What does this C code mean?

Question

I know that typedef'ing allows you to refer to a struct without using the struct keyword. However, with this:

typedef struct SYSTEM SYSTEM;

I've seen this typedef in a C API, but struct SYSTEM has not been defined previously.

In fact, a .c file containing only that line will compile by itself.

What does the compiler think SYSTEM (or struct SYSTEM) actually is?

Many thanks!

possible duplicate of [What is the effect of typedef'ing a struct to itself?](http://stackoverflow.com/questions/17515787/what-is-the-effect-of-typedefing-a-struct-to-itself) — , Jul 07 '13 at 20:17
***You asked this question 18 minutes ago.*** It has been closed. Why are you reposting it, seriously? — , Jul 07 '13 at 20:18
@H2CO3: it's because he updated the question later and nobody answered that question... the updated question isn't a duplicate, and it shouldn't have been closed. and because there's no answer there this one shouldn't be closed as a duplicate either. oh well, it's complicated. — Karoly Horvath, Jul 07 '13 at 20:29
@haccks one should take time before to vote for close question and down vote. — Grijesh Chauhan, Jul 07 '13 at 20:37
@Grijesh; I did't vote down bro....and also I have not yet gained privilege to close questions :) — haccks, Jul 07 '13 at 20:39
@haccks I asked you so that you can support my comment ;) , sorry misinterpreted — Grijesh Chauhan, Jul 07 '13 at 20:44
@GrijeshChauhan; :D, very smart. Yes you are right, one should take time to down vote. — haccks, Jul 07 '13 at 20:47

Kerrek SB · Accepted Answer · 2013-07-07T20:19:02.143

5

The struct doesn't need to be defined. It only needs to be declared. The typedef declaration you present subsumes the declaration of struct SYSTEM.

So it's equivalent to:

struct SYSTEM;

typedef struct SYSTEM SYSTEM;

Thanks to this typedef, SYSTEM now names an incomplete type, so you can use it like SYSTEM * in interface definitions, without needing to expose the actual definition of the struct, which can be kept private:

public_header.h:

typedef struct SYSTEM SYSTEM;

SYSTEM * create_system();

int do_useful_thing_with(SYSTEM * system, int count, void * addr);

void destroy_system(SYSTEM * system);

private_impl.c:

#include <public_header.h>

struct SYSTEM { int a; char b[12]; float q; void * next; };

SYSTEM * create_system()
{ 
    SYSTEM * p = malloc(sizeof *p);
    if (p) { p->a = 2; p->q = 1.5; }
    return p;
}

void destroy_system(SYSTEM * system)
{
    free(system);
}

// etc.

edited Jul 07 '13 at 20:19

answered Jul 07 '13 at 20:13

Kerrek SB

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Thanks! So presumably the API programmers have defined this somewhere on the API-side of the interface? – James Jul 07 '13 at 20:14
@James: It's not empty. It's undefined. An empty struct is a defined struct like `struct Foo {};`. I added an explanation; you can use it as an incomplete type. – Kerrek SB Jul 07 '13 at 20:15
1

The purpose of being allowed to reference undefined structs is so that you can create pointers to that struct - perhaps from within that struct (this is most useful for declaring linked-lists, for example). – PP. Jul 07 '13 at 20:16

score 0 · Answer 2 · answered Jul 07 '13 at 20:19

This is a typical C construct, as opposed to C++ and Java etc, that directly define types in the global namespace. C structs cannot be accessed by their name only, so the typedef is needed to be able to call a SYSTEM just SYSTEM.

struct SYSTEM {
  ...
};
struct SYSTEM foo;   //legal
SYSTEM bar;   //illegal
typedef struct SYSTEM SYSTEM;   //give it a global name
SYSTEM baz;   //now it's legal

score -1 · Answer 3 · answered Jul 07 '13 at 20:14

-1

I'd say this is generally ill-advised behaviour but the gist is:

typedef type name

..where here the type is struct SYSTEM and the name assigned is SYSTEM.

Usually one would give a different name to the typedef than the type the typedef is derived from. Hence the low opinion many code maintainers might have to this kind of behaviour.

answered Jul 07 '13 at 20:14

PP.

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It is good if you use it right, it is bad if you abuse it. Good usage is usually precisely what the OP asked about (getting rid of unnecessary `struct` clutter), bad usage is precisely what you are talking about (giving arbitrary names to stuff already named). – cmaster - reinstate monica Jul 07 '13 at 20:23
1

Calling `struct` "clutter" is a gross misunderstanding of the type system in C and/or pure laziness. – PP. Jul 08 '13 at 07:11
*Usually one would give a different name to the typedef than the type the typedef is derived from.* Um, no. `typedef struct NAME NAME` is an extremely common idiom in older C code. It's DIY syntactic sugar to avoid having to type "struct" over and over. Nothing wrong with it. – dodgethesteamroller Jul 08 '13 at 14:26

What does this C code mean?

3 Answers3