using typedef for structures in C

[Ravikant]

Hello,
Here I have written two types of declaring a structure.
Type:1
typedef struct node
{
1. U32 age;
2. char name[100];
3. struct node *rlink;
4. struct node *llink;
}Data_node;
Type:2
typedef struct
{
1. U32 age;
2. char name[100];
3. ..............;
4. ..............;
}Data_node;

Now I am not knowing what should be written in second type of
structure declaration at lines 3 and 4.


Please let me know about it.Thank you.
bye
Ravi

 

[Horst Kraemer]

Hello,
Here I have written two types of declaring a structure.
Type:1
typedef struct node
{
1. U32 age;
2. char name[100];
3. struct node *rlink;
4. struct node *llink;
}Data_node;

An equivalent variation

typedef
struct node Data_node;

struct node
{
U32 age;
char name[100];
Data_node *rlink;
Data_node *llink;
};

Type:2
typedef struct
{
1. U32 age;
2. char name[100];
3. ..............;
4. ..............;
}Data_node;

Now I am not knowing what should be written in second type of
structure declaration at lines 3 and 4.

Impossible. If you define a struct type without tag name (like 'node'
in your first example), i.e. an unnamed struct type, then you can't
define a pointer to this struct type inside the struct because the
struct has no name to refer to and its typedef name 'Date_node' is
only introduced textually *after* the complete declaration.

 

[Richard Bos]

Type:1
typedef struct node
{
1. U32 age;
2. char name[100];
3. struct node *rlink;
4. struct node *llink;
}Data_node;
Type:2
typedef struct
{
1. U32 age;
2. char name[100];
3. ..............;
4. ..............;
}Data_node;

Now I am not knowing what should be written in second type of
structure declaration at lines 3 and 4.

You can't do it like that. You can't use a Data_node *, because
Data_node hasn't been written yet. You can't use a struct <something> *,
because you've removed the <something>. So you need a trick. This one,
for example:

typedef Data_node *Data_node_ptr;
typedef struct {
U32 age; /* BTW, it's probably better not to use an explicitly sized
integer here, unless you're sure that it'll always be
large enough. And if it's a human age, a simple char
would suffice...*/
char name[100];
Data_node_ptr rlink;
Data_node_ptr llink;
} Data_node;

See also the FAQ, posted just this morning.

Richard

 

[Sean Kenwrick]

Type:1
typedef struct node
{
1. U32 age;
2. char name[100];
3. struct node *rlink;
4. struct node *llink;
}Data_node;
Type:2
typedef struct
{
1. U32 age;
2. char name[100];
3. ..............;
4. ..............;
}Data_node;

Now I am not knowing what should be written in second type of
structure declaration at lines 3 and 4.

You can't do it like that. You can't use a Data_node *, because
Data_node hasn't been written yet. You can't use a struct <something> *,
because you've removed the <something>. So you need a trick. This one,
for example:

typedef Data_node *Data_node_ptr;
typedef struct {
U32 age; /* BTW, it's probably better not to use an explicitly sized
integer here, unless you're sure that it'll always be
large enough. And if it's a human age, a simple char
would suffice...*/

Hey, I plan to live to around 129 years old which could crash his program if
he followed your advice above...

Sean

 

[CBFalconer]

Here I have written two types of declaring a structure.
Type:1
typedef struct node
{
1. U32 age;
2. char name[100];
3. struct node *rlink;
4. struct node *llink;
}Data_node;
Type:2
typedef struct
{
1. U32 age;
2. char name[100];
3. ..............;
4. ..............;
}Data_node;

Now I am not knowing what should be written in second type of
structure declaration at lines 3 and 4.

Yup, that is a problem. I always have difficulties referring to
nameless entities. With that in mind, which form do you think you
should actually use?

 

[Mark A. Odell]

Hey, I plan to live to around 129 years old which could crash his
program if he followed your advice above...

Plain char can be unsigned but if signed, then unsigned char should do you
fine.

 

[Dave Thompson]

(e-mail address removed) (Ravikant) wrote:

(selfreferential struct with and without tag)

You can't do it like that. You can't use a Data_node *, because
Data_node hasn't been written yet. You can't use a struct <something> *,
because you've removed the <something>. So you need a trick. This one,
for example:

typedef Data_node *Data_node_ptr;
typedef struct { [using Data_node_ptr]

Nope. You can have, or typedef, a pointer to incomplete type, but not
to completely unknown typename as in Pascal or Ada. There's no way to
make a struct type selfreferential without giving it a tag.

- David.Thompson1 at worldnet.att.net