Thread: Passing arrays between functions

Hello everyone, I'm an amateur C programmer working on what every amateur programmer wants to do: make a game. In my case a text adventure, though it's not very far along yet. Anyways, I'm having trouble with passing a pointer to a static array from one function to another. Here's the important parts of my code:

Code:

//test_run.c
#include <stdio.h>
#include "my_functions.h"

int main() {
   long *array_var=my_function();
   printf("%ld\n",array_var[0][1]);
   return 0;
}

//my_functions.c
#include <stdio.h>
#include "my_functions.h"

long *my_function() {

   static long array_var[10][2];
   /*some other declarations*/

   /*values are added to the array by use of a while loop*/

   return array_var;
}

//my_functions.h
long *my_function()

Here's what happens when I compile "my_functions.c."

Code:

>c99 -c my_functions.c -I.
>warning: return from incompatible pointer type

If I change the last line of my_functions.c from "return array_var" to "return *array_var", the warning goes away. This doesn't make sense to me.

And in either case when I try and compile "test_run.c":

Code:

>c99 test_run.c my_functions.o -I.
>error: subscripted value is neither array nor pointer

Needless to say, when I remove the subscripts it compiles, but I print out some useless number.

Can someone help me out?

Originally Posted by jtay

Here's what happens when I compile "my_functions.c."

Code:

>c99 -c my_functions.c -I.
>warning: return from incompatible pointer type

Because my_function() is returning a pointer to an array of long while in the caller array_var is a simple pointer to long.

Originally Posted by jtay

If I change the last line of my_functions.c from "return array_var" to "return *array_var", the warning goes away. This doesn't make sense to me.

Yep! because now the return type matches the receiving type.

Originally Posted by jtay

And in either case when I try and compile "test_run.c":

Code:

>c99 test_run.c my_functions.o -I.
>error: subscripted value is neither array nor pointer

Needless to say, when I remove the subscripts it compiles, but I print out some useless number.

Can someone help me out?

Again, in the caller array_var is a pointer to long so it can't be double-scripted.

I've fiddled with the code a bit more, but I still can't find what I need to change to get this to work. I don't see how my_function is returning a pointer to an array, since it's declared as long *my_function(). The reason I was trying to say return array_var instead of return *array_var, is because I was under the impression that the name of the array is more or less equivalent to a pointer to the first element in the array. I'm afraid I'm going to need a few more hints from some people before I can figure this out.

Okay, I'm starting to understand how this works. So since my_function is a pointer to long, array_var can only be subscripted once. How would I go about making this so I can subscript it twice? Right now when I print array_var[1], I get the value in row 0, column 1; array_var[2] gives me row 1, column 0; array_var[3] gives me row 1, column 1; etc.

Thanks for that! It took me a little while to figure out how to put the typedef in my header file, but my code is working now. I still only understand maybe 70% how this works, but I expect I'll run into this problem several more times, so that should get me some practice. This whole array of pointers to arrays thing is tough to understand the first time around.