2D arrays of structs

[Dr Sup]

Hey all,

I am having some issues trying to initialize and access a 2-dimensional array of structures (PageTableEntry's).

I have:

page_map = safe_malloc(num_processes * sizeof(PageTableEntry));
    for(i=0;i<num_processes;i++)
    {
        page_map[i] = safe_malloc(num_virtual_pages * sizeof(PageTableEntry));
    }
 
for(i=0;i<num_processes;i++)
    {
        for(j=0;j<num_virtual_pages;j++)
        {
            page_map[i][j] = NULL;
        }
    }

on compilation i get: error: incompatible types in assignment; at the line 11. I get a similar error when i attempt to assign:
page_map[i][j]->permissions = READ_WRITE; or any other assignment for that matter.

Any help would be greatly appreciated.

[AdrianH]

Hey all,

I am having some issues trying to initialize and access a 2-dimensional array of structures (PageTableEntry's).

I have:

    page_map = safe_malloc(num_processes * sizeof(PageTableEntry));
    for(i=0;i<num_processes;i++)
    {
        page_map[i] = safe_malloc(num_virtual_pages * sizeof(PageTableEntry));
    }
 
    for(i=0;i<num_processes;i++)
    {
        for(j=0;j<num_virtual_pages;j++)
        {
            page_map[i][j] = NULL;
        }
    }

on compilation i get: error: incompatible types in assignment; at the line 11. I get a similar error when i attempt to assign:
page_map[i][j]->permissions = READ_WRITE; or any other assignment for that matter.

Any help would be greatly appreciated.

Not knowing the type that you declared page_map as, I am assuming that you declared it as a 2d array of structs? If so, if you dereference the array twice with the [] operator, it becomes a user defined struct type, not a pointer to a user defined struct type.

I.e.

struct A {
  int x;
};
 
A a[1][4];
a[0][2] = NULL; // invalid since NULL cannot be assigned to a struct A type.
 
// Similarly...
A** a2;
a2 = (struct A**)malloc(sizeof(A**)*1);
a2[0] = (struct A*)malloc(sizeof(A*)*4);
a2[0][2] = NULL; // invalid since NULL cannot be assigned to a struct A type.
 

Adrian