What is char ** in C or C++? Explaining Functions that Return char **

I just came across a weird API method in a C library for Windows that returned a list of strings as char **. Obviously, to read this back you need to know when to stop enumerating, so it told me this list is NULL terminated (terminated by a NULL pointer). Pretty cool, but I had no clue how to even allocate or to free this data type properly so I set up to create a proper example in a Visual C++ console application, since I couldn’t find any good articles on this online. So, effectively this list lives as a pointer to a pointer to a string. Here is how you can create one, read one back, and free one:

#include "stdafx.h"
#include <malloc.h>
#include <string.h>
 
char ** get_list() {
    char **result = (char **)malloc(3 * sizeof(char *));
    char *valueOne = new char[14];
    strcpy_s(valueOne, 14, "Hello, world.");
    result[0] = valueOne;
    char *valueTwo = new char[14];
    strcpy_s(valueTwo, 14, "Hello, again.");
    result[1] = valueTwo;
    result[2] = NULL;
    return result;
}
 
void free_list(char **list) {
    int i = 0;
    while (true) {
        char *value = *(list + i);
        if (value == NULL)
            break;
        delete[] value;
        i++;
    }
    free(list);
}
 
int main()
{
    char **list = get_list();
    int i = 0;
    while (true) {
        char *value = *(list + i);
        if (value == NULL)
            break;
        printf(value);
        printf("\n");
        i++;
    }
    free_list(list);
    return 0;
}

Hopefully you won’t have to spend a few hours on this like I just did. In addition, you would only use such a structure in production if you knew that none of your strings are ever to be NULL, or else they’d be mistaken for the NULL terminator and your deallocation method will fail miserably.

Alexandru

"To avoid criticism, say nothing, do nothing, be nothing." - Aristotle

"It is wise to direct your anger towards problems - not people; to focus your energies on answers - not excuses." - William Arthur Ward

"Science does not know its debt to imagination." - Ralph Waldo Emerson

"Money was never a big motivation for me, except as a way to keep score. The real excitement is playing the game." - Donald Trump

"All our dreams can come true, if we have the courage to pursue them." - Walt Disney

"Mitch flashes back to a basketball game held in the Brandeis University gymnasium in 1979. The team is doing well and chants, 'We're number one!' Morrie stands and shouts, 'What's wrong with being number two?' The students fall silent." - Tuesdays with Morrie

I'm not entirely sure what makes me successful in general programming or development, but to any newcomers to this blood-sport, my best guess would be that success in programming comes from some strange combination of interest, persistence, patience, instincts (for example, someone might tell you that something can't be done, or that it can't be done a certain way, but you just know that can't be true, or you look at a piece of code and know something doesn't seem right with it at first glance, but you can't quite put your finger on it until you think it through some more), fearlessness of tinkering, and an ability to take advice because you should be humble. Its okay to be wrong or to have a bad approach, realize it, and try to find a better one, and even better to be wrong and find a better approach to solve something than to have had a bad approach to begin with. I hope that whatever fragments of information I sprinkle across here help those who hit the same roadblocks.

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