y] = temp;
}
}
}
for (i = 0; i < n; i++)
{
printf("%d: %s\n", i, new_list[i]);
}
return new_list;
}
int main(){
char *ar[5]={
"eee", "aaa", "bbb", "ccc", "ddd",
};
bubble_sort(ar, 5);
return (0);
}

Answer 3

filenames is a pointer to pointer to char, therefore on this line...
new_list = malloc(sizeof(filenames));
...you're allocating the amount of the size of the pointer (to pointer), which isn't what you want.
You probably want malloc(sizeof(filenames) * n); which will give you the space for n pointers.

Answer 4

This is somewhat of a duplicate. See: Creating `char ***data`?
There are serious memory performance issues with respect to allocating at char** for a 2D array. Better to use a char* and a indexing scheme. In this way you get a contiguous chunk of memory. You can also use a one-D std::vector with such a scheme.
If you must allocate a char**, the for loop is all you can do AFAIK. But at least make a subroutine out of that! :)