Memory allocation of an array of struct in a C function - c

Question

I am trying to pass the pointer to a struct to a function to create an array of the struct there. The overall idea is:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
typedef struct items
{
int number;
char *name;
char *description;
} ITEMS;
int process(ITEMS **items)
{
int i = 0, something = 200;
// for loop is for the representation.
// The actual data come from MySQL row loop
for (int j = 100; j < something; j++)
{
// Growing the array of struct
items = realloc(items, (i + 1) * sizeof(*items));
// Adding items here
items[i]->number = j;
strcpy(items[i]->name, "Some name"); // it comes from a variable
strcpy(items[i]->description, "Some text");
i++;
}
return i;
}
int main()
{
ITEMS *items;
int num_items = process(&items);
for (int i = 0; i < num_items; i++)
{
printf("%d - %s - %s\n", items[i].number, items[i].name,
items[i].description);
}
return 0;
}
but I am struggling with reallocating the memory for the struct (and with the pointers too).

Answer 1

The pointer to pointer must be de-referenced in the function so the allocation is visible in the calling function.
strdup is use to allocate memory to the pointers in the structure.
It is better to use a temporary variable for the reallocation.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
typedef struct items
{
int number;
char *name;
char *description;
} ITEMS;
int process(ITEMS **items)
{
int i = 0;
// Growing the array of struct
*items = realloc(*items, (i + 1) * sizeof(**items));
// repeating this step in a loop of adding items
// Adding items here
(*items)[i].number = 72;
(*items)[i].name = strdup( "Some name"); // it comes from a variable
(*items)[i].description = strdup ("Some text");
i++;
*items = realloc(*items, (i + 1) * sizeof(**items));
// another item
(*items)[i].number = 88;
(*items)[i].name = strdup( "Some name"); // it comes from a variable
(*items)[i].description = strdup ("Some text");
i++;
return i;
}
int main()
{
ITEMS *items = NULL;
int num_items = process(&items);
for (int i = 0; i < num_items; i++)
{
printf("%d - %s - %s\n", items[i].number, items[i].name, items[i].description);
}
return 0;
}
With better error detection
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
typedef struct items
{
int number;
char *name;
char *description;
} ITEMS;
int process(ITEMS **items)
{
ITEMS *temp = NULL;
int i = 0;
// Growing the array of struct
if ( NULL == ( temp = realloc(*items, (i + 1) * sizeof(**items)))) {
fprintf ( stderr, "realloc problem\n");
return i;
}
*items = temp;
// repeating this step in a loop of adding items
// Adding items here
(*items)[i].number = 72;
(*items)[i].name = strdup( "Some name"); // it comes from a variable
(*items)[i].description = strdup ("Some text");
i++;
if ( NULL == ( temp = realloc(*items, (i + 1) * sizeof(**items)))) {
fprintf ( stderr, "realloc problem\n");
return i;
}
*items = temp;
// another item
(*items)[i].number = 88;
(*items)[i].name = strdup( "Some name"); // it comes from a variable
(*items)[i].description = strdup ("Some text");
i++;
return i;
}
int main()
{
ITEMS *items = NULL;
int num_items = process(&items);
for (int i = 0; i < num_items; i++)
{
printf("%d - %s - %s\n", items[i].number, items[i].name, items[i].description);
}
return 0;
}