Returned struct array to variable in main() - c

Question

I have the following code to read an array of rectangles (defined as a struct from an input file). I now wish to call the read_shapes_rpt() function and save the returned value to a struct array in the main() function.
I am a rookie in C, and have only just started to understand pointers and the like, and I have encountered a bunch of errors in trying to get this done. Any help will be appreciated!
typedef struct Rectangle
{
float ll_x;
float ll_y;
float ur_x;
float ur_y;
} Rectangle;
struct Rectangle read_shapes_rpt()
{
FILE *f = fopen("check_pg_stapler_shapes.rpt", "r") ;
struct Rectangle shape_list[100000];
float temp_ll_x, temp_ll_y, temp_ur_x, temp_ur_y;
int i = 0;
while (fscanf(f, "%f,%f,%f,%f", &temp_ll_x, &temp_ll_y, &temp_ur_x, &temp_ur_y) != EOF) {
shape_list[i].ll_x = temp_ll_x;
shape_list[i].ll_y = temp_ll_y;
shape_list[i].ur_x = temp_ur_x;
shape_list[i].ur_y = temp_ur_y;
printf("%f,%f,%f,%f\n", temp_ll_x, temp_ll_y, temp_ur_x, temp_ur_y);
i++;
}
fclose(f);
return *shape_list;
}
int main()
{
struct Rectangle *rect_array[100000];
rect_array = read_shapes_rpt();
return 0;
}
The error message is as follows:
optimize_via_pgStapler.c: In function 'main':
optimize_via_pgStapler.c:38: error: incompatible types in assignment
Thank you!

Answer 1

You defined read_shapes_rpt to return a single struct Rectangle, but you're attempint to assign it to a array of pointers to struct Rectangle. You can't do an assignment like that.
What you probably want to do is pass rect_array (along with the size of the array) to read_shapes_rpt and work on that, and get rid of shape_list.
Returning a pointer to a local variable (in this case return *shape_list) is never a good thing, as the memory for that structure lives on the stack and is unusable once the function returns.

Answer 2

You declare the function to return a single structure; you can't assign that to a whole array of pointers to the structure type. You need to look at the memory management hard. You can't return arrays from functions. You should probably pass an array of the structure (not of pointers to the structure) into the function, and the return value from the function should be the number of valid entries in the array. You should also pass the size of the array to the function so it does not write out of bounds, causing a stack overflow.
#include <stdio.h>
typedef struct Rectangle
{
float ll_x;
float ll_y;
float ur_x;
float ur_y;
} Rectangle;
int read_shapes_rpt(const char *filename, int n, struct Rectangle shape_list[n])
{
FILE *f = fopen(filename, "r") ;
if (f == 0)
{
fprintf(stderr, "Failed to open file %s for reading\n", filename);
return -1;
}
float temp_ll_x, temp_ll_y, temp_ur_x, temp_ur_y;
int i = 0;
while (i < n && fscanf(f, "%f,%f,%f,%f", &temp_ll_x, &temp_ll_y, &temp_ur_x, &temp_ur_y) == 4) {
shape_list[i].ll_x = temp_ll_x;
shape_list[i].ll_y = temp_ll_y;
shape_list[i].ur_x = temp_ur_x;
shape_list[i].ur_y = temp_ur_y;
printf("%f,%f,%f,%f\n", temp_ll_x, temp_ll_y, temp_ur_x, temp_ur_y);
i++;
}
fclose(f);
return i;
}
int main(void)
{
enum { MAX_SHAPES = 100000 };
struct Rectangle rect_array[MAX_SHAPES];
int n_shapes = read_shapes_rpt("check_pg_stapler_shapes.rpt", MAX_SHAPES, rect_array);
if (n_shapes > 0)
{
/* Use the shapes that were read */
}
return 0;
}
Note that passing the file name to the function both generalizes the function and also (coincidentally) makes it easier to report errors meaningfully without repeating yourself (or the name of the file). Passing the array size as shown uses a C99 feature. You can change the function parameter to struct Rectangle *shape_list if you are stuck with a C89/90 compiler — which you might be if you work on Windows.

Answer 3

struct Rectangle read_shapes_rpt() -- this function returns instance of type struct Rectangle which you are assigning to pointer to array of struct Rectangle.