How can I create a dynamic array in a procedure in C? - c

Question

I'm new to this fantastic site and I found it very useful in many occasions, but now I'm dealing with a problem which I can't seem to solve.
I'm a beginner C student and I'm studying dynamic memory allocation.
I want to create a dynamic array of, let's say, integers, allocating memory for it inside a procedure, not in the main function.
This works:
#include <stdio.h>
#include <stdlib.h>
int main()
{
int *numbers; // pointer to create a dynamic array
int *test; // pointer to test realloc function
int c;
size_t i = 0;
unsigned int dim; // array's length
// allocate memory for the 1st number
numbers = malloc(sizeof(*numbers));
if (numbers == NULL) {
fputs("Error while allocating memory!\n", stderr);
} else {
printf("Insert the 1 number: ");
scanf("%d", &numbers[0]);
++i;
/* allocate memory for the other numbers,
* until the user inputs EOF
*/
while (!feof(stdin)) {
test = realloc(numbers, (sizeof(*numbers) * (i + 1)));
if (test == NULL) {
fputs("Error while allocating memory!\n", stderr);
} else {
numbers = test;
printf("Insert the %u number: ", i + 1);
scanf("%d", &numbers[i]);
++i;
}
}
dim = --i;
// print the array, 5 numbers per line
for (i = 0; i < dim; ++i) {
if (i % 5 == 0) {
puts("");
}
printf("%d ", numbers[i]);
}
puts("");
}
getchar();
return 0;
}
But I want to do this (which doesn't work):
#include <stdio.h>
#include <stdlib.h>
void get_numbers(int **numbers, unsigned int *dim);
int main()
{
int *numbers; // pointer to create a dynamic array
size_t i = 0;
unsigned int dim; // array's length
get_numbers(&numbers, &dim);
// print the array, 5 numbers per line
for (i = 0; i < dim; ++i) {
if (i % 5 == 0) {
puts("");
}
printf("%d ", numbers[i]);
}
puts("");
getchar();
return 0;
}
void get_numbers(int **numbers, unsigned int *dim)
{
int *test; // pointer to test realloc function
int c;
size_t i = 0;
// allocate memory for the 1st number
*numbers = malloc(sizeof(*numbers));
if (*numbers == NULL) {
fputs("Error while allocating memory!\n", stderr);
} else {
printf("Insert the 1 number: ");
scanf("%d", &numbers[0]); // Maybe the error is here
++i;
/* allocate memory for the other numbers,
* until the user inputs EOF
*/
while (!feof(stdin)) {
test = realloc(*numbers, (sizeof(*numbers) * (i + 1)));
if (test == NULL) {
fputs("Error while allocating memory!\n", stderr);
} else {
*numbers = test;
printf("Insert the %u number: ", i + 1);
scanf("%d", &numbers[i]);
++i;
}
}
*dim = --i;
}
}
Do I need to use a pointer to a pointer to int, right?
I know I made some mistakes but I can't figure out how to fix them.
Thanks in advance for your replies!
Bye,
Fabio Nardelli

Answer 1

In the first block of code, numbers is of type int*.
In the second block of code, numbers is of type int**.
You have not fixed all the places that need to be fixed because of that change.
My suggestion:
Change the input argument to numbersPtr.
Create a local variable int* numbers.
Before you return from the function, set *numbersPtr to number.
void get_numbers(int **numbersPtr, unsigned int *dim)
{
int* numbers;
...
*numbersPtr = numbers;
}
It will be better to change get_numbers to return numbers.
int* get_numbers(unsigned int *dim)
{
int* numbers;
...
return numbers;
}
and change its usage as:
numbers = get_numbers(&dim);