Access a 2D array[x][y] as a 1D array[z] in C

Question

I recently started writing chunks of C code as part of my university's programming lessons (so you can freely assume that I am a complete idiot). I'm trying to write a function that writes a 2D array's data to a file, but I'm having difficulties. I declare the array in main, I have its x and y dimensions saved as #defines, and I want to call my function() like so;

include "function.h"
#define /* x_res, y_res */
int main(){
  static unsigned char pic[x_res][y_res];
  /* do some operations on pic*/
  function(pic,x_res,y_res);
}

The function itself is saved in a header file and is intended to be included at the very top of my main .c file. It goes something like this;

void function(unsigned char arry,int x_res,int y_res){
    /* some calculations, declaring file pointer with fopen() */
    for(int i=0;i<y_res;i++){
        for(int j=0;j<x_res;j++){
            fprintf(f,"%c",arry[i][j]);
        }
    }
}

I'm greeted with an error in the line fprintf(f,"%c",arry[i][j]); saying that the "subscripted value is neither array nor pointer nor vector", which is false since I know that arry is an array. Furthermore, if I try to replace said line with something like fprintf(f,"%c",arry[i*j+j]);, the error goes away, but the file output is gibberish (I'm assuming I'm only printing the addresses of the first-dimension elements of arry).

The question, then; Why can't 2D arrays be accessed like their 1D counterparts, and how do I work around this? I would imagine that an int array[][]={{0,1},{2,3}}; would give an output of

array[0] -> 0
array[1] -> 1
array[2] -> 2
array[3] -> 3

, but this is not the case -- it prints 0, 2, and then two memory addresses.

I've tried declaring my function to accept arguments as void function(unsigned char arry[*value of x_res*][*value of y_res*],x_res,y_res), which works but is not how I would like the function to work.

I've looked at some other online examples but it seems few people have had a similar problem. I tried some answers from this question but again things do not work. For example, using void function(unsigned char **arry,x_res,y_res) works with accessing the array as 2D (arry[i][j]), but again, like with the example above, most values (all that aren't in the first column) are trash.

Answer 1

In C99 and later, it is possible to have a VLA

 void function(int x_res, int y_res, int char[][y_res])
 {
     for(int i=0;i<x_res;i++)
     {
          for(int j=0;j<y_res;j++)
          {
              fprintf(f,"%c",arry[i][j]);
          }
     }
 }

The problem is that support of an implementation for VLAs was made optional in C11 (i.e. a C11 compiler is not required to support them). And VLAs are definitely not supported in C90 (the ISO C standard of 1990).

An declared array is contiguous in memory, so can be treated like a flat 1D array. For example;

 void function2(int x_res, int y_res, unsigned char *arr)
 {
     for(int i=0;i<x_res;i++)
     {
          for(int j=0;j<y_res;j++)
          {
              fprintf(f,"%c",arr[i*y_res + j]);
          }
     }
 }

 int main()
 {
      unsigned char x[10][20];
      unsigned char y[10*20];
      unsigned char *z = malloc(10*20*sizeof(*z));

      /*  initialise elements x, y, and z  */

      function2(10,20, (unsigned char *)x);
      function2(10,20, &x[0][0]);

      function2(10,20, y);
      function2(10,20, z);
 }

The type conversion in the first call of function() is needed since a 2D array of unsigned char cannot be implicitly converted to a unsigned char *. However, the address of x and the address of x[0][0] have the same value, even though they have different types.

A gotcha with this technique is that the dimensions passed (first two arguments of function2()) are not checked at compile time. For example;

   int xx[5][6];

   function2(10, 20, (unsigned char *)xx);   /* danger, Will Robinson!! */
   function2(10, 20, &xx[0][0]);             /*  danger, danger!! */

will compile but, since the dimensions of xx are less than the first two arguments tell function2() to expect, will cause function2() to have undefined behaviour for both calls.