How to initialize to NULL a 2D array of pointer structs in C?

Question

I have the following struct:

typedef void (*HSM_State_Fcn)(void*, size_t);
typedef HSM_State_Fcn HSM_Dest_State_Fcn;
typedef uint8_t (*HSM_Guard_Fcn)(void*, size_t);
typedef void (*HSM_Action_Fcn)(void*, size_t);

typedef struct HSM_Transition_Tag
{
    HSM_Dest_State_Fcn dest_fcn;
    HSM_Guard_Fcn      guard_fcn;
    HSM_Action_Fcn     action_fcn;
}HSM_Transition_T;

And I have the following 2D array that I want every element to be NULL :

static HSM_Transition_T Transition_Table_Array_Test[3][3] = {NULL};

As you can see I tried to equal to {NULL} but I get the following warning:

What would be the correct approach to initialize every element to NULL ?

If I make it:

static HSM_Transition_T Transition_Table_Array_Test[3][3] = {{NULL}};

I get the same error.

Answer 1

static HSM_Transition_T* Transition_Table_Array_Test[3][3]={{NULL}};

NULL should be used with pointers not structure.

If you want to initialize struct fields in 2d array of structs, then

static HSM_Transition_T Transition_Table_Array_Test[3][3]={{{NULL,NULL,NULL}}};

Answer 2

The reason you're getting a warning is because NULL is a primitive, but the elements of your array are aggregates; they need to either be initialized with an aggregate variable (of the right type), or with yet a third braced expression (constructing an aggregate in-place).

The type of the array does not admit using NULL to initialize the elements, because structure values cannot be NULL , only pointers. Structures exist in-place, so it's not possible for them to be "absent" from the array (the elements of the array are the structures; if the array exists, so do they).

The compiler is complaining that braces are still missing, rather than telling you a structure value cannot be NULL , because it thinks you're trying to initialize the function pointers within the HSM_Transition_T to NULL (which is an operation that would make sense).

Answer 3

You can write:

static HSM_Transition_T Transition_Table_Array_Test[3][3] = {
    {NULL, NULL, NULL},
    {NULL, NULL, NULL},
    {NULL, NULL, NULL}};

But NULL is mostly used with pointer and not struct.

You would have written (note the '*' before Transition_Table_Array_Test :

static HSM_Transition_T *Transition_Table_Array_Test[3][3] = {
    {NULL, NULL, NULL},
    {NULL, NULL, NULL},
    {NULL, NULL, NULL}};