Why does “typdef struct { struct S *s; } S;” containing a pointer to same type compile?

by

In the first case, your struct has two equivalent names: struct Element (where Element is a struct tag) and Element (where Element is a typedef, an alias for an existing type).

In the second case, you just didn’t define a tag for the struct. Normally that would be perfectly valid, but here you’re referring to the nonexistent type struct Element in the declaration of the next member.

In that context, struct Element is an incomplete type. You can’t declare objects of incomplete types, but you can declare pointers to them.

The declaration

typedef struct
{
    char value;
    struct Element *next;
} Element;

is legal, but it doesn’t make next a pointer to the enclosing type. It makes it a pointer to some incomplete type, and you won’t be able to refer to it until and unless you declare the full type.

Your second declaration is one of the plethora of things that don’t make sense, but are still legal C.

You might consider just omitting the typedef and consistently referring to the type as struct Element. As lot of people like the convenience of having a one-word name for a structure type, but my own personal opinion is that there’s not much benefit to that (unless the type is truly opaque, i.e., users of the type don’t even know it’s a struct). It’s a matter of style.

Note that you need to refer to the type as struct Element, not Element, within the definition itself, since the typedef name Element isn’t visible yet.

The fact that the struct tag and the typedef have the same name may seem confusing, but it’s perfectly legititimate. Struct tags and typedefs are in separate namespaces (in the C sense, not the C++ sense); a struct tag can only appear immediately after the struct keyword.

Another alternative is to separate the typedef from the struct definition:

typedef struct Element Element;

struct Element {
    char value;
    Element *next;
};

(You can use an incomplete type name in a typedef.)

More Related Contents:

  1. Is there a way to use a typedef struct variable in a function…? [closed]
  2. typedef struct vs struct definitions [duplicate]
  3. self referential struct definition?
  4. Why should we typedef a struct so often in C?
  5. How to define a typedef struct containing pointers to itself?
  6. Opaque C structs: various ways to declare them
  7. What is ‘forward declaration’ and the difference between ‘typedef struct X’ and ‘struct X’?
  8. C typedef of pointer to structure
  9. C Typedef and Struct Question
  10. C : typedef struct name {…}; VS typedef struct{…} name;
  11. C Typedef – Incomplete Type
  12. dereferencing pointer to incomplete type
  13. C pointer to another struct
  14. Are typedef and #define the same in c?
  15. When are anonymous structs and unions useful in C11?
  16. Is an empty initializer list valid C code?
  17. C: pointer to struct in the struct definition
  18. Passing a 2D array of structs
  19. uint8_t vs unsigned char
  20. C struct initialization using labels. It works, but how?
  21. What is a designated initializer in C?
  22. Incompatible implicit declaration of built-in function ‘malloc’
  23. Enforce strong type checking in C (type strictness for typedefs)
  24. Are there any downsides to passing structs by value in C, rather than passing a pointer?
  25. Need help using qsort with an array of structs
  26. Rationale behind the container_of macro in linux/list.h
  27. sizeof single struct member in C
  28. Array of structs replacing values over itself
  29. How to create a typedef for function pointers
  30. Why does my function not modify the value of variable in a struct in C?

Leave a Comment