Question

I’m not sure how to explain this but this piece of code bellow can compile perfectly but when you run it, SIGSEV.
Please, can anyone tell precisely where I got things wrong?
The fact is I want to be able to access elements by index as below and at the same time to be able to work with struct.

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

/* This is a struct describing properties of an element */
struct element{
    int age;
    char* name;
};

/* This struct contains a pointer to a pointer on a element "struct element" */
struct person{
    struct element** p;
    int id;
};

/* Thus function initializes a struct person by allocation memory for it */
struct person* init(int size)
{
    struct person* sample = (struct person* )malloc(size*sizeof(struct person));
    sample->p = NULL;
    sample->id = 0;
    return sample;
}

/* use this function to insert a new element in the struct */
void insert(struct person* sample, char* _name, int _age)
{
    sample->p[sample->id]->name = _name; /* the program crashes here  according to the debugger , but why?? */
    sample->p[sample->id]->age = _age;  /* of course, this will cause trouble too because it has the same construct as the previous one */
    sample->id++;
}


/* main entry */
int main()
{
    struct person* student = init(10); /* Allocating space for 10 students */
    insert(student, "kido", 8);
    printf("Your name is %s and your age is %d", student->p[0]->name, student->p[0]->age); /* we can't write student->p->name */
    return 0;
}

Answer 1

The problem is in the insert method at the line of code you flagged in the question

sample->p[sample->id]->name = _name;

Nowhere in your program do you allocate memory for the p array inside of the person struct. Hence this value will always be NULL. Attempting to assign to this value will rightfully lead to a crash of your program.

To fix this you need to ensure the p array is large enough to accommodate the index provided by the expression sample->id. Best way to accomplish this is to use the realloc function and add a field to person to store the size of the p array

Here’s a quick sample. Note: Error checking and 0 initialization of memory omitted for bevity.

struct person{
    struct element** p;
    size_t length;
    int id;
};

void insert(struct person* sample, char* _name, int _age)
{
  if (sample->id >= sample->length) {
    sample->p = realloc(sample->p, sizeof(element*) * sample->id);
  }
  ...
}

It does seem odd though that the name and age are always indexed via the sample->id field. This indicates that it’s always placed in the same location in which case an array is not needed. Can you elaborate on how this is supposed to function?