Return a pointer to a 2-D array from a C-function

Question

I know this question has been asked here, here and here and many more times, but something's not working yet. I am trying to make a karnaugh map solver in C and I want one of my functions to pass a 2D array back to the main function, so here is my program:

    typedef char (*arr)[][4];

    int rows;

    int main(void){
      int noVar;
      char *grid;
      printf("Please Enter the number of variables: ");


      scanf("%d",&noVar);
      grid = setMap(noVar);
      feedOnes(grid);
    }

    arr setMap(int noVar){

    rows = pow(2,noVar)/4;
    char grid[rows][4];
    for(int i = 0; i<rows; i++){
        for(int j = 0; j<4; j++){
            grid[i][j]='0';
        }
    }
    printGrid(grid);
    return &grid;
}

This gives me an 4 warnings while it does the job for now:

    In file included from kmap.c:9:
./setMap.h:33:13: warning: address of stack memory associated with local
      variable 'grid' returned [-Wreturn-stack-address]
    return &grid;
            ^~~~
./setMap.h:56:1: warning: control reaches end of non-void function
      [-Wreturn-type]
}
^
kmap.c:16:8: warning: incompatible pointer types assigning to 'char *' from
      'arr' (aka 'char (*)[][4]') [-Wincompatible-pointer-types]
  grid = setMap(noVar);
       ^ ~~~~~~~~~~~~~
kmap.c:17:12: warning: incompatible pointer types passing 'char *' to parameter
      of type 'char (*)[4]' [-Wincompatible-pointer-types]
  feedOnes(grid);
           ^~~~
./setMap.h:36:19: note: passing argument to parameter 'grid' here
int feedOnes(char grid[][4]){
                  ^

My question is, can I resolve these warnings? and will these warnings cause any problem in future as I do not know why are they appearing

Also, I am a newbie so please don't be harsh at me if this question was not asked properly..

Thank You.

and yes, feedOnes is just a program that prints the output.. nothing else — sameer, Dec 03 '16 at 22:15
Did you read the possible duplicates? You cannot return a pointer to an array. Usually, you create the array where you want and pass it to functions as a pointer, do anything you want with the array in the scope you created it, including functions called in the same scope. But returning it is a problem, because it's allocated in a way that will cause deallocation when it goes out of scope. — Iharob Al Asimi, Dec 03 '16 at 22:18
You could try actually doing the things suggested in your links "here, here and here" — M.M, Dec 03 '16 at 23:38
I did try them, but they all generated a bunch of errors, or segmentation faults if grid sizes increased beyond 128 — sameer, Dec 07 '16 at 13:20

score 0 · Accepted Answer · edited May 23 '17 at 12:24

The arraygrid[][] is local to the setMap() function. Once this function returns, the variable is no longer available and is out-of-scope. Local variables are declared on the stack memory.

In order for something like this to work you will need to allocate and deallocate memory from the heap using the malloc() and free()function calls respectively.

Refer to this link for clarification on Stack vs Heap Memory:

What and where are the stack and heap?

Refer to this link for scoping in C:

https://www.tutorialspoint.com/cprogramming/c_scope_rules.htm

Happy Programming!

Return a pointer to a 2-D array from a C-function

1 Answers1