Accessing members of dynamically allocated array of pointers to structs

Question

I need to have a global dynamic array of pointers, in which I will store my structs, beacuse later I will need to iterate through this array to list all the stored information, I also need to be able to read the name, age and job variables from the console, and store them in a person_t in the iterator array.

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

typedef struct Person
{
    char name[30];
    int age;
    char job[30];
} person_t;

person_t **iterator;
int capacity = 10;
int size = 0;

int main()
{
    int i;
    *iterator = (person_t *)malloc(capacity * sizeof(person_t));
    for (i = 0; i < capacity; ++i)
    {
        person_t p;
        p.age = i;
        *iterator[i] = p;
    }
    return 0;
}

I get no errors/warnings compiling this code (gcc -ansi -pedantic -Wall -Wextra), but when I try to run it, I get a Segmentation fault immediately.

Answer 1

When you do this:

*iterator = (person_t *)malloc(capacity * sizeof(person_t));

You're deferencing iterator, however as a file-scope pointer variable it's initialized to NULL. Attempting to dereference a NULL pointer invokes undefined behavior.

I suspect what you really want is an array of structs, not an array of pointers to structs. That being the case, define iterator as:

person_t *iterator;

Then you allocate memory for it like this:

iterator = malloc(capacity * sizeof(person_t));

Then assign to array elements like this:

iterator[i] = p;

Answer 2

Your stated purpose is to create a "global dynamic array of pointers, in which I will store my structs". The following modification of your code (see comments) will do this:

person_t p[10] = {0};

int main()
{
    int i;
    // with declaration: person_t **iterator = NULL;, 
    //following is all that is needed to create an array of pointers:
    iterator = malloc(capacity * sizeof(person_t *));//no need to cast return of malloc
   
        
        for (i = 0; i < capacity; ++i)
        {
            //person_t p;//moved to scope that will exist outside of main()
            p[i].age = i;
            iterator[i] = &p[i];//assign the address of the object to the pointer
                             //iterator[i] is the ith pointer in a collection of 
                             //pointers to be assigned to point to 
                             //instances of struct person_t 
        }
        //Once all fields are populated (to-do), the following will display the results:
        for (i = 0; i < capacity; ++i)
        { 
           printf("%d) Name: %s Age: %d  Job: %s\n",  i, iterator[i]->name,iterator[i]->age,iterator[i]->job);
        }

    return 0;
}

Answer 3

you are not allocating memory correctly

First you need to allocate memory for a pointer which can store capacity number of address i.e done through iterator = malloc(capacity * sizeof(person_t*)); and then you need to allocate memory for holding each structure element i.e iterator[i] = malloc(sizeof(person_t));

all the malloc'ed memory should be free'd once we are done with it.

Also, have not done the error check for malloc's , that is left as an exercise for you.

int main()
{
    int i;
    // test data
    char *names[] = {"ABC", "DEF"};
    char *jobs[] = {"Accountant", "Security"};
    int ages[] = {50, 60};
    
    // first allocate memory for iterator , which can hold pointers to store iterator poniters
    iterator = malloc(capacity * sizeof(person_t*)); 
    
    for (i = 0; i < capacity; ++i)
    {
        // now allocate memory for individual iterator
        iterator[i] = malloc(sizeof(person_t)); 
        strcpy(iterator[i]->name,names[i]);
        iterator[i]->age = ages[i];
        strcpy(iterator[i]->job, jobs[i]);
    }
    
    for (i = 0; i < capacity; ++i)
    {
        printf("name = %s ", iterator[i]->name);
        printf("Age = %d ", iterator[i]->age);
        printf("Job = %s\n", iterator[i]->job);
    }
    return 0;
}