error: request for member data in something not a structure or union despite using arrow operator

Question

I am facing an error: request for member data in something not a structure or union. I am passing the address of a structure receiver_data in a structure global_vals to a function in another file.

The init_func function receives the address as a pointer. I used the arrow operator since its a pointer but I cant seem to access the structure. Both memory addresses &val.receiver_data and *ptr are the same so I'm not quite sure where went wrong

Would appreciate if someone can point out my mistake.

The components are split across various source/ header files with the following structure.

main.c
func_init.c
datatypes.c

main.c

global_vals val;
void main(void)
{
    val.receiver_data.data = 10;
    init_func(&val.receiver_data);
}

func_init.c

void init_func(int *ptr_data)
{
    printf("%d\n", ptr_data->data);
}

datatypes.h

typedef struct Receiver {
    int data;
} Receiver;
    
typedef struct {
    Receiver receiver_data;
    // truncated
} global_vals;

As far as I know, `int* ptr_data` cannot have any members, like `->data`. It's an integer. — Captain Trojan, Mar 03 '21 at 11:35
That you don't get more compiler messages is because you don't declare the `init_func` function before you call it. If you did then you would be getting messages that you try to pass invalid arguments to the function. Always make it a habit to declare the functions you call, before you call them. — Some programmer dude, Mar 03 '21 at 11:37
@Someprogrammerdude Or, you know, simply don't use a compiler which hasn't been updated in the past 22 years... implicit function declarations were removed from the C language during the previous millennium. — Lundin, Mar 03 '21 at 11:48

Vlad from Moscow · Accepted Answer · 2021-03-03T11:47:52.577

The function parameter

void init_func(int *ptr_data)
{
    printf("%d\n", ptr_data->data);
}

has the type int * that is it is a pointer to a scalar object. So this construction ptr_data->data is invalid because integers do not have data members like data.

It is not clear why the function is not declared like

void init_func( Receiver *ptr_data);

An alternative approach is to declare and define the function like

void init_func(int *ptr_data)
{
    printf("%d\n", *ptr_data);
}

and call it like

init_func(&val.receiver_data.data);

I hope that actually the function does something useful instead of just outputting an integer.

score 0 · Answer 2 · answered Mar 03 '21 at 11:43

The compiler says:

error: request for member 'data' in something not a structure or union

The compiler points at the exact location of the bug:

|     printf("%d\n", ptr_data->data);
|                            ^~

The programmer thinks: Is ptr_data really a structure or union?
The programmer checks the declaration int *ptr_data. No, it is not.

error: request for member data in something not a structure or union despite using arrow operator

2 Answers2