string_append

main.c

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

// You need to pass a pointer since the pointer itself is being modified
void append_string(const char *** array, const char * string)
{
    if(*array == NULL)  // If the array is empty, allocate it with size 2 for the current string and an ending null marker
    {
        *array = malloc(sizeof(const char *)*2);
        if(*array == NULL)  // if *array is NULL, then malloc failed because it couldn't find enough memory
        {
            printf("Failed to malloc!\n");
            return;
        }
        else
        {
            (*array)[0] = string;
            (*array)[1] = NULL;  // the null marker allows it to find the size: just go over the array until you find it
        }
    }
    else  // If the array isn't empty, search for the null marker to find the size and reallocate it bigger
    {
        size_t size = 0;
        while((*array)[size] != NULL)  // search for the null marker, and stop once we found it
        {
            size++;
        }

        const char ** new_array = realloc(*array, sizeof(const char *)*(size+2));  // size only counts the number of strings already in the array, so you need +1 for the null marker, and +1 for new null marker
        if(new_array == NULL)  // if new_array is NULL, then realloc failed because it couldn't find enough memory
        {
            printf("Failed to realloc!\n");
            return;
        }
        else
        {
            *array = new_array;
            (*array)[size] = string;  // add the new string right before the end, over the null marker
            (*array)[size+1] = NULL;  // and a new null marker at the end
        }
    }
}

int main()
{
    const char ** array = NULL;  // starts as NULL so the function knows it's empty
    append_string(&array, "HELLO");  // pass a pointer to the array so the NULL can get changed into something else
    append_string(&array, "WORLD");
    append_string(&array, "!!!");

    for(int i = 0; array[i] != NULL; i++)
    {
        printf("String %d in array is: %s\n", i+1, array[i]);
    }

    free(array);  // don't forget to free, or else you leak memory!

    return 0;
}