C: sizeof single struct member

Question

Hi,

I am trying to declare a struct that is dependent upon another struct. I want to use sizeof to be safe/pedantic.

typedef struct _parent
{
  float calc ;
  char text[255] ;
  int used ;
} parent_t ;

Now I want to declare a struct child_t that has the same size as parent_t.text.

How can I do this? (Pseudo-code below.)

typedef struct _child
{
  char flag ;
  char text[sizeof(parent_t.text)] ;
  int used ;
} child_t ;

I tried a few different ways with parent_t and struct _parent, but my compiler will not accept.

As a trick, this seems to work:

parent_t* dummy ;
typedef struct _child
{
  char flag ;
  char text[sizeof(dummy->text)] ;
  int used ;
} child_t ;

Is it possible to declare child_t without the use of dummy?

Thanks.

Answer 1

Use a preprocessor directive, i.e. #define:

#define TEXT_LEN 255

typedef struct _parent
{
  float calc ;
  char text[TEXT_LEN] ;
  int used ;
} parent_t ;

typedef struct _child
{
  char flag ;
  char text[TEXT_LEN] ;
  int used ;
} child_t ;

Answer 2

You can use a preprocessor directive for size as:

#define TEXT_MAX_SIZE 255

and use it in both parent and child.

Answer 3

I am not on my development machine right now, but I think you can do one of the following:

sizeof(((parent_t *)0)->text)

sizeof(((parent_t){0}).text)

---

Edit: I like the member_size macro Joey suggested using this technique, I think I would use that.

Answer 4

Although defining the buffer size with a #define is one idiomatic way to do it, another would be to use a macro like this:

#define member_size(type, member) sizeof(((type *)0)->member)

and use it like this:

typedef struct
{
    float calc;
    char text[255];
    int used;
} Parent;

typedef struct
{
    char flag;
    char text[member_size(Parent, text)];
    int used;
} Child;

I'm actually a bit surprised that sizeof((type *)0)->member) is even allowed as a constant expression. Cool stuff.

Answer 5

Another possibility would be to define a type. The fact that you want to ensure the same size for the two fields is an indicator that you have the same semantics for them, I think.

typedef char description[255];

and then have a field

description text;

in both of your types.