structure

[koder]

i have different structure definitions.cou ld any one tell me why it is not taking the folowing variations

ex1:
#include<stdio.h>

struct{
int i;              // 4 bytes
char c;          // 1 byte
char b;          // 1 byte
};

main()
 {
struct a p;



  printf("%d",sizeof(p));
}

gcc produces the given error :storage size of `p' isn't known

ex2.

#include<stdio.h>

struct {
int i;              // 4 bytes
char c;          // 1 byte
char b;          // 1 byte
}a;

main()
 {
struct a p;



  printf("%d",sizeof(p));
}

gcc produces "storage size of `p' isn't known"

could any one tell me why this above are wrong?

[tyreld]

You are using sizeof wrong. It expects a type to be passed to it, but you are passing an actual variable.

struct a p;

// Wrong
sizeof(p);

// Correct
sizeof(struct a);

[Banfa]

You are using sizeof wrong. It expects a type to be passed to it, but you are passing an actual variable.

struct a p;

// Wrong
sizeof(p);

// Correct
sizeof(struct a);

No this is wrong sizeof can take a type or a variable and most people suggest using the variable name because then if the type changes the code stays correct with any further changes.

The problem lies in the way the structure is declared

struct {
  int i;              // 4 bytes
  char c;          // 1 byte
  char b;          // 1 byte
}a;

struct a p;

This does not define a type of structure named a, it defines a variable a that is a structure you need

struct [b]a[/b] {
  int i;              // 4 bytes
  char c;          // 1 byte
  char b;          // 1 byte
};

struct a p;

This defines a type of structure named a which can then be used to define a variable p of that type.


P.S.

main()

should be

int main()

to avoid undefined behaviour

[tyreld]

Banfa, you are very much correct. Thanks for catching that. Past my bedtime it would seem.

[koder]

No this is wrong sizeof can take a type or a variable and most people suggest using the variable name because then if the type changes the code stays correct with any further changes.

The problem lies in the way the structure is declared

struct {
  int i;              // 4 bytes
  char c;          // 1 byte
  char b;          // 1 byte
}a;

struct a p;

This does not define a type of structure named a, it defines a variable a that is a structure you need

struct [b]a[/b] {
  int i;              // 4 bytes
  char c;          // 1 byte
  char b;          // 1 byte
};

struct a p;

This defines a type of structure named a which can then be used to define a variable p of that type.


P.S.

main()

should be

int main()

to avoid undefined behaviour

hai Banfa, really glad to see u explain the things from basics.But we have a rule that we dont need to give structure name,after the keyword struct,..in that case how i will have a structure definition and have a variable if that structure type

[Banfa]

hai Banfa, really glad to see u explain the things from basics.But we have a rule that we dont need to give structure name,after the keyword struct,..in that case how i will have a structure definition and have a variable if that structure type

If you don't put a name after the struct keyword you have not named the structure type and will not be able to declare variables as

struct a p;

However if you don't want to name you structures you can use a typedef to define a new type for the structure

typedef struct {
  int i;              // 4 bytes
  char c;          // 1 byte
  char b;          // 1 byte
} a;

a is now a new type (although very poorly named) that is the structure, you can then declare variables

a p;