Question

I have a structure , which present my element of data

struct myElement
{
    int field1;
    int field2;
    int field3;    
};

another structure, which contain array of this elements and some another data

struct myArray 
{
    struct myElement *elements;
    int someData;
};

and I need to have array of this arrays like that

struct myArray *myMatrix;

But I have a problem with memory allocation. Count of elements in myArray’s can be different, in myMatrix too, so I need to allocate memory dynamicaly. What is the corret way to allocate and deallocate memory in this situation?

Answer 1

Here’s a small example of how you would allocate (malloc) and deallocate (free) a dynamic struct myElement array in a struct myArray. Note also that you will need to keep track of the size of the array, so I added size_t elements_len; to struct myArray (excuse the combination of camelCase and underscores – I use underscores in C, but didn’t want to modify your identifiers):

#include <stdlib.h>

struct myElement
{
    int field1;
    int field2;
    int field3;    
};

struct myArray 
{
    struct myElement *elements;
    size_t elements_len;
    int someData;
};

void allocate_elements(struct myArray *m, size_t length)
{
   m->elements = malloc(length * sizeof( *m->elements) );
   m->elements_len = length;
}

void free_elements(struct myArray *m)
{
   free(m->elements);
   m->elements = NULL;  /* point to NULL to signify no array allocated */
   m->elements_len = 0; /* length of 0 also signifies no elements/no array */
}

int main(void)
{
   struct myArray ma;

   allocate_elements(&ma, 5);
   free_elements(&ma);

   return 0;
}

Apply similar logic in order to have a dynamic array of struct myArray. You would malloc enough memory for X amount of struct myArrays, then for each struct myArray element in that array, you would call allocate_elements. Then iterate through each element in the array once you’re done with it and call free_elements.