function my_function()
{
...
  if (my_structarray[0] != NULL)
  {
    than do blablabla
  }
...
}

#include <acknex.h>

typedef struct {
	int x;
	int y;
} data;

data *d;

int main()
{
	if( d == NULL ) printf("Caught!");
	
	return 0;
}

...
MY_STRUCT* mystruct[20][20];

function struct_startup()
{
        //empty pointer first
	var i, j;
	for (i = 0; i < 20; i++)
	{  
		for (j = 0; j < 20; j++) mystruct[i][j] = NULL; 
	} 
	
	//than fill
        ...
}

//store filter for 20 types
var filter[20] = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };

function next_filter (var buttonnumber, PANEL* p_panelofbutton)
{
  //set to current type & filter
  var type = p_panelofbutton.skill_x; //e.g. 0
  var filter = filter[type]; //e.g. 1

                //till struct that is not NULL
		while(1)
		{
			filter[type] += buttonnumber * 2 - 3; //either +1 or -1
			if (filter[type] > 19) filter[type] = 0;
			if (filter[type] < 0) filter[type] = 19;	
			
			if (mystruct[type][filter] != NULL) break; //it always breaks here for some reason
                }		
  
  refresh_page(); //refresh a window with debug vars & digits etc.
}