Include Guards in C

Question

I have 2 header files that have to include one other.

config.h:

#ifndef CONFIG
#define CONFIG

#include "debug.h"

typedef struct Config_t {
    /* some stuff */
} Config;
#endif

debug.h

#ifndef DEBUG
#define DEBUG

#include "config.h"

void somePrintingFunction(Config* conf);
#endif

Here is the error I get:

debug.h: error: unknown type name 'Config'

config.c: warning: implicit declaration of function 'somePrintingFunction'

debug.h: error: unknown type name 'Config'

I guess it's looping in the header declaration?

Edit:

Fixed in merging both files so simplify project design. If you want a real fix check in comments.

You get warning message from config.c, but you haven't shown it. Please create [mcve]. — user694733, Nov 22 '18 at 13:35
There exists no scenario where 2 header files need to include each other. Your program design is confused and the only correct solution is to fix the program design. — Lundin, Nov 22 '18 at 13:51

Petr Skocik · Accepted Answer · 2018-11-22T13:56:48.167

4

When config.h includes debug.h, debug.h will try to include config.h but since the CONFIG guard macro will already have been defined, the retroactive "include" of config.h will get skipped and the parsing will continue on the next line:

void somePrintingFunction(Config* conf);

without the Config type having been defined.

As StoryTeller has pointed out, you can break the mutual dependency by forward declaring the Config_t struct:

struct Config_t;
void somePrintingFunction(struct Config_t* conf);

Since C11, you can also do the typedef since C11 can handle duplicated typedefs as long as they refer to the same type.

typedef struct Config_t Config;
void somePrintingFunction(Config* conf);

(Forward declarations of aggregates (structs or unions) don't allow you to declare objects of those types (unless a full definition has already been provided) but since C guarantees that all pointers to structs or unions must look the same, they are enough to allow you to start using pointers to those types.)

edited Nov 22 '18 at 13:56

answered Nov 22 '18 at 13:44

Petr Skocik

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I did not know C11 had that improvement. Nice. – StoryTeller - Unslander Monica Nov 22 '18 at 13:46
But I'll get a redefinition of Config? – BeGreen Nov 22 '18 at 13:49
@BeGreen You won't in C11 and newer is what I'm saying. Repeating a forward declaration is perfectly OK in all C's. Repeating a typedef to the same time is OK since C11 and in previous versions you might get a warning (especially if you compile with -pedantic). – Petr Skocik Nov 22 '18 at 13:53
`struct Config` should be `struct Config_t` to match OP's original code. @BeGreen , `Config` is being used as a structure tag in this case, which is in a different namespace to the `Config` defined by `typedef`. `struct Config_t;` is an incomplete type that will be completed once the full `struct Config_t { ... };` is encountered. The `void somePrintingFunction(struct Config_t *conf);` declaration is fine even if `struct Config_t` is incomplete. – Ian Abbott Nov 22 '18 at 13:55

score 3 · Answer 2 · edited Nov 22 '18 at 14:13

The problem with this is the circular inclusion. You are actually including your function before you declare your structure. Lets say you include config.h(assuming you removed the wrong include a.h the preprocessor does this:

#include "debug.h"

typedef struct Config_t {
    /* some stuff */
} Config;

and defines the symbol CONFIG so that this file will not be included twice. Then it evaluates the remaining include:

#include "config.h"

void somePrintingFunction(Config* conf);


typedef struct Config_t {
    /* some stuff */
} Config;

and defines the symbol DEBUG. Since symbol CONFIG is defined it will not include config.ha second time and thus it is finished. Now notice how the declaration of the function is before the declaration of the structure. To solve this use a forward declaration like this

#include "config.h"

struct Config_t;
void somePrintingFunction(struct Config_t* conf);

So the compiler knows what Config is before you use it. Just keep in mind to define all functions that use a predeclared structure or class in the c file because the predeclared object is not a defined one yet but will be in the c file.

EDIT: I should mention that circular inclusion is not a good thing to do and you can usually find another solution which is less sanity risking.

@Lundin whoops i corrected the fily types. The C-Preprocessor should remain the same anyway. — Yastanub, Nov 22 '18 at 13:58

StoryTeller - Unslander Monica · Answer 3 · 2018-11-22T13:35:59.153

2

debug.h doesn't need the other header. You can define that function just fine with a forward declaration of Config_t alone

struct Config_t;
void somePrintingFunction(struct Config_t* conf);

edited Nov 22 '18 at 13:35

answered Nov 22 '18 at 13:32

StoryTeller - Unslander Monica

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And the compiler will understand that it's not Config_t from debug.h but from a.h? – BeGreen Nov 22 '18 at 13:32
@BeGreen The compiler doesn't need to care. All struct pointers are guaranteed to look the same. – Petr Skocik Nov 22 '18 at 13:34
@BeGreen - You will need to include the other header when calling the function or defining it. So yes, it will. There can be only one structure definition with a given tag in a valid C program. If there isn't exactly one, you'll have much bigger problems. – StoryTeller - Unslander Monica Nov 22 '18 at 13:34
When I do this I get some issues in my function. "deferencing pointer to incomplete 'Config " {aka struct Config_t}' – BeGreen Nov 22 '18 at 13:42
@BeGreen - I did say you need to include the header with the structure definition when **defining the function**. In the corresponding *.c file. – StoryTeller - Unslander Monica Nov 22 '18 at 13:43
@BeGreen as i mentioned in my answer if you predeclare objects you can not use them before you declare the rest of it so your definition of your function needs to happen after the full declaration in the `cpp` file – Yastanub Nov 22 '18 at 13:44

Include Guards in C

3 Answers3