How do I access data from a dynamic array in c? - c

Question

I've got two different arrays that I'm using. With one, I'm getting the exact results that I want, the other, not so much. I'm filing the arrays with by reading from a text file similar to this:
2597
283
4
723
21
82
426
The first five lines would be the customer IDs. There is always 5 lines but they don't always have a value. The next line is the number of vendors, then followed by the vendor ids.
void use_arrays()
{
int
i,
customer_count,
*customer_ids,
vendor_count,
*vendor_ids;
customer_ids = malloc(sizeof(int));
vendor_ids = malloc(sizeof(int));
fill_arrays(&customer_count, customer_ids, &vendor_count, vendor_ids);
for (i = 0; i < customer_count; i++)
{
printf("Customer[%d]: %d\n", i, customer_ids[i]);
}
for (i = 0; i < vendor_count; i++)
{
printf("Vendor[%d]: %d\n", i, vendor_ids[i]);
}
free(customer_ids);
free(vendor_ids);
}
void fill_arrays(int *customer_count, int *customer_ids, int *vendor_count, int *vendor_ids)
{
int
i,
*temp,
customer_id,
vendor_id,
num_cust = 0;
FILE
*inp_file;
char
*endptr = NULL,
buffer[500];
inp_file = fopen(g_filename, "r");
for (i = 0; i < 5; i++) /* Can't be more than 5 customers */
{
fgets(buffer, sizeof(buffer), inp_file);
customer_id = strtol(buffer, &endptr, 0);
if (customer_id != 0)
{
customer_ids[i] = customer_id;
temp = realloc(customer_ids, (i+2)*sizeof(int));
if (temp != NULL)
{
customer_ids = temp;
}
else
{
printf("Couldn't allocate memory\n");
}
num_cust++;
}
}
*customer_count = num_cust;
/* Next is number of vendor ids*/
fgets(buffer, sizeof(buffer), inp_file);
*vendor_count = strtol(buffer, &endptr, 0);
temp = realloc(vendor_ids, *vendor_count*sizeof(int));
if (temp != NULL)
{
vendor_ids = temp;
}
else
{
printf("Couldn't allocate memory\n");
}
for (i = 0; i < *vendor_count; i++)
{
fgets(buffer, sizeof(buffer), inp_file);
vendor_id = strtol(buffer, &endptr, 0);
if (vendor_id != 0)
{
vendor_ids[i] = vendor_id;
}
}
fclose(inp_file);
}
Once the arrays print out, customer_ids is showing the correct numbers but vendor_ids is printing out random numbers from memory. To be more frustrating, it prints the vendors correctly from inside fill_arrays.

Answer 1

If you want to modify vendor_ids the way you do in fill_arrays, then you have to pass it in as a pointer to a pointer:
fill_arrays(int *customer_count, int *customer_ids, int *vendor_count, int **vendor_ids)
Call it like this:
fill_arrays(&customer_count, customer_ids, &vendor_count, &vendor_ids);
Then you can realloc like so:
temp = realloc(*vendor_ids, *vendor_count*sizeof(int));
if (temp != NULL)
{
*vendor_ids = temp;
}
Also, at the end of your function:
vendor_ids[i] = vendor_id;
will have to change to
(*vendor_ids)[i] = vendor_id;
You will also have to make the same changes to customer_ids. The fact that customer_ids was working while vendor_ids wasn't was probably due to your use of realloc. If realloc decides that the memory block has to be reallocated in a new location, you'll run into these problems but if reallocates the memory in the same location, your pointer that you passed in is still pointing there. Since you never know if realloc is going to make that descision or not, both customer_ids and vendor_ids should be passed in as pointers to pointers.

Answer 2

You seem a bit confused how to return memory from a function.
if you have a function that looks like this
void foo(int a);
you can not change a inside of foo, foo only gets a copy of a.
void foo(int *a);
otoh gives foo the address of a, i.e. "i know where you live" it lets you change what a points to. if a points to an array then you can change the contents of that array, if a points to a single integer you can change that.
void foo(int **a);
lets you change not only what a points to, but also where it points. so if you want a to point to somewhere else you can. this is what you need in your function, if you do a malloc/calloc/realloc in your function to return the result you need this