I am very sure that someone told me once that NULL is defined as a pointer to void. I leafed through the K&R, and NULL was just said to be interchangeable with zero.

But either way, if I have a function that returns a 3d char array (array of arrays of strings), can I then return NULL if something goes wrong? will it be a valid return type?

Many functions that return pointers return NULL if something goes wrong - fopen for instance where you check for NULL and then perror.

But I am confused about multi-dimensional pointers.

I mean, I know that they are technically just pointers. I am unsure what the multiple asterisks do except tell the programmer how many dimensions there are. Hmmm Is this the solution?

Comments?

Source: https://cs50.harvard.edu/x/2023/labs/5/

Here is the portion that I need help with:

// Create a new individual with `generations`
person *create_family(int generations)
{
    // TODO: Allocate memory for new person
    person *new_person = malloc(sizeof(person));
    if (new_person == NULL)
    {
        return 1;
    }

Here is what ChatGPT says on my return 1 usage:

The TODO comment indicates that you need to allocate memory for a new person structure. In the provided code, you're using malloc(sizeof(person))to allocate memory dynamically. However, when memory allocation fails (i.e., malloc returns NULL), you're returning the integer 1 from the create_family function. Typically, a function returning a pointer should return NULL to indicate a failure.

Next, I rephrased the code:

// Create a new individual with `generations`
person *create_family(int generations)
{
    // TODO: Allocate memory for new person
    person *new_person = malloc(sizeof(person));
    if (!new_person)
    {
        return 1;
    }

ChatGPT reply:

Your updated code checks if new_person is NULL using the condition if (!new_person).

However, it's better to return NULL*(a pointer) in case of memory allocation failure, rather than the integer* 1.

Here's the modified code:

// Create a new individual with `generations`
person *create_family(int generations)
{
    // TODO: Allocate memory for new person
    person *new_person = malloc(sizeof(person));
    
    // Check if memory allocation was successful
    if (!new_person)
    {
        fprintf(stderr, "Memory allocation failed.\n");
        return NULL;
    }
    
    // Rest of the code for creating the family tree
    // ...

    return new_person;
}

In this modification:

• If memory allocation fails, the function prints an error message to the standard error stream using fprintf(stderr, "Memory allocation failed.\n");

• return NULL;
  is used to indicate that the function failed to create a new person due to a memory allocation failure.

• If memory allocation is successful, the function proceeds with the rest of the code for creating the family tree.

Returning NULL in case of memory allocation failure is a common practice in C, as it allows the calling code to check for errors by examining the returned pointer.

Full project code provided by CS50:

// Simulate genetic inheritance of blood type

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

// Each person has two parents and two alleles
typedef struct person
{
    struct person *parents[2];
    char alleles[2];
} person;

const int GENERATIONS = 3;
const int INDENT_LENGTH = 4;

person *create_family(int generations);
void print_family(person *p, int generation);
void free_family(person *p);
char random_allele();

int main(void)
{
    // Seed random number generator
    srand(time(0));

    // Create a new family with three generations
    person *p = create_family(GENERATIONS);

    // Print family tree of blood types
    print_family(p, 0);

    // Free memory
    free_family(p);
}

// Create a new individual with `generations`
person *create_family(int generations)
{
    // TODO: Allocate memory for new person
    person *new_person = malloc(sizeof(person));
    if (new_person == NULL)
    {
        return 1;
    }

    // If there are still generations left to create
    if (generations > 1)
    {
        // Create two new parents for current person by recursively calling create_family
        person *parent0 = create_family(generations - 1);
        person *parent1 = create_family(generations - 1);

        // TODO: Set parent pointers for current person

        // TODO: Randomly assign current person's alleles based on the alleles of their parents
    }

    // If there are no generations left to create
    else
    {
        // TODO: Set parent pointers to NULL

        // TODO: Randomly assign alleles
    }

    // TODO: Return newly created person
    return NULL;
}

// Free `p` and all ancestors of `p`.
void free_family(person *p)
{
    // TODO: Handle base case

    // TODO: Free parents recursively

    // TODO: Free child
}

// Print each family member and their alleles.
void print_family(person *p, int generation)
{
    // Handle base case
    if (p == NULL)
    {
        return;
    }

    // Print indentation
    for (int i = 0; i < generation * INDENT_LENGTH; i++)
    {
        printf(" ");
    }

    // Print person
    if (generation == 0)
    {
        printf("Child (Generation %i): blood type %c%c\n", generation, p->alleles[0], p->alleles[1]);
    }
    else if (generation == 1)
    {
        printf("Parent (Generation %i): blood type %c%c\n", generation, p->alleles[0], p->alleles[1]);
    }
    else
    {
        for (int i = 0; i < generation - 2; i++)
        {
            printf("Great-");
        }
        printf("Grandparent (Generation %i): blood type %c%c\n", generation, p->alleles[0], p->alleles[1]);
    }

    // Print parents of current generation
    print_family(p->parents[0], generation + 1);
    print_family(p->parents[1], generation + 1);
}

// Randomly chooses a blood type allele.
char random_allele()
{
    int r = rand() % 3;
    if (r == 0)
    {
        return 'A';
    }
    else if (r == 1)
    {
        return 'B';
    }
    else
    {
        return 'O';
    }
}