C11: Typedef of an anonymous struct works with pointers to the typedef as a member?

Question

Here is how I would usually define a Node for a linked list in C:

typedef struct _Node {
    int value;
    struct _Node * next;
} Node;

With some testing, I have found that I am able to get this definition to work via an anonymous struct, like this:

typedef struct {
    int value;
    struct Node * next;
} Node;

Is there a reason why this works? I feel like it shouldn't work since Node isn't a type yet in the struct definition.

My hypothesis is that the struct Node * next is an anonymous struct itself that only has a pointer to a Node. But if this is the case then how am I able to use the Node type here if the typedef isn't resolved yet?

Answer 1

This only seems to work. The "tag" name space and the name space of "ordinary identifiers" are completely distinct in C. (C++ has slightly different rules)

Your member next of the anonymous struct points to an incomplete struct Node that is forward-declared by that same declaration.

Your typedef names an anonymous struct that is distinct from that type struct Node . So your pointer next points to a type for which the compiler knows nothing more than that it is a struct .

A good strategy to avoid such headaches is to forward declare struct Node and the typedef to be the same:

typedef struct Node Node;
               ^^^^ ^^^^
               tag  typedef name

and then

struct Node {
  ...
};

Answer 2

This declaration

typedef struct {
    int value;
    struct Node * next;
} Node;

introduces two structure specifiers. The first one is the unnamed structure with the typedef name Node . The second one is the named structure struct Node .

These two structure specifiers are different.

So for example the compiler will issue a warning or an error when you will try to assign a pointer of the type Node * to a pointer of the type struct Node * or vice versa because there are used incompatible pointer types.

Consider the following demonstration program.

#include <stdlib.h>
#include <stdio.h>

typedef struct {
    int value;
    struct Node * next;
} Node;

int main( void )
{
    Node *head = malloc( sizeof( Node ) );
    head->value = 1;
    head->next = NULL;

    Node *next = malloc( sizeof( Node ) );
    next->value = 2;
    next->next = NULL;

    head->next = next;
}

For the statement

head->next = next;

for example the compiler gcc 11.2 issues the following warnings

<source>:18:16: warning: assignment to 'struct Node *' from incompatible pointer type 'Node *' [-Wincompatible-pointer-types]
   18 |     head->next = next;
      |                ^
ASM generation compiler returned: 0
<source>: In function 'main':
<source>:18:16: warning: assignment to 'struct Node *' from incompatible pointer type 'Node *' [-Wincompatible-pointer-types]
   18 |     head->next = next;
      |               

Moreover you will be unable to dereference a pointer of the type struct Node * because the type struct Node is an incomplete type. The definition of struct Node is unknown.

For example if you will append the above demonstration program with statement

printf( "%d\n", head->next->value );

then the compiler at this time will issue the following error message

<source>:21:31: error: invalid use of undefined type 'struct Node'
   21 |     printf( "%d\n", head->next->value );
      |                               ^~

Instead you could write for example

typedef struct Node Node;

struct Node {
    int value;
    Node * next;
};